Three-dimensional finite element model construction method for study and judgment of skull chopping case

Abstract

The invention relates to a three-dimensional finite element model construction method for research and judgment of a skull chopping case, which comprises the following steps of: constructing a personalized skull geometric model, and manufacturing a skull physical model by using a 3D printing technology; using a head physical model and an infrared light point motion capture system to perform a simulated chopping experiment and collect data, and calculating to obtain a wound morphological index; establishing a skull finite element model according to the personalized skull geometric model, and performing computer simulation on the formation of a cut skull wound; and comparing a simulation result with a wound morphological index, and verifying the effectiveness of the finite element model of the head. According to the method, a simulated chopping experiment and a finite element simulation experiment are carried out aiming at a chopping head, and the two experiments are organically unified; a simulation experiment result of the finite element model is effectively calibrated and verified through the simulated chopping experiment, the verified finite element model can be repeatedly subjected to a computer simulation experiment, forward deduction verification is carried out on various possibilities of a chopping case, and reverse study and judgment are achieved.

Description

Technical Field

[0001] The present invention relates to a method for constructing a three-dimensional finite element model for the study and judgment of head chopping cases, belonging to the technical fields of biomechanical modeling, computer simulation technology, and computer visualization technology. Background Art

[0002] The harm caused by criminals using knives to chop victims is a relatively common criminal act in civil disputes or terrorist attacks. After the occurrence of a case, criminal investigators such as forensic doctors usually examine the chop wounds (wounds) of the victim, and combine the physical evidence collected at the scene to comprehensively analyze and infer the process of the case, including information such as the characteristics of the chopping behavior, the type of knife, and the individual characteristics of the suspect, so as to provide more complete evidence for case investigation, prosecution, and court trial.

[0003] In chopping cases, the morphological characteristics of chop wounds, the discrimination of chopping tools, the action characteristics of chopping behavior, and the individual characteristics of the perpetrator are the four key elements in criminal investigation and court trial of chopping cases. Through scientific research, establishing a quantitative relationship between the morphological characteristics of chop wounds and the characteristics of knives and chopping behavior will greatly improve the scientific level of criminal investigation and court trial of chopping cases.

[0004] The inventor's research team has applied for multiple related patents and published related papers, as follows.

[0005] (1) On April 10, 2020, an invention patent "System and Method for Reappearing Chopping Injury Cases" (application number CN202010278731.8, authorization announcement number CN111523265B) was applied for. Among them, the system is composed of an input unit, a simulation analysis unit, and an output unit connected in sequence; the input unit includes a case scene clue module and an artificial inference module; the simulation analysis unit includes a machete digitization module, a chopping action digitization module, a multi-body system dynamics analysis module for the human body being chopped, and a finite element analysis module for the chopped part of the human body; the output unit includes a chopping event visualization module; this method uses the above system to conduct a reappearance demonstration in animation. The technical solution of this patent is based on the basic laws of human physiology, anatomy, and biomechanics, and realizes the reappearance of chopping injury cases through computational simulation technology and computer visualization technology, with the advantages of being scientific, reliable, intuitive, realistic, and flexible.

[0006] (2) On January 27, 2021, an invention patent "Upper Limb Movement Space Range Deduction System, Its Construction Method and Usage Method" was applied for (Application No. CN202110112278.8, Authorization Publication No. CN112949031B). Among them, in this construction method, the subject is used as the action performer to conduct a simulated kinematic experiment on the upper limb movement. Through the motion capture of the upper limb movement, kinematic data is obtained; a full-body human model is established; combined with the human model, the kinematic data is analyzed and processed to calculate the dynamic change values of the three-dimensional space coordinates of the human body node marker points, the tool or non-tool instrument or bare-handed hand marker points during the upper limb movement; the upper limb movement space range index is calculated; and an upper limb movement space range deduction system is established. The upper limb movement space range deduction system established by this patent technical solution can calculate the upper limb movement space range index of the assailant, help criminal investigators judge the compatibility between the movement trajectory of the assailant and the crime scene space in an assault case, and provide a scientific basis for inferring the evolution process of the case or event.

[0007] (3) On March 8, 2022, an invention patent "Reverse Research and Judgment System for the Formation of Chop Wounds, Its Construction Method and Research and Judgment Method" was applied for (Application No. CN202210220552.8, Authorization Publication No. CN114580241B). Among them, this construction method is based on the biomechanical theory of the damage of human tissues under chopping loads, and adopts the ergodic simulation experiment method of the finite element method to establish a reverse research and judgment scale for the formation factors of chop wounds, and then constructs a reverse research and judgment system; this reverse research and judgment system takes the chop wound characteristics, chopping angle, tool type and sharpness as inputs, and takes the chopping speed and chopping force of the chopping action as outputs, with clear meaning, easy to learn and low implementation difficulty. Using this patent technical solution, it is very easy to combine the on-site investigation evidence to conduct research and judgment on various possible situations. With the assistance of computer software, a cycle of research and judgment can be completed in a very short time, and it can quickly conduct combined research and judgment of various different factors, saving a lot of time for case investigation or court trial evidence presentation.

[0008] (4) Published paper: Hao Weiya, Shan Songjunjie, Shi Yi, etc. Influence of Knife Sharpness on Forearm Injury in Chopping Cases. Journal of Medical Biomechanics, 2020, 35(05): 546-552. Among them, by establishing a finite element model of the knife and the human forearm, the influence of knife sharpness on forearm injury in knife chopping cases was quantitatively explored. This paper established a method for quantitatively evaluating the process of the formation of upper limb wounds caused by knife chopping. The research results deepened the understanding of the biomechanical mechanism of the formation of knife chop wounds and provided a reference basis for forensic identification and court trial of chopping cases.

[0009] Based on the above technical means, the inventor's research team can already study the correlation between the action behavior characteristics of the attacker in a hacking case (including hacking speed, hacking force, action range), the characteristics of the attacker (height, arm length, etc.), the type of hacking tool (machete or kitchen knife), and the morphological characteristics of the victim's wound (depth, circumference, etc.) through human motion capture technology, anthropometric data, human imaging data, and human three-dimensional finite element analysis methods.

[0010] However, the above research results are all described by taking the hacking of the victim's arm as an example. In knife homicide cases, the head is a common injured part. The existing technical materials lack specific technical solutions for the construction and verification of the head finite element model. In particular, none of the above research results point out the technical solutions for the experimental verification of the human finite element model constructed through medical imaging, so it is very difficult to conduct experimental verification on the head finite element model.

[0011] After a head hacking case occurs, criminal investigation and court trials need to reverse-infer the relevant characteristics of the case based on the victim's head wound (i.e., hacking wound) and on-site evidence, such as the knife, hacking speed and hacking force level, and the characteristics of the suspect (such as gender, age, physical characteristics), etc., to improve the relevance level of case evidence and provide a scientific level for case investigation and court trials. However, there is still a lack of technical means directly for the study of head hacking. Summary of the Invention

[0012] The purpose of the present invention is: aiming at the problems existing in the above-mentioned prior art, based on the latest research results of the inventor's research group, the present invention proposes a three-dimensional finite element model construction method for the study of head hacking cases. For the anatomical characteristics of the head, a head finite element model can be constructed to study the formation of wounds in head hacking cases.

[0013] The technical solution for the present invention to solve its technical problems is as follows:

[0014] A three-dimensional finite element model construction method for the study of head hacking cases, characterized by including the following steps:

[0015] First step, conduct on-site inspection of the head hacking case, measure the data of the victim's head, and accordingly construct a personalized head geometric model, and use 3D printing technology to make a head physical model.

[0016] Second step, use the head physical model and the infrared light point motion capture system to conduct simulated hacking experiments and collect data; obtain the coordinate transformation relationship between the infrared light point motion capture system coordinate system and the CAD system coordinate system according to the preset fusion algorithm; use this coordinate transformation relationship to process and calculate the collected data to obtain simulated wound morphological indicators; the simulated wound morphological indicators include wound circumference and wound depth.

[0017] Step 3: Establish a finite element model of the head based on the personalized head geometric model and assemble it with the finite element model of the cutting tool, and perform computer simulation on the formation of the head cut wound; the simulation results include the wound circumference and the wound depth.

[0018] Step 4: Compare the simulation results obtained in Step 3 with the simulated wound morphological indexes obtained in Step 2 to verify whether the finite element model of the head is effective. If not, calibrate the finite element model of the head and then transfer it to the computer simulation step of Step 3. If so, use this finite element model of the head as the finally constructed finite element model of the head.

[0019] Based on the victim's personalized geometric model, this construction method establishes two different types of models: a 3D printed physical model and a finite element model. By comparing the results of physical experiments and finite element simulation experiments, the effectiveness of the finite element model is verified, providing a scientific and effective finite element model for further case study. (Note: For further case study, the technical solutions of the applicant's existing patents CN111523265B and CN114580241B can be adopted.)

[0020] The further improved technical solution of the present invention is as follows:

[0021] Preferably, in the first step, the measured data of the victim's head include: the position of the wound on the victim's head and the morphological indexes of the wound, and the three-dimensional geometric data of the victim's head; wherein, the morphological indexes of the wound include the length and depth of the wound, and the three-dimensional geometric data include the length, height and width of the head.

[0022] Preferably, in the first step, the specific process of constructing the personalized head geometric model is as follows:

[0023] Import the THUMS head model into computer-aided engineering grid processing software, scale the model in a 1:1 ratio according to the three-dimensional geometric data of the victim's head, and check the quality of the 1D, 2D and 3D grids respectively. If there are grid units that do not meet the standards, perform preset processing. The specific process of the preset processing is: first re-divide the grid of the local area of the head by using the automatic grid division function, then delete the unqualified units and regenerate qualified grids; then the personalized head geometric model of the victim is obtained.

[0024] The specific process of making the head physical model by 3D printing technology is as follows:

[0025] Reverse reconstruct the cranial vault, facial cranium, and teeth of the personalized cranial geometric model of the victim; the cranial vault includes the frontal bone, parietal bone, occipital bone, temporal bone, sphenoid bone, and ethmoid bone; the facial cranium includes the maxilla, mandible, zygomatic bone, lacrimal bone, nasal bone, palatine bone, inferior nasal concha, and vomer; during reverse reconstruction, use surface treatment software to fill holes, smooth, optimize, and perform three-dimensional surface reconstruction; then, import the reconstructed model into medical image processing software to convert it into a 3D printing special model, and then use 3D printing technology to make a cranial physical model according to a 1:1 size.

[0026] More preferably, the material for printing the parietal bone of the cranium is polyetheretherketone (PEEK) material; the material for printing the remaining parts of the cranium is polylactic acid (PLA) material.

[0027] Adopting the above preferred technical solution can further optimize the specific technical features of the first step, which is conducive to achieving better technical effects for the overall technical solution.

[0028] Preferably, in the second step, during the simulated hacking experiment, the subject holds a preset tool to hack at a preset part of the cranial physical model according to the preset hacking requirements; several geometric marking points are respectively set on the subject, the cranial physical model, and the preset tool in advance, and then reflective marking points are respectively set on each geometric marking point; during the process of the subject hacking at the cranial physical model, use an infrared light point motion capture system to continuously collect the spatial coordinates of each marking point at a preset sampling frequency.

[0029] Preferably, in the second step, the specific process of the preset fusion algorithm is as follows:

[0030] Assume there are N marking points, and collect the spatial coordinates of each marking point at M time points within a period of time; assume the i-th marking point at the j-th time point is P ij point; i = 1, 2,..., N; j = 1, 2,..., M; assume P ij The coordinate of the point in the coordinate system of the infrared light point motion capture system is And the coordinate of this point in the CAD system coordinate system is The relationship between the two coordinates is Equation (I):

[0031]

[0032] Among them, Represents the rotation relationship between the two coordinate systems; Represents the translation relationship between the two coordinates; Solve the T matrix and the b matrix to achieve the coordinate conversion under the above two coordinate systems;

[0033] Define the cost function for the optimization calculation:

[0034]

[0035] This transforms the solution of the T matrix and the b matrix into finding the optimal solution of equation (II), that is:

[0036]

[0037] Let Then equation (III) becomes

[0038]

[0039] The steepest descent method is used for iterative solution to obtain the optimal solution. The specific process is as follows:

[0040] Define the gradient of the function J(T cb ) as:

[0041]

[0042] Then the unit vector of the gradient of the function J(T cb ) is:

[0043]

[0044] Where is the modulus of the gradient of the function J(T cb ), that is:

[0045]

[0046] Let:

[0047]

[0048] Where α k is the optimal step size;

[0049] Using equation (VIII), the iterative calculation is carried out according to the following steps to obtain the optimal solution of the function J(T cb ):

[0050] (i), Arbitrarily select the initial point Give the convergence accuracy ε > 0;

[0051] (ii), Calculate the gradient and the gradient modulus of the point, and determine the search direction

[0052] (iii), Judge whether the convergence criterion is satisfied. If it is satisfied, stop the iteration and output the optimal solution Otherwise, continue to the next iteration;

[0053] (iv), Starting from a point, perform a one-dimensional search along the search direction. After determining the optimal step size, obtain the next iteration point. Let k = k + 1, and go back to step (ii) to continue the iteration.

[0054] According to the obtained optimal solution, obtain the T matrix and the b matrix, and obtain the coordinate transformation relationship between the infrared light spot motion capture system coordinate system and the CAD system coordinate system.

[0055] Preferably, in the second step, the specific process of obtaining the simulated wound morphological index is as follows: first obtain the hacking speed, then obtain the instantaneous spatial position of the tool plane at the end of the hack and the instantaneous spatial position of the cutting edge at the end of the hack, and then calculate to obtain the wound circumference and the wound depth.

[0056] Among them, the hacking speed is the linear speed of the tool tip when the tool contacts the physical model of the skull in the simulated hacking experiment, and is obtained by calculating the filtered spatial coordinates by the infrared light spot motion capture system.

[0057] The instantaneous spatial position of the tool plane at the end of the hack is obtained according to the following process:

[0058] Using a preset fusion algorithm, import the data collected by the infrared light spot motion capture system into the CAD system, and use the position when the hacking speed of the tool is 0 as the instantaneous spatial position at the maximum hacking depth of the tool, which is the instantaneous spatial position of the tool plane at the end of the hack.

[0059] Select 3 marked points on the tool: the tool tip K-ST, the tool back K-BOL, and the centroid K-C; assume that the spatial coordinates of these points in the CAD system are Then the spatial equation of the tool plane is:

[0060]

[0061] Simplify this formula to:

[0062] A K x + B K y + C K z + D K = 0(X)

[0063] Equation (X) is the plane equation of the tool plane in the CAD system.

[0064] The instantaneous spatial position of the cutting edge at the end of the hack is obtained according to the following process:

[0065] Assume that the spatial coordinates of the rear point K-B1 and the front point K-B2 of the tool cutting edge are respectively

[0066] For K-B1, according to K-B1 being in the tool plane and the distance between K-B1 and the tool back K-BOL being the tool width d k, the line connecting the knife ridge K-BOL and K-B1 and the line connecting the knife ridge K-BOL and the knife tip K-ST are orthogonal and perpendicular, and a system of equations is established:

[0067]

[0068] Based on the known coordinates of the knife ridge K-BOL and the knife tip K-ST, solve the system of equations (XI) to obtain the spatial coordinates of K-B1; similarly, obtain the spatial coordinates of K-B2;

[0069] Approximate the spatial position of the blade at the end of the hacking as the line connecting K-B1 and K-B2, and determine the equation of the spatial position of the blade at the end of the hacking as:

[0070]

[0071] The circumference of the wound is obtained as follows:

[0072] The circumference of the wound is the length of the surface curve of the skull wound caused by the knife at the end of the hacking; after determining the spatial position of the knife plane at the end of the hacking, measure the length of the contour curve formed by the intersection of the skull surface and the knife plane in the CAD system, which is the circumference of the wound; when measuring, set N Qk points Q i in the CAD system, and their spatial coordinates are The interval between two adjacent points Q i is 1±0.3 mm. Approximate the length W L of the above contour curve as the sum of the lengths of the broken lines formed by Q i , specifically the following formula:

[0073]

[0074] The depth of the wound is obtained as follows:

[0075] Take the spatial position of the blade at the end of the hacking as the deepest part of the skull wound; use the N Qk points Q i of the above contour curve to calculate the perpendicular distance between each point Q i and the line connecting K-B1 and K-B2 in turn, and take the maximum value as the depth of the wound.

[0076] The specific calculation process is as follows: First, set the vector S KB as:

[0077] S KB =(x KB2 -x KB1 )i+(y KB2 -y KB1 )j+(z KB2 -zKB1 ) k = m KB i + n KB j + p KB k(XIV)

[0078] Wherein, i, j, and k are unit vectors in the x, y, and z axis directions respectively;

[0079] Vector S KB The modulus of is the distance between the line connecting K - B1 and K - B2, that is:

[0080]

[0081] Let the vector S connecting K - B1 and Q i be: KQ as follows:

[0082] S KQ = (x Qi - x KB1 )i + (y Qi - y KB1 )j + (z Qi - z KB1 )k = m QK i + n QK j + p QK k(XVI)

[0083] According to the method of spatial analytic geometry, the perpendicular distance from point Q i to the line connecting K - B1 and K - B2 is:

[0084]

[0085] The numerator part on the right side of the above formula is the modulus of the cross product of two vectors, and the denominator is the calculation result of formula (XV);

[0086]

[0087] The modulus of the above vector is calculated according to the following formula:

[0088]

[0089] Calculate the perpendicular distance from each point Q i to the line connecting K - B1 and K - B2 in sequence through the above formulas (XV), (XIX), and (XVII), and take the maximum value among them as the wound depth.

[0090] Adopting the above preferred technical solution can further optimize the specific technical features of the second step, which is beneficial to achieving better technical effects for the overall technical solution.

[0091] Preferably, in the third step, the specific process of establishing a finite element model of the skull based on the personalized skull geometric model and assembling it with the finite element model of the tool includes:

[0092] S1. Use computer-aided engineering grid processing software to perform element mesh division on the personalized skull geometric model to establish a finite element model of the personalized skull; during the establishment process, use the hierarchical modeling function to divide the skull into several layers with a thickness of 0.1 mm, and respectively construct the outer cortical bone, middle cancellous bone, and the thickness of the inner cortical bone to match the actual anatomical structure. Among them, the thickness of the outer cortical bone is 2.4 ± 0.1 mm, the thickness of the middle cancellous bone is 3.2 ± 0.1 mm, and the thickness of the inner cortical bone is 2.4 ± 0.1 mm.

[0093] S2. In the computer-aided engineering grid processing software, use hexahedral elements to encrypt the element mesh in the contact area between the tool and the skull, and use progressive tetrahedral elements to process the mesh in the transition area between the contact area and the non-contact area. The main steps of the above processing include: using the mesh subdivision function in the contact area between the tool and the skull to encrypt the mesh in the contact area, and the minimum mesh size after encryption is 0.09 mm; using the mesh refinement strategy in the transition area between the contact area and the non-contact area to divide the transition area mesh into progressive tetrahedral elements; the finite element model of the skull after the above processing includes 5 major types of tissues: outer cortical bone, middle cancellous bone, inner cortical bone, gray matter, and white matter.

[0094] S3. Use the node merging strategy to process the co-nodes of the mesh of the finite element model of the skull to make the finite element model of the skull have mechanical behavior consistency and geometric continuity.

[0095] S4. If the actual tool has been found in the skull hacking case, establish a finite element model of the tool based on the actual tool. The main steps include: using a vernier caliper to measure the geometric parameters of the actual tool, using the blade geometric data obtained by previously measuring the preset tool used in the second step of the simulated hacking experiment with a scanning electron microscope to perform reverse reconstruction on the outer contour of the actual tool, applying the CAD system to establish a tool geometric model, and then importing it into the computer-aided engineering grid processing software for mesh division and local encryption processing of the blade area;

[0096] If the actual tool has not been found in the skull hacking case, preset the possible tool types in the case, and then select the corresponding finite element model of the tool from the existing tool database.

[0097] S5. In the finite element software, assemble the finite element model of the tool and the finite element model of the skull based on the tool hacking angle and the contact part between the tool and the skull in the second step of the simulated hacking experiment.

[0098] S6. In the finite element software, set the parameters of the constitutive equations of the cranial finite element model and the parameters of the constitutive equations of the cutter finite element model. The parameters include density, elastic modulus or Young's modulus, Poisson's ratio, ultimate stress, and ultimate strain. According to the cadaver experiment data in previous literature, assign corresponding material properties to the constitutive equations of the cranial finite element model. Assign the constitutive equation of the cutter finite element model as a steel isotropic material.

[0099] More preferably, in the third step, the specific process of computer simulation for the formation of the cranial wound by hacking includes:

[0100] Use the solver of the finite element software for computer simulation. Its boundary conditions are determined by the hacking position, hacking angle, and hacking speed obtained from the simulated hacking experiment in the second step, and the bottom of the cranial finite element model is fixed. In the cranial finite element model, set the failure mode of the bone tissue element as ductile fracture failure and set the failure mode of the soft tissue element as shear failure. During the computer simulation process, when the loads on the bone tissue element and the soft tissue element in the contact area between the tool and the skull reach the failure conditions, set the element as detached and removed from the calculation.

[0101] Adopting the above preferred technical solutions can further optimize the specific technical features of the third step, which is conducive to achieving better technical effects for the overall technical solution.

[0102] Preferably, the specific process of the fourth step is:

[0103] Compare the wound circumference of the simulation result obtained in the third step with the wound circumference of the wound morphological index obtained in the second step, and compare the wound depth of the simulation result obtained in the third step with the wound depth of the wound morphological index obtained in the second step. If the differences are both less than 15%, it is determined that the cranial finite element model is effective; otherwise, calibrate the cranial finite element model and then transfer to the computer simulation step of the third step to obtain the simulation result again and continue the above comparison until it is determined that the cranial finite element model is effective. Among them, the calibration measures include adjusting the element mesh division and / or adjusting the boundary conditions. Finally, use the effective cranial finite element model as the finally constructed cranial finite element model.

[0104] Adopting the above preferred technical solutions can further optimize the specific technical features of the fourth step, which is conducive to achieving better technical effects for the overall technical solution.

[0105] Compared with the prior art, the present invention conducts simulated hacking experiments and finite element simulation experiments for hacking the skull and organically unifies the two. The simulated hacking physical experiment, as a limited experiment, effectively calibrates and verifies the results of the finite element model simulation experiment, and the verified finite element model can then be repeatedly used for computer simulation experiments to positively deduce and verify various possibilities of hacking cases and achieve reverse research and judgment. Brief Description of the Drawings

[0106] Figure 1 It is a schematic flow chart when the specific implementation of the present invention is carried out.

[0107] Figure 2 It is a schematic diagram of a 3D printed human skull model with a 1:1 ratio in Embodiment 1 of the present invention. Among them, Figure (a) is the CAD skull model; Figure (b) is the 3D printed skull model.

[0108] Figure 3 It is a schematic diagram of the chopping experiment test of the 3D printed skull model in Embodiment 2 of the present invention.

[0109] Figure 4 It is a schematic diagram of the microstructure of the experimental kitchen knife and its blade geometric features in Embodiment 2 of the present invention. Among them, Figure (a) is the external view schematic diagram of the experimental kitchen knife, and Figure (b) is the electron microscope measurement picture of the experimental kitchen knife. Note: The experimental kitchen knife in the figure is equipped with an acceleration and force sensor (see the utility model patent of the applicant: "A Motion and Mechanics Measurement Tool for Simulating Chopping and Stabbing", application number 202120686829.7, authorization announcement number CN214372664U).

[0110] Figure 5 It is a schematic diagram of the chopping experiment process and data acquisition in Embodiment 2 of the present invention. Among them, Figure (a) is the schematic diagram of the action of chopping the 3D printed skull, and Figure (b) is the schematic diagram of the stick figure of the chopping action parsed by the QTM software.

[0111] Figure 6 It is a schematic diagram of the CAD model of the kitchen knife chopping the skull in Embodiment 2 of the present invention. Among them, Figure (a) is the initial contact moment of chopping, and Figure (b) is the moment when the chopping kitchen knife stops moving and forms a chopping wound.

[0112] Figure 7 It is a diagram of the positional relationship of the key points of the kitchen knife in Embodiment 2 of the present invention. Note: d k represents the width of the tool, d k = 9.0 cm.

[0113] Figure 8 It is a schematic diagram of calculating the length and depth of the chopping wound in Embodiment 2 of the present invention. Among them, Figure (a) is the moment when the kitchen knife chopping is the deepest; Figure (b) is the calculation of the perimeter length and depth of the wound.

[0114] Figure 9 It is a schematic diagram of the measured calculation of the perimeter and depth of the wound of the 3D printed skull exemplified in Embodiment 2 of the present invention.

[0115] Figure 10Schematic diagram of the finite element head model and the grid resolution of the encryption area in Embodiment 3 of the present invention. Among them, (a) shows the layered structure and encryption area of the head finite element model; (b) shows the head gray matter model; (c) shows the head white matter model.

[0116] Figure 11 Schematic diagram of the finite element model of the kitchen knife as an example in Embodiment 3 of the present invention.

[0117] Figure 12 Schematic diagram of the assembly of the finite element models of the kitchen knife and the head in Embodiment 3 of the present invention.

[0118] Figure 13 Schematic diagram of the computer simulation of head chopping based on the finite element model in the example of Embodiment 3 of the present invention. Among them, (a) shows the wound formed by the chopping of the knife; (b) shows the von Mises stress distribution of the chopped skull. Detailed implementation manners

[0119] The flow schematic diagram during the specific implementation of the present invention is as Figure 1 shown. During the specific implementation, the three-dimensional finite element model construction method for head chopping case research and judgment of the present invention includes all the technical features (including the features of the preferred solutions) recorded in the foregoing invention content. When in use, the model obtained by using this construction method is adopted, and the specific content will not be repeated here.

[0120] It should be noted that the software programs such as computer-aided engineering grid processing software, surface processing software, medical image processing software, and finite element software used in the present invention are all mature existing technology products. Those skilled in the art can easily find existing software programs with corresponding functions and there is more than one according to the functions of each software recorded in the present invention. Those skilled in the art can select a suitable product from them. The present invention does not need to clearly specify these software programs, which are technical means that those skilled in the art can easily understand and easily implement, and there is no situation where the content recorded in the present invention cannot constitute a clear and complete technical solution due to this.

[0121] The present invention will be further described in detail below with reference to the accompanying drawings and in combination with embodiments. However, the present invention is not limited to the given examples.

[0122] Embodiment 1

[0123] This embodiment is to use 3D printing technology to make a physical model of the head.

[0124] The specific content of this embodiment is as follows:

[0125] (1) On-site inspection

[0126] After a hacking case occurs, conduct an on-site inspection to measure the location and morphological indicators of the wounds on the victim's head, including the length and depth of the wounds. These indicators can be used for subsequent judgment after the finite element model is constructed (for details, see the applicant's existing patents CN111523265B and CN114580241B, which will not be elaborated in this invention).

[0127] Use a ruler or vernier caliper to measure the main morphological characteristic indicators of the victim's personalized head, including the three-dimensional geometric data of the head (including length L, height H, width W, etc.).

[0128] (2) Construction of personalized head geometric model

[0129] According to the geometric data of the victim's head obtained from the above measurements, perform personalized processing on the publicly available human body whole model (Total human model for safety, THUMS, developed by Toyota Motor Corporation of Japan) www.toyota.co.jp / thums / ) to obtain the personalized head geometric model of the victim. The specific process is as follows: Import the THUMS head model into computer-aided engineering mesh processing software (for example: Gmsh, v4.11, University of Liège, Belgium) (Note: It can be imported directly or after preprocessing, and it is determined according to the software used). Scale the model in a 1:1 ratio according to the geometric size of the victim's head. In this way, the geometric differences between the THUMS model and the victim's head can be excluded as much as possible.

[0130] After the scaling is completed, check the quality of the 1D, 2D, and 3D meshes respectively, and process the mesh elements that do not meet the standards as follows: 1) Re-divide the mesh of the local area of the head by using the automatic mesh generation function; 2) Delete the unqualified elements and regenerate the qualified mesh.

[0131] After completing the scaling and quality inspection of the head model, the personalized head geometric model of the victim is obtained, and the model file is saved.

[0132] (3) Making a physical model with 3D printing technology

[0133] (3.1) Data model for 3D printing

[0134] Using the personalized cranial geometric model of the victim, a 3D printed cranial physical model is made. The geometric features of this model are basically the same as those of the victim. Reverse reconstruction is carried out on the cranial bones (frontal bone, parietal bone, occipital bone, temporal bone, sphenoid bone and ethmoid bone) and facial bones (maxilla, mandible, zygomatic bone, lacrimal bone, nasal bone, palatine bone, inferior nasal concha and vomer) and teeth of the cranial part, and hole filling, smoothing and optimization processing are carried out using surface processing software (such as: MeshLab, v2022.02, ISTI-CNR, Pisa, Italy), and three-dimensional surface reconstruction is carried out. Then the constructed three-dimensional solid model is imported into medical image processing software (such as: 3DSlicer, v5.4.0, Boston, MA, USA) and converted into a model dedicated for printing.

[0135] (3.2) Material selection

[0136] In this embodiment, two polymers are used as the materials for printing the cranium, and they have similar mechanical properties to the human cranial bone: ① Polyether ether ketone (PEEK) material is used to print the parietal bone of the cranium (see the (b) figure in Figure 2 ; the brown area in the figure); PEEK is a medical-grade material and has similar mechanical properties to the human cranial bone; ② Polylactic acid (PLA) is used for other parts and support areas of the cranial part (see the (b) figure in Figure 2 ; the white area in the figure). These polymers are provided in the form of 3D printing filaments, each with a diameter of 1.75 ± 0.05 mm. Table 1 presents the material properties of the printing filaments used.

[0137] Table 1. Material properties of 3D printed model and real cranial model

[0138]

[0139] (3.3) 3D cranial printing process

[0140] Based on a comprehensive consideration of experimental purposes, cost - effectiveness, and experimental feasibility, the parietal bone of the cranial part and other parts of the cranial part were separated through the coronal suture, lambdoid suture, and squamous suture. The parietal bone and other parts of the cranial part were 3D - printed at a 1:1 scale using PEEK and PLA materials respectively. The printing method was layer - by - layer printing using the fused deposition modeling (FDM) technique. Considering that the 3D - printed parietal bone and other parts of the cranial part need to withstand chopping impacts, a "mortise - tenon structure" was used to enhance the stability and strength between the structures, that is, columns (tenons) were added to the left and right parietal bones, and grooves (mortises) were added to the underlying temporal bones. By inserting the tenons into the mortises, a tight connection between the two parts was achieved. Table 2 shows the printing parameters used for 3D - printing the skull in this embodiment.

[0141] Table 2. 3D - printing parameter settings

[0142]

[0143] In addition, to enhance the adhesion between the printing bed and the molten layer and reduce the risk of warping, a special coating was applied to the printing bed. To ensure the consistency of the final results, all samples were fabricated following the same printing direction. After printing, the 3D - printed cranium was subjected to post - processing such as drying, ultraviolet curing, polishing, and coloring engraving, and the parietal bone and other parts of the cranial bone were colored differently in yellow - brown and white respectively.

[0144] The model of this embodiment is as Figure 2 shown. Among them, the CAD cranial model is shown in Figure (a), and the 3D - printed cranial model is shown in Figure (b). This model is from an adult male with a height of 175 cm, a weight of 77 kg, and a 50th percentile body size. The dimensions of the model cranium are length × width × height: 152 mm × 200 mm × 256 mm. Considering that the cranium needs to be fixed on a fixture in the experiment, a cuboid column (length × width × height: 50 mm × 75 mm × 110 mm) was added at the foramen magnum of the cranium.

[0145] Example 2

[0146] In this example, the 3D - printed cranial model obtained in Example 1 was used to collect data through a simulated chopping experiment and processed using a fusion algorithm to calculate the wound morphological indexes.

[0147] The specific content of this example is as follows:

[0148] (1) Chopping experiment on the 3D - printed cranial physical model

[0149] This experiment aims to provide verification data for the three - dimensional finite - element model.

[0150] (1.1) The striker selects

[0151] According to the monitoring data published in the "Fifth China National Physical Fitness Monitoring Bulletin", the average height of Chinese young men (aged 20 - 29) is 172.4 cm, and the average weight is 71.6 kg. Based on this, the inventor's research team recruited a right-handed and healthy young man with a height of 173.0 cm and a weight of 68.5 kg as the subject (i.e., the striker).

[0152] (1.2) Instruments and equipment

[0153] The hacking experiment was carried out in a laboratory environment (length × width × height: 9.52 m × 5.08 m × 3.12 m) equipped with an 8-lens three-dimensional infrared light point motion capture system (Oqus 700, Qualisys TrackManager, Gothenburg, Sweden).

[0154] The 3D printed skull model was fixed on a bench vice fixture (MYTEC, China), and the bench vice fixture was fixed on an adjustable landing gear (TN, China) through bolts. By adjusting the height of the landing gear, the height of the 3D printed skull model can be adjusted. In order to simulate the average height of adult men in actual situations as much as possible, the height of the 3D printed skull model was set to 1.70 m. Two high-speed cameras (CASIO EX-F1, Japan) were placed on the left and right sides of the subject for shooting ( Figure 3 ).

[0155] In this embodiment, an experimental kitchen knife equivalent to a household kitchen knife was used as the test cutter. The length, width and weight of the kitchen knife are 35.8 cm, 9.0 cm and 1.16 kg respectively ( Figure 4 Figure (a)). Since the sharpness of the blade has a direct impact on the wound morphology and the severity of casualties in hacking cases (see the paper mentioned in the background technology: Hao Weiya, Shan Songjunjie, Shi Yi, etc. The influence of the sharpness of the cutter on forearm injuries in hacking cases. Journal of Medical Biomechanics, 2020, 35(05): 546 - 552). It is very important to accurately collect the geometric shape and sharpness of the blade, especially for fine modeling of the cutter geometry in finite element analysis. For this reason, in this embodiment, a scanning electron microscope (SEM, JSM-IT200, JEOL, Japan) was used to measure the blade of the kitchen knife, and the blade thickness and angle of the kitchen knife are 6.4 μm and 22.8° respectively ( Figure 4 Figure (b)).

[0156] (1.3) The process of the hacking experiment and data collection

[0157] Before the experiment, the subjects were informed of the test content. The subjects were required to change into tight clothes and warm up. Subsequently, reflective markers were placed on the subjects, the bony landmark points of the 3D skull, and the geometric landmark points of the experimental kitchen knife (Table 3). The subjects were informed of the chopping action, and they could adjust their standing positions and directions according to their own situations before chopping the 3D-printed skull. After the subjects understood the chopping requirements, they held the experimental kitchen knife and chopped the right parietal bone of the 3D-printed skull with all their strength ( Figure 5 Figure (a) of Figure 5 Figure (b) of

[0158] Table 3. List of geometric landmark point names and positions of subjects, 3D-printed skull models, and kitchen knives

[0159]

[0160] (2) Motion capture data and CAD data fusion algorithm

[0161] During the chopping process, the coordinates of the reflective markers on the cutter and the skull can be obtained by the QTM software of the motion capture system and filtered using a fourth-order Butterworth low-pass filter with a cut-off frequency of 25 Hz. In order to convert the spatial coordinates between the two systems, a fusion algorithm for the spatial coordinates of the two systems needs to be constructed.

[0162] (2.1) Coordinate conversion calculation between motion capture data and CAD system

[0163] The coordinate conversion between different coordinate systems in the same space can be achieved by applying rotation and translation between coordinates. Let be the coordinates of point A in the QTM coordinate system, and be the coordinates of point A in the CAD software system (e.g., FreeCAD, v0.21.2). According to spatial analytic geometry, the coordinates of point A in the two coordinate systems have the following relationship:

[0164]

[0165] where represents the rotation relationship between the two coordinate systems, and is the translation between the two coordinates. According to spatial analytic geometry, the T matrix is a 3rd-order symmetric invertible matrix. Obviously, if the T matrix and the b matrix are obtained, the coordinate conversion relationship between the two different coordinate systems can be achieved.

[0166] (2.2) Solution algorithm for rotation matrix and translation matrix based on the steepest descent method

[0167] To improve the conversion accuracy and stability between two coordinates, in this embodiment, based on the least squares method, the coordinates of N points at different positions on the 3D printed physical model of the skull (when selecting N points, points at different positions in the front, back, left, and right of the skull can be selected) at M time points within a period of time t are used.

[0168] Since the sampling frequency of the motion capture system in this embodiment is 250 Hz, if the time t = 1 s is selected, each fiducial point will have three-dimensional coordinates at 250 moments, and M = 250.

[0169] Let be the coordinates of point P (the point at the j-th moment of the i-th point) in the QTM coordinate system, while ij is the coordinate of this point within the CAD software system. Then i = 1, 2,..., N; j = 1, 2,..., M.

[0170] According to Equation (I'), the relationship between the above two coordinates is as follows:

[0171]

[0172] Among them, represents the rotation relationship between the two coordinate systems; represents the translation relationship between the two coordinates. Solving the T matrix and the b matrix can realize the conversion relationship of coordinates under two different coordinate systems.

[0173] At this time, the cost function of the optimization calculation can be defined as

[0174]

[0175] In the above formula, the square root is the root mean square error between the calculation results and the actual results of the coordinates of all points at different moments through coordinate conversion.

[0176] In this way, solving the two matrices T and b becomes an optimization calculation problem of solving J(T, b). The values of the two matrices can be obtained through the most common optimization calculation methods. Since the matrix T is a symmetric matrix, the T matrix has a total of 6 independent variables (T 11 , T 12 , T 13 , T 22 , T 23 , T 33 ). Plus the b matrix, there are a total of 9 independent variables. In this way, the conversion of the motion capture space coordinate data and the CAD space coordinate data becomes the optimal solution of solving Equation (II), that is

[0177]

[0178] Let Then formula (III) becomes

[0179]

[0180] The steepest descent method (also known as the optimal gradient method) is used for iterative solution to obtain the optimal solution. The algorithm is as follows:

[0181] Define the gradient of the function J(T cb ) as:

[0182]

[0183] Then the gradient unit vector of the function J(T cb ) is:

[0184]

[0185] Where is the modulus of the gradient of the function J(T cb ), that is

[0186]

[0187] Let:

[0188]

[0189] The above formula is the algorithm for iterative solution according to the steepest descent method, where α k is the optimal step size. After constructing the above algorithm, the optimal solution T cb of the function J(T cb ) can be solved. The specific iterative calculation steps include:

[0190] (i) Arbitrarily select an initial point Give a convergence accuracy ε > 0;

[0191] (ii) Calculate the gradient and gradient modulus of the point, and determine the search direction

[0192] (iii) Judge whether the convergence criterion is satisfied. If it is satisfied, stop the iteration and output the optimal solution Otherwise, continue to the next iteration;

[0193] (iv) Starting from the point, perform a one-dimensional search along the search direction. After determining the optimal step size, obtain the next iteration point Let \(k = k + 1\), and go to step (ii) to continue the iteration.

[0194] Through the above steps, the optimal solutions of formulas (III) and (IV) can be obtained, thereby obtaining the rotation matrix \(T\) and the translation matrix \(b\) between the QTM coordinate system and the CAD system coordinate system, and realizing the coordinate transformation calculation of the motion capture data and the CAD system.

[0195] (3) Measurement and calculation of wound morphology indexes

[0196] (3.1) Hacking speed

[0197] The hacking speed is defined as the linear velocity of the tip of the knife when it contacts the 3D printed skull model instantaneously in the hacking experiment, and is obtained by calculating the spatial coordinates after filtering by the QTM system (such as Figure 6 the moment shown in Figure a).

[0198] (3.2) Spatial position of the tool plane at the end of hacking

[0199] In the QTM system, the dynamic change of the tip speed of the knife can also be obtained during the whole process of hacking. Obviously, when the hacking speed of the knife becomes 0, the knife stops moving, and at this time, the hacking wound is formed. In order to measure the wound morphology indexes, it is necessary to first determine the spatial position relationship between the knife and the skull at the end of hacking (such as Figure 6 the moment shown in Figure b).

[0200] Using the previous coordinate transformation calculation method between the motion capture data and the CAD system, all the motion capture data are imported into the CAD system to determine the spatial position at the instant of the maximum hacking depth of the knife (the position when the hacking speed is 0).

[0201] Select the spatial coordinates of three points (K-ST, K-BOL, and K-C) on the knife in the CAD system According to spatial analytic geometry, the spatial equation of the tool plane is determined as determinant (IX):

[0202]

[0203] The above equation can be simplified to:

[0204] A K x + B K y + C K z + D K = 0 (X)

[0205] In this way, the plane equation of the tool plane in the CAD system is established.

[0206] (3.3) Spatial position of the blade at the end of hacking

[0207] LetFigure 7 The spatial coordinates of two points K-B1 and K-B2 at the front and back of the blade of the middle knife are respectively and To obtain the spatial coordinates of point K-B1, a system of equations can be established according to the following three conditions: According to that point K-B1 is in the tool plane and the distance between point K-B1 and point K-BOL is d k (Note: d k represents the tool width, d k = 9.0 cm), the line connecting point K-BOL and K-B1 and the line connecting point K-BOL and K-ST are orthogonal to each other( Figure 7 ), that is, there is the following system of equations:

[0208]

[0209] Since the coordinates of point K-BOL and K-ST are known, the above system of equations has a unique solution. After solving, the spatial coordinates of point K-B1 can be obtained; in the same way, the spatial coordinates of point K-B2 can also be calculated.

[0210] The spatial position of the blade at the end of the hacking can be approximately regarded as the line connecting points K-B1 and K-B2. According to spatial analytic geometry, the equation for determining the spatial position of the blade at the end of the hacking is:

[0211]

[0212] (3.4) Calculation of the wound circumference

[0213] The wound circumference refers to the length of the surface curve of the skull wound caused by the tool at the end of the hacking of the skull (as shown in figure b of Figure 8 ). After determining the spatial position of the tool plane, the length of the contour curve formed by the intersection of the skull surface in the skull model and the tool plane can be measured in the CAD software system. Simply, the spatial coordinate data of points are generated at approximately 1 mm intervals by manually clicking the mouse in the CAD software system. Suppose there are a total of N Qk points Q i ( Figure 8 ), and their spatial coordinates are In this way, the length W L of the wound contour curve can be approximately calculated as the sum of the lengths of the broken lines composed of Q i , specifically the following formula:

[0214]

[0215] (3.5) Calculation of the wound depth

[0216] The spatial position reached by the blade at the end of the hacking is basically the deepest part of the wound( Figure 8) By successively calculating the vertical distance between each point Q on the surface curve of the skull wound and the line connecting two points K - B1 and K - B2 at the instant of the end of the cut [Equation (XII)], and taking the maximum value as the wound depth. i To calculate the vertical distance from Q to the line connecting K - B1 and K - B2, first set the vector S

[0217] For calculating Q i to the vertical distance of the line connecting K - B1 and K - B2, first set the vector S KB as:

[0218] S KB =(x KB2 -x KB1 )i+(y KB2 -y KB1 )j+(z KB2 -z KB1 )k = m KB i + n KB j + p KB k (XIV)

[0219] where i, j, and k are the unit vectors in the x, y, and z coordinate axes directions respectively.

[0220] The modulus of the vector S KB is the distance of the line connecting K - B1 and K - B2, that is:

[0221]

[0222] Then set the vector S i connecting K - B1 and Q KQ as:

[0223] S KQ =(x Qi -x KB1 )i+(y Qi -y KB1 )j+(z Qi -z KB1 )k = m QK i + n QK j + p QK k (XVI)

[0224] According to the method of spatial analytic geometry, the vertical distance from point Q i to the line connecting K - B1 and K - B2 is:

[0225]

[0226] The numerator part on the right side of the above formula is the modulus of the cross product of the two vectors, and the denominator is the calculation result of formula (XV).

[0227]

[0228] The modulus of the above vector is calculated by the following formula:

[0229]

[0230] In this way, Q can be calculated successively through formulas (XV), (XIX) and (XVII). i The vertical distance from the point to the line connecting K - B1 and K - B2, and the maximum value among them is the depth d of the chopping wound. This index can be used to verify the results of subsequent finite element simulations.

[0231] Example: Calculation of the circumference and depth of the experimental wound

[0232] The research group conducted a chopping experiment on a 3D printed skull physical model by a young man. During the experiment, a three - dimensional motion capture system (Oqus700, Qualisys Track Manager, Gothenburg, Sweden) was used to collect the dynamic changes of the three - dimensional spatial positions of the infrared marker points on the chopper, the knife and the skull. The skull physical model was fixed on a vise fixture (MYTEC, China), and the vise fixture was fixed on an adjustable landing gear through bolts. By adjusting the height of the landing gear, the height of the 3D skull can be adjusted.

[0233] Through the " (2) Motion capture data and CAD data fusion algorithm" mentioned above to process the collected data, and finally 54 - point data of the wound curve are obtained through the CAD software system ( Figure 9 ), and the three - dimensional coordinates of each point in space are successively:

[0234] Q1=(289.29, 280.58, 1645.49),

[0235] Q2=(289.61, 281.04, 1646.40),

[0236] ……,

[0237] Q 54 =(322.25, 295.53, 1605.71).

[0238] 1. Calculation of the wound circumference

[0239] Calculate the wound circumference according to formula (XIII),

[0240]

[0241] After substituting the coordinates of each point, we get:

[0242] W L ≈52.3mm.

[0243] 2. Wound Depth Calculation

[0244] The line connecting the two ends of the wound coincides with the straight line of the blade. Therefore, the wound depth can be calculated using points Q1 and Q2 and Q i (i = 2, …, 53) to calculate the wound depth, that is, to find the maximum perpendicular distance from Q i to the line connecting Q1 and Q2.

[0245] S KB = Q 54 - Q1 = 32.96i + 14.95j - 39.78k

[0246] According to equations (XV) to (XIX), the distance from Q i (i = 2, …, 53) to the vector S KB can be calculated. The maximum value among them is the hacking depth.

[0247] Specifically, Q 26 = (310.51, 284.26, 1629.87), then

[0248] S Q26 = Q 26 - Q1 = 21.23i + 3.68j - 15.62k

[0249] Subsequently, calculate the vector cross product of d Q26 and S KB according to equation (XVIII)

[0250] S KB × S Q26 = -86.95i - 330.97j - 196.06k

[0251] Calculate the magnitudes of the above two vectors respectively to obtain

[0252]

[0253] Finally, substitute into equation (XVII) to obtain the wound depth at point Q 26 :

[0254]

[0255] Example 3

[0256] Based on Example 1 and Example 2, in this example, a finite element model of the human head is established to perform computer simulation of the formation of hacking wounds on the head, and the finite element model of the head is verified.

[0257] The specific content of this example is as follows:

[0258] (1) Establishment of the Finite Element Model of the Head

[0259] (1.1) Structural Reconstruction and Element Division of Personalized Cranial Finite Element Model

[0260] Based on the cranial geometric model of the case victim established in “(2) Construction of Personalized Cranial Geometric Model” in Embodiment 1, its structure is refined and discretized to construct a personalized cranial finite element model. Specifically as follows:

[0261] Since the existing THUMS model is mainly applied to the research on macroscopic large-scale injuries such as vehicle and train collisions, the structure of cortical bone and cancellous bone in the cranial part is not considered in model construction. Therefore, the model based on THUMS cannot accurately analyze and calculate the injuries caused by sharp weapon injuries to the local part of the head. In this embodiment, a fine model of the “sandwich” structure of “outer cortical bone - cancellous bone - inner cortical bone” of the skull is constructed in computer-aided engineering mesh processing software, and its average thickness is constructed as 6.4 mm (wherein, the outer cortical bone: 1.82 mm; the inner cortical bone: 1.75 mm; the middle cancellous bone: 2.83 mm) to enhance the accuracy of the model in local injury assessment. After completing the cranial geometric modeling, the computer-aided engineering mesh processing software is used to further divide the unit mesh of the cranial three-dimensional geometric model to establish a cranial finite element model. During the establishment process, the layered modeling function is used to divide the skull into several layers with a thickness of 0.1 mm, and referring to the cadaver experiment data in the references, the thicknesses of cortical bone and cancellous bone are respectively constructed to match the actual anatomical structure (see Table 4).

[0262] Table 4. Tissue Structure and Finite Element Model

[0263]

[0264] In order to improve the calculation accuracy and reduce the number of elements in the whole model, in the discretization process of the cranial geometric model in this embodiment, the areas subjected to hacking (such as the frontal bone, left and right parietal bones, and occipital bone regions) are encrypted with hexahedral elements, and the minimum size reaches 0.09 mm, while other parts use larger-sized elements approximately, which effectively speeds up the requirements of the finite element model for computer hardware and also speeds up the solution speed.

[0265] When the head is injured by a knife hacking, the cranial bone and skin are the main load-bearing tissues, and the internal structure of the head (cerebrospinal fluid, white matter, and gray matter, etc.) plays a relatively small role. Therefore, larger-sized elements are used for these tissues during element division to reduce the number of elements and speed up the solution speed of the finite element model. The cranial finite element model after element division is as Figure 10 shown.

[0266] (1.2) Structural Reconstruction and Element Division of Knife Finite Element Model

[0267] The establishment of the finite element model of the knife can refer to the existing patent technical solutions of the inventor's research team (Hao Weiya, Xiao Xiaofei. Reverse research and judgment system for chopping wounds, its construction method and research and judgment method [P]. Beijing: CN202210220552.8, October 1, 2024). The general steps include: Select the actual knife as the representative prototype, measure the geometric parameters with a vernier caliper, and after reverse reconstructing the outer contour of the knife by combining the geometric data of the knife edge obtained by scanning electron microscope in Embodiment 2, use CAD software to establish the geometric model of the knife. After completing the geometric modeling, the model is exported and saved in STP format. Import the knife model in STP format into the computer-aided engineering mesh processing software for mesh generation. In particular, local refinement is performed on the knife edge area to ensure the accuracy of the simulation. For example, Figure 11 is a finite element model of a kitchen knife, and the model includes 53,304 elements and 10,656 nodes.

[0268] When there is no actual knife, the possible types of knives in the case can be preset in advance, and then the corresponding finite element model of the knife is selected from the knife database (see the patent: Hao Weiya, Xiao Xiaofei. Reverse research and judgment system for chopping wounds, its construction method and research and judgment method [P]. Beijing: CN202210220552.8, October 1, 2024) as the knife model of this embodiment.

[0269] (1.3) Mesh processing of the contact area and the transition area

[0270] In the computer-aided engineering mesh processing software, after completing the skull layering, in order to more accurately capture the wound changes in the contact area between the knife and the skull (taking the frontal bone as an example in this embodiment), the mesh (skin, cancellous bone and compact bone) in this contact area is refined using hexahedral elements (see Figure 10 ). The operation steps are as follows: Determine the contact position between the knife and the skull according to the experimental data obtained by the three-dimensional motion capture system in the chopping experiment, then select the contact area and use the mesh subdivision function to refine the mesh in the contact area. The minimum mesh size after refinement is 0.09 mm. In order to optimize the calculation amount, the non-contact area is not refined. In the transition area between the contact area and the non-contact area, the mesh is divided into progressive tetrahedral elements using the mesh refinement strategy. Finally, the finite element model of the skull includes five major types of tissues: the outer compact bone, the middle cancellous bone, the inner compact bone, the gray matter and the white matter. The model elements are of two types: hexahedron and tetrahedron, and there are approximately 4.04 million elements in total (see Table 4 above).

[0271] (1.4) Model assembly

[0272] Since the original skull was stratified into three parts and there were no common nodes between the parts, a node merging strategy was used to handle the mesh common nodes to ensure the mechanical behavior consistency and geometric continuity of the model. After the mesh generation was completed, the model file was saved, and the tool model file was also imported into the computer-aided engineering mesh processing software. Finally, in the finite element software (e.g., MSC Marc (v2007, MSC Software Corporation, USA)), the model was assembled based on the hacking angles and contact parts of the "tool - skull" obtained from the three-dimensional motion capture system during the hacking experiment. The assembled model is as shown in Figure 12 as shown.

[0273] (1.5) Material properties of the skull and tool models

[0274] In finite element simulation, selecting a constitutive equation suitable for skull tissues is one of the important factors affecting the calculation accuracy. In this embodiment, based on the cadaver experiment data in previous literature, the material parameters of various parts and tissues of the human head were assigned values.

[0275] Specifically, the parameters of each constitutive equation were set in the finite element software, including density, elastic modulus (Young's modulus), Poisson's ratio, ultimate stress, and ultimate strain, etc.; subsequently, according to the literature data, these constitutive equations were assigned to the material properties of each component of the tool and the skull to ensure the physical consistency and accuracy of the simulation.

[0276] The tool was set as a steel isotropic material and modeled using the linear elastic model in the finite element software. The scalp was also modeled using the linear elastic model. The bone parts of the head, such as the skull, facial bone, and mandible, were modeled using an elastic-viscous-plastic material model. The viscoelastic material model was used to simulate the cerebrospinal fluid and the gray and white matter in the brain tissue (Table 5).

[0277] Table 5. Mechanical properties of each tissue in the tool and finite element skull model

[0278]

[0279] Note: E, elastic modulus; σγ, yield stress; K, bulk modulus; G0, initial shear modulus; GI, infinite shear modulus; λ, attenuation constant.

[0280] (2) Computer simulation of the formation of the hacking wound on the skull

[0281] (2.1) Boundary conditions

[0282] After completing the construction of the cranial finite element model and determining the material mechanical properties and assignment of the knife and each tissue of the cranium (compact bone, cancellous bone, cerebrospinal fluid, and nerve tissue) in the model, a computer simulation experiment of cranial hacking can be carried out using the solver of the finite element software. The boundary conditions of the computer simulation experiment are determined by parameters such as the hacking position, hacking angle, and hacking speed obtained from the hacking 3D printed cranial simulation experiment in Example 2. The bottom (neck) of the cranial finite element model is fixed. These conditions constitute the boundary conditions of the finite element model.

[0283] (2.2) Computer simulation of wound formation

[0284] In a paper published by the inventor's research team (Hao Weiya, Shan Songjunjie, Shi Yi, et al. Influence of Knife Sharpness on Forearm Injury in Hacking Cases. Journal of Medical Biomechanics, 2020, 35(05): 546 - 552), the study on knife hacking of the human forearm was conducted. This example follows the relevant damage patterns and theories of bone tissue and soft tissue under the action of knife hacking in this study. The failure mode of bone tissue is set as ductile fracture failure, and the failure mode of soft tissue is set as shear failure.

[0285] During the computer simulation process, when the load on the bone tissue and soft tissue units in the knife hacking cutting area (i.e., the area where the tool contacts the skull) reaches the failure condition, the unit is set by the software to fall off and removed from the calculation.

[0286] (3) Finite element model verification

[0287] Compare the wound circumference and wound depth formed by the hacking 3D printed cranial simulation experiment and the finite element model simulation experiment. If the difference between the two is less than 15%, the finite element model can be considered valid. If it is ≥15%, the finite element model is calibrated by adjusting measures such as the element division and boundary conditions of the model, so that the difference between the finite element simulation result and the hacking 3D printed simulation experiment result is within 15%.

[0288] Example: This example combines a case to illustrate the implementation process of the technical solution of the present invention in constructing a personalized finite element model and its application. The basic situation of the case is that a male in a residential community was hacked to death at home in the living room. The kitchen knife used in the crime was left at the scene, and the perpetrator escaped, but the police locked the suspect as a 1.73m tall, 74kg heavy, and physically strong man. The case investigation requires sorting out and analyzing the crime process, especially the hacking process; and confirming whether the characteristics of the murderer match the suspect.

[0289] According to the technical solution of the present invention, the following steps are carried out to construct the head finite element model of this case:

[0290] T1. After the case occurred, the police conducted an on-site inspection in accordance with the law to determine the identity of the victim, the chopping tool, and the identity and physical characteristics of the suspect.

[0291] T2. Measure the key geometric dimensions of the victim's head, and obtain the length × width × height of the victim's skull as 152 mm × 200 mm × 256 mm.

[0292] T3. According to the key geometric dimensions of the victim, scale the THUMS model and construct a personalized skull geometric model of the victim in CAD.

[0293] T4. Based on the personalized skull model of the victim in CAD software, conduct 3D printing to produce a 3D printed physical model of the skull.

[0294] T5. Find a strong man similar to the suspect in height and weight as a test volunteer to conduct a simulated chopping experiment on the 3D printed physical model. Use an infrared motion capture system to collect the dynamic change process of the three-dimensional spatial coordinate values of the infrared reflective marker points attached to the human body, the tool, and the skull physical model.

[0295] T6. Conduct fusion calculation of the motion capture data and the CAD data, solve the conversion and translation of the three-dimensional spatial coordinates in the motion capture coordinate system and the three-dimensional spatial coordinate system in the CAD software system, and realize the mutual conversion of the two spatial coordinates.

[0296] T7. Use the QTM system to extract the chopping speed in the experiment of the volunteer simulating the chopping of the 3D printed skull physical model as 8.7 m / s; then determine the instantaneous tool plane and the spatial position of the blade at the end of the chopping in turn, and finally calculate the wound circumference as 54.2 mm and the depth as 7.9 mm.

[0297] T8. Check the element quality in the skull model scaled based on the THUMS model in the computer-aided engineering grid processing software, eliminate the pathological elements caused by scaling; construct the geometric model of the on-site tool and establish the corresponding finite element model; refine the grid in the contact area between the tool and the skull; then conduct model assembly, assemble the tool model and the skull model in the finite element software; finally, check the parameter of the material properties of the skull and the tool in the software.

[0298] T9. Use the data such as the chopping part and the chopping speed obtained from the chopping simulation experiment, and use the solver of the finite element software to conduct computer simulation of the wound morphology and the skull stress distribution characteristics caused by chopping the skull.

[0299] T10. The wound circumference and depth obtained through computer simulation are within the range of ±15% of the results of the actual simulation experiment (circumference: 46.1 - 62.3 mm; depth: 6.7 - 9.1 mm); this result indicates that the constructed finite element model is effective.

[0300] T11. After verifying the effectiveness of the finite element model, the technical solutions of the applicant's existing patents CN111523265B and CN114580241B can be followed to conduct further computer simulation experiments on the victim's hacking site and the suspect in this case, assisting criminal investigation and courtroom technical personnel in judging various possibilities of the case occurrence.

[0301] Combining the above embodiments and their examples, the main key points of the technical solution of the present invention are:

[0302] 1. To establish a finite element model for assisting in hacking judgment, the present invention is based on the publicly available THUMS finite element model, and scales the THUMS model according to the geometric characteristics of the victim's head to construct a personalized geometric model of the victim.

[0303] 2. Based on the personalized geometric model of the victim, two different types of models are established: a 3D printed physical model and a finite element model. By comparing the results of physical experiments and finite element simulation experiments, the effectiveness of the finite element model is verified, providing a scientific and effective finite element model for further case judgment (the technical solutions of the applicant's existing patents CN111523265B and CN114580241B can be adopted for further case judgment).

[0304] 3. For the first time, a finite element model of the head under the condition of knife hacking is constructed. The head has a different structure from other structures such as the forearm. The head has skin, skull and intracranial tissues; the skull has a three-layer structure of compact bone - cancellous bone - compact bone, and there are neural tissue structures such as gray matter and white matter inside the skull.

[0305] 4. Motion capture data and CAD data fusion algorithm. In order to realize the mutual conversion between the three-dimensional space of motion capture data and the three-dimensional space of CAD data, iterative solution is carried out through spatial analytic geometry and the steepest descent method (also known as the optimal gradient method) to obtain the rotation and translation matrices between the two spatial coordinate systems.

[0306] 5. Wound morphology index measurement and calculation method. Based on spatial analytic geometry and curve length integral calculation method, ① the lengths of the line segments formed by the scattered points of the wound are summed to approximately calculate the wound circumference; ② the maximum vertical distance between the scattered points and the straight line of the knife edge is found as the hacking depth.

[0307] 6. Infrared light spot pasting technical solution in the hacking head simulation experiment, including the pasting positions of infrared light spots on the human body, knife, 3D printed head and dummy.

[0308] The specific advantages of the present invention are:

[0309] 1. The present invention for the first time conducts simulated hacking physical experiments and finite element simulation experiments on hacking the head, and organically unifies the two. The simulated hacking physical experiment, as a limited experiment, effectively calibrates and verifies the results of the finite element model simulation experiment, and the verified finite element model can be repeatedly used for computer simulation experiments to positively deduce and verify various possibilities of hacking cases, achieving reverse research and judgment.

[0310] 2. When developing and processing the 3D printed physical model of the head, the mechanical properties of the materials used are similar to those of human bone tissue, and the printed geometric shape is processed according to the CAD geometric model, realizing the homologous and isomeric properties of the finite element model and the physical model, forming two different modal models, which provides extremely favorable conditions for physical model experiments.

[0311] 3. The present invention proposes a fusion mutual conversion algorithm based on the space analytic geometry method, which fuses the space coordinates of the motion capture system and the CAD system.

[0312] 4. The present invention proposes a calculation method for collecting morphological data of hacking wounds in the physical model experiment of hacking the head based on the space analytic geometry and calculus methods.

[0313] 5. For the first time, a finite element model of the head under the condition of hacking with a knife tool is constructed. The head has a different structure from other structures such as the forearm. The head has skin, skull and intracranial tissues; the skull has a "sandwich" structure of compact bone - cancellous bone - compact bone, and there are gray matter and white matter structures inside the skull.

[0314] 7. For the first time, through finite element model simulation, the destruction and failure of human skin, bone and nerve tissues under the hacking of a knife tool are achieved, which has great innovation.

[0315] 8. In constructing the finite element model, the present invention refines the elements in the hacking area, and gradually increases the element scale during the process of moving away from the hacking area. This element division strategy not only ensures the accuracy of the calculation, but also ensures the continuity of the model and improves the calculation speed.

[0316] 9. In the process of constructing the geometric model of the experimental knife tool, for the first time, the geometric scale of the blade part is observed and measured through an electron microscope, ensuring the authenticity of the constructed finite element model.

[0317] Except for the above embodiments, the present invention may have other implementation manners. All technical solutions formed by equivalent replacement or equivalent transformation fall within the protection scope required by the present invention.

Claims

1. A method for constructing a three-dimensional finite element model for the study and judgment of head chopping cases, characterized in that It includes the following steps: First step: Conduct on-site inspection of the head chopping case, measure and obtain the data of the victim's head, construct a personalized head geometric model based on this, and produce a head physical model using 3D printing technology; Second step: Use the head physical model and the infrared light point motion capture system to conduct a simulated chopping experiment and collect data; obtain the coordinate transformation relationship between the infrared light point motion capture system coordinate system and the CAD system coordinate system according to the preset fusion algorithm; use this coordinate transformation relationship and combine the collected data for processing and calculation to obtain the simulated wound morphological indexes; the simulated wound morphological indexes include the wound circumference and the wound depth; Third step: Establish a head finite element model based on the personalized head geometric model and assemble it with the knife finite element model to conduct computer simulation on the formation of the head chopping wound; the simulation results include the wound circumference and the wound depth; Fourth step: Compare the simulation results obtained in the third step with the simulated wound morphological indexes obtained in the second step to verify whether the head finite element model is effective. If not, calibrate the head finite element model and then transfer it to the computer simulation step in the third step. If so, use this head finite element model as the finally constructed head finite element model.

2. The method for constructing a three-dimensional finite element model for analyzing and judging head chopping cases according to claim 1, characterized in that, In the second step, in the simulated chopping experiment, the subject holds a preset knife to chop a predetermined part of the head physical model according to the predetermined chopping requirements; several geometric marker points are respectively set on the subject, the head physical model, and the preset knife in advance, and then reflective marker points are respectively set on each geometric marker point; during the process of the subject chopping the head physical model, use the infrared light point motion capture system to continuously collect the spatial coordinates of each marker point at the preset sampling frequency.

3. The method for constructing a three-dimensional finite element model for analyzing and judging a head chopping case according to claim 1, characterized in that, In the second step, the specific process of the preset fusion algorithm is as follows: Suppose there are N marked points, and the spatial coordinates of each marked point are collected at M time points within a period of time; let the i-th marked point at the j-th time point be P ij point; i = 1, 2, …, N; j = 1, 2, …, M; let P ij The coordinates of the point in the coordinate system of the infrared light point motion capture system are And the coordinates of the point in the CAD system coordinate system are The relationship between the two coordinates is Equation (I): Among them, represents the rotation relationship between the two coordinate systems; represents the translation relationship between the two coordinates; Solving the T matrix and the b matrix can realize the coordinate transformation under the above two coordinate systems; Define the cost function for the optimization calculation: In this way, the solution of the T matrix and the b matrix is converted into the optimal solution of formula (II), that is: Let Then Equation (III) becomes The steepest descent method is used for iterative solution to obtain the optimal solution. The specific process is as follows: Define the gradient of the function J(T cb ) as: Then the gradient unit vector of the function J(T cb ) is as follows: Among them is the modulus of the gradient of the function J(T cb ), that is: Let: where α k is the optimal step size; Using Equation (VIII), perform iterative calculations according to the following steps to obtain the optimal solution of the solution function J(T cb ): (i), Optionally select an initial point Give a convergence accuracy ε > 0; (ii), Calculate the gradient and the gradient norm of the point to determine the search direction (iii), Determine whether the convergence criterion is satisfied If it is satisfied, stop the iteration and output the optimal solution Otherwise, continue to the next iteration; (iv), Starting from a point, perform a one-dimensional search along the search direction. After determining the optimal step size, obtain the next iteration point. Let k = k + 1, and go back to step (ii) to continue the iteration. According to the obtained optimal solution, obtain the T matrix and the b matrix, and obtain the coordinate transformation relationship between the infrared light point motion capture system coordinate system and the CAD system coordinate system.

4. A method for constructing a three-dimensional finite element model for analyzing and judging head chopping cases according to claim 2, characterized in that, In the second step, the specific process of obtaining the simulated wound morphological indexes is as follows: First, obtain the chopping speed, then obtain the spatial position of the tool plane at the instant of the end of the chop and the spatial position of the blade at the instant of the end of the chop, and then calculate to obtain the wound circumference and the wound depth; Among them, the chopping speed is the linear speed of the tool tip at the instant when the tool contacts the head physical model in the simulated chopping experiment, and is obtained by calculating the filtered spatial coordinates using the infrared light point motion capture system; The spatial position of the tool plane at the instant of the end of the chop is obtained according to the following process: Using the preset fusion algorithm, import the data collected by the infrared light point motion capture system into the CAD system, and use the position when the chopping speed of the tool is 0 as the spatial position at the instant of the maximum chopping depth of the tool, which is the spatial position of the tool plane at the instant of the end of the chop; Select 3 marked points on the cutting tool: the tool tip K-ST, the tool edge K-BOL, and the center of mass K-C; let the spatial coordinates of these points in the CAD system be Then the spatial equation of the cutting tool plane is: Simplify this formula to: A K x + B K y + C K z + D K = 0 (Equation (X)), where Equation (X) is the plane equation of the cutting tool plane in the CAD system; The spatial position of the blade at the instant of the end of the chop is obtained according to the following process: Let the spatial coordinates of the rear point K-B1 and the front point K-B2 of the tool edge be respectively For K-B1, since K-B1 is in the tool plane, the distance between K-B1 and the tool edge K-BOL is the tool width d k , the line connecting the tool edge K-BOL and K-B1 and the line connecting the tool edge K-BOL and the tool tip K-ST are perpendicular to each other. Set up a system of equations: According to the known coordinates of the knife ridge K-BOL and the tip of the knife K-ST, solve the system of equations (XI) to obtain the spatial coordinates of K-B1; similarly, obtain the spatial coordinates of K-B2; Approximate the spatial position of the blade at the instant of the end of the hacking as the line connecting K-B1 and K-B2, and determine the equation of the spatial position of the blade at the instant of the end of the hacking as follows: The perimeter of the wound is obtained through the following process: The wound circumference is the length of the surface curve of the skull wound caused at the instant when the tool strikes the skull; after determining the spatial position of the tool plane at the instant of the end of the strike, measure the length of the contour curve formed by the intersection of the skull surface and the tool plane in the CAD system to obtain the wound circumference; when measuring, set N Qk points Q i in the CAD system, and their spatial coordinates are The interval between two adjacent points Q i is 1±0.3 mm. Approximate the length W L of the above contour curve by the sum of the lengths of the broken lines formed by Q i . Specifically, the formula is as follows: The depth of the wound is obtained through the following process: Take the spatial position of the blade at the instant of the end of the hacking as the deepest part of the head wound; use the N Qk points Q i of the above contour curve, and successively calculate the vertical distance between each point Q i and the connecting line between the above K-B1 and K-B2, and take the maximum value as the wound depth; The specific calculation process is as follows: First, set the vector S KB as: S KB =(x KB2 - x KB1 )i+(y KB2 - y KB1 )j+(z KB2 - z KB1 )k = m KB i + n KB j + p KB k (In formula (XIV), i, j, and k are unit vectors in the x, y, and z axis directions respectively; Vector S KB The modulus of is the distance between the connecting lines of K - B1 and K - B2, that is: Set K-B1 and Q again i The vector S of the connection line KQ is as follows: S KQ =(x Qi - x KB1 )i+(y Qi - y KB1 )j+(z Qi - z KB1 )k = m QK i + n QK j + p QK k(XVI) According to the method of spatial analytic geometry, Q i The perpendicular distance from the point to the line connecting K-B1 and K-B2 is: The numerator part on the right side of the above formula is the magnitude of the cross product of two vectors, and the denominator is the calculation result of formula (XV); The magnitude of the above vector is calculated according to the following formula: Calculate each Q in sequence through the above formulas (XV), (XIX), and (XVII). i The perpendicular distance from the point to the line connecting K - B1 and K - B2, and take the maximum value as the wound depth.

5. A method for constructing a three-dimensional finite element model for analyzing head chopping cases, according to any one of claims 1 to 4, characterized in that, in the first step, the data of the victim's head measured includes: The position of the wound on the victim's head and the morphological indexes of the wound, the three-dimensional geometric data of the victim's head; wherein, the morphological indexes of the wound include the length and depth of the wound, and the three-dimensional geometric data include the length, height and width of the head.

6. A method for constructing a three-dimensional finite element model for judging a head hacking case according to claim 5, characterized in that, in the first step, the specific process of constructing the personalized head geometric model is as follows: Import the THUMS head model into computer-aided engineering grid processing software, and scale the model in a 1:1 ratio according to the three-dimensional geometric data of the victim's head; check the quality of 1D, 2D and 3D meshes respectively. If there are mesh elements that do not meet the standards, perform preset processing. The specific process of the preset processing is as follows: first re-divide the mesh of the local area of the head by using the automatic mesh generation function, then delete the unqualified elements and regenerate qualified meshes; after that, the personalized head geometric model of the victim is obtained; The specific process of making the head physical model by 3D printing technology is as follows: Reverse reconstruct the cranial vault, facial skull and teeth of the personalized head geometric model of the victim; the cranial vault includes the frontal bone, parietal bone, occipital bone, temporal bone, sphenoid bone and ethmoid bone; the facial skull includes the maxilla, mandible, zygomatic bone, lacrimal bone, nasal bone, palatine bone, inferior nasal concha and vomer; during reverse reconstruction, use surface treatment software for hole filling, smoothing and optimization processing, and perform three-dimensional surface reconstruction; after that, import the reconstructed model into medical image processing software to convert it into a 3D printing special model, and then use 3D printing technology to make the head physical model in a 1:1 size.

7. A method for constructing a three-dimensional finite element model for the study and judgment of head hacking cases according to claim 6, characterized in that, The material for printing the parietal bone of the head is polyetheretherketone (PEEK) material; the material for printing the remaining parts of the head is polylactic acid (PLA) material.

8. A method for constructing a three-dimensional finite element model for analyzing head chopping cases according to any one of claims 1 to 4, characterized in that In the third step, the specific process of establishing a head finite element model based on the personalized head geometric model and assembling it with the knife finite element model includes: S1. Use computer-aided engineering grid processing software to perform unit mesh division on the personalized head geometric model to establish a personalized head finite element model; during the establishment process, use the hierarchical modeling function to divide the skull into several layers with a thickness of 0.1 mm, and respectively construct the outer cortical bone, middle cancellous bone and the thickness of the inner cortical bone to match the actual anatomical structure. Among them, the thickness of the outer cortical bone is 2.4 ± 0.1 mm, the thickness of the middle cancellous bone is 3.2 ± 0.1 mm, and the thickness of the inner cortical bone is 2.4 ± 0.1 mm; S2. In the computer-aided engineering grid processing software, hexahedral elements are used to encrypt the element grid in the contact area between the tool and the skull, and progressive tetrahedral elements are used to process the grid in the transition area between the contact area and the non-contact area. The main steps of the above processing include: using the grid subdivision function in the contact area between the tool and the skull to encrypt the grid in the contact area, and the minimum grid size after encryption is 0.09 mm; using the grid refinement strategy in the transition area between the contact area and the non-contact area to divide the grid in the transition area into progressive tetrahedral elements; the finite element model of the skull after the above processing includes five major types of tissues: outer compact bone, middle cancellous bone, inner compact bone, gray matter, and white matter. S3. Use the node merging strategy to process the co-nodes of the grid of the finite element model of the skull, so that the finite element model of the skull has mechanical behavior consistency and geometric continuity. S4. If the actual tool has been found in the skull hacking case, a finite element model of the tool is established according to the actual tool. The main steps include: using a vernier caliper to measure the geometric parameters of the actual tool, using the blade geometric data obtained by previously measuring the preset tool used in the second-step simulated hacking experiment with a scanning electron microscope to reversely reconstruct the outer contour of the actual tool, establishing a tool geometric model using a CAD system, then importing it into the computer-aided engineering grid processing software for grid division, and locally encrypting the blade area. If the actual tool has not been found in the skull hacking case, the possible types of tools in the case are preset first, and then the corresponding finite element model of the tool is selected from the existing tool database. S5. In the finite element software, the finite element model of the tool and the finite element model of the skull are assembled based on the tool hacking angle and the contact part between the tool and the skull in the second-step simulated hacking experiment. S6. In the finite element software, set the parameters of each constitutive equation of the finite element model of the skull and the parameters of the constitutive equation of the finite element model of the tool. The parameters include density, elastic modulus or Young's modulus, Poisson's ratio, ultimate stress, and ultimate strain; according to the cadaver experiment data in previous literature, assign the corresponding material properties to each constitutive equation of the finite element model of the skull; assign the constitutive equation of the finite element model of the tool to an isotropic steel material.

9. The method for constructing a three-dimensional finite element model for analyzing and judging a skull chopping case according to claim 8, wherein In the third step, the specific process of computer simulation for the formation of the hacking wound on the skull includes: Use the solver of the finite element software for computer simulation. Its boundary conditions are determined by the hacking position, hacking angle, and hacking speed obtained from the second-step simulated hacking experiment, and the bottom of the finite element model of the skull is fixed; in the finite element model of the skull, set the failure mode of the bone tissue unit to ductile fracture failure, and set the failure mode of the soft tissue unit to shear failure. During the computer simulation process, when the load on the bone tissue unit and soft tissue unit in the contact area between the tool and the skull reaches the failure condition, set the unit to fall off and remove it from the calculation.

10. A method for constructing a three-dimensional finite element model for analyzing and judging head chopping cases according to any one of claims 1 to 4, characterized in that, The specific process of the fourth step is: Compare the wound circumference of the simulation results obtained in the third step with the wound circumference of the wound morphological indexes obtained in the second step, and compare the wound depth of the simulation results obtained in the third step with the wound depth of the wound morphological indexes obtained in the second step; if the differences are both less than 15%, it is determined that the finite element model of the skull is valid; otherwise, calibrate the finite element model of the skull and then transfer to the computer simulation step in the third step to obtain the simulation results again, and continue the above comparison until it is determined that the finite element model of the skull is valid. Among them, the calibration measures include adjusting the element mesh division and / or adjusting the boundary conditions; finally, use the valid finite element model of the skull as the finally constructed finite element model of the skull.

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