Lower limb simplification method of human body finite element model

Abstract

The invention relates to the technical field of collision testing, in particular to a lower limb simplification method of a human body finite element model, which comprises the following steps: S1, analyzing an original lower limb model and formulating a simplification strategy; s2, reconstructing and modifying a grid of the original lower limb model; s3, performing shell extraction treatment on the skeleton part; s4, performing simplification treatment on the ligament part; and S5, carrying out weight balancing and verification on the simplified part. According to the method, the mesh number and contact of the lower limbs can be reduced while the biomechanical characteristics of the lower limbs of the human body finite element model are kept and the flexibility and degree of freedom of joints are maintained, and the operation time of a large-number mesh human body finite element model is greatly shortened; meanwhile, a general mode of lower limb model simplification is provided, the method can be applied to various human body models for lower limb simplification, and research on human body injury biomechanics can be better promoted.

Description

Technical Field

[0001] The present invention relates to the technical field of vehicle crash tests, and particularly to a method for simplifying the lower limbs of a human finite element model. Background Art

[0002] Vehicle crash tests are one of the core steps in the vehicle R & D process. When conducting research on human crash safety, using a finite element model (Finite Element Method, FEM) to simulate the response of the human body during a crash is a common method. Due to the rapid development of the automotive industry in recent years, the application of human models has become increasingly frequent, and it has become increasingly important to improve the usage efficiency of human models.

[0003] However, due to the complexity of the human body structure and the limitation of computing resources, it is usually unrealistic to directly perform high-precision modeling of the entire human body. It is necessary to provide a simplified lower limb model based on the human finite element model that can be used to optimize component-level verification. By combining the original lower limb model with the simplified lower limb model, while requiring the kinematic response of the simplified result to be accurate, in order to improve the computational efficiency and stability of component-level verification of the human model.

[0004] Therefore, it is necessary to simplify some parts of the finite element model. The main purpose is to improve the computational efficiency, reduce the computing resource requirements, and facilitate the calibration and verification of the model on the premise of ensuring certain accuracy and mechanical properties. Through a reasonable simplification strategy, efficient and reliable simulation can be achieved in complex human crash simulations, thereby providing strong support for fields such as vehicle safety design and medical device development. Summary of the Invention

[0005] The present invention provides a method for simplifying the lower limbs of a human finite element model, which can reduce the number of meshes and contacts of the lower limbs while maintaining the biomechanical characteristics of the lower limbs of the human finite element model and maintaining the joint flexibility and degrees of freedom, greatly shortening the operation time of the human finite element model with a large number of meshes; at the same time, it can provide a general mode for simplifying the lower limbs, which is applied to the simplification of the lower limbs of various human models, and is conducive to better promoting the research on human injury biomechanics.

[0006] The present application provides the following technical solutions:

[0007] A method for simplifying the lower limbs of a human finite element model, comprising the following steps:

[0008] S1. Analyze the original lower limb model and formulate a simplification strategy;

[0009] S2. Reconstruct and modify the meshes of the original lower limb model;

[0010] S3. Perform a shelling process on the bone part;

[0011] S4. Simplify the ligament part;

[0012] S5. Weigh and verify the simplified part.

[0013] Technical principle: After confirming the correct geometric features of the original human body model, analyze each part of the original human body model and formulate targeted simplification strategies to effectively reduce the number of elements. Finally, make local weight adjustments to the parts with structural changes during the simplification process, compare the kinematic responses of the adjusted simplified model with the original model, and adjust the parameters of each joint connection part based on the comparison results.

[0014] Beneficial effects: The method of the present invention can reduce the number of meshes and contacts in the lower limbs while maintaining the biomechanical characteristics of the lower limbs of the human finite element model and maintaining the joint flexibility and degrees of freedom, greatly shortening the operation time of the human finite element model; at the same time, it can provide a general mode for simplifying the lower limbs, which can be applied to the simplification of the lower limbs of various human body models, and is conducive to better promoting the research on human injury biomechanics.

[0015] Furthermore, the simplification strategy includes enlarging the mesh size of the bones and subcutaneous soft tissues in the original lower limb model, shelling the bones in the original lower limb model and rigidifying the materials, and optimizing the ligament connections at the joints, and ensuring that the simplified lower limb model can maintain the same motion trend and mechanical response as the original lower limb model through segment verification.

[0016] Furthermore, S2 includes the following steps:

[0017] S21. Confirm the total number of solid meshes of the subcutaneous soft tissues in the original lower limb model. When the total number exceeds the preset threshold, execute S22; if the total number of meshes does not exceed the preset threshold, perform bone shelling;

[0018] S22. Increase the mesh size of the subcutaneous soft tissues to reduce the total number of meshes below the preset threshold;

[0019] S23. Appropriately enlarge the meshes of the middle sections of the femur and tibia;

[0020] S24. Retain the meshes at both ends of the femur and tibia and perform co-node transition processing;

[0021] S25. Repack the meshes of the subcutaneous tissues of the tetrahedral elements at the knee and ankle.

[0022] Furthermore, the preset threshold for the total number of solid meshes is 150,000.

[0023] Furthermore, S3 includes the following steps:

[0024] S31. Determine whether the bone part to be simplified belongs to a rigid body. If it belongs to a rigid body, execute S32; if it does not belong to a rigid body, simplify the femur and tibia into 2D rigid bodies, delete the cancellous bone at both ends of the bone, and replace it with a layer of 2D mesh.

[0025] S32. Perform 2D shelling on the entire bone.

[0026] Furthermore, the S4 includes the following steps:

[0027] S41. Hinge the ligaments at the joints, where the joints include the hip joint, knee joint, and ankle joint;

[0028] S42. Perform 1D processing on the foot ligaments.

[0029] Furthermore, the S5 includes:

[0030] S51. Add weights to the simplified part;

[0031] S52. Compare and verify the kinematic response data of the simplified lower limb model and the original lower limb model;

[0032] S53. Adjust the simplified lower limb model to improve its accuracy and ensure that its kinematic response matches that of the original lower limb model.

[0033] Furthermore, the S52 includes comparing the displacement results at key points and the stress in key areas between the simplified lower limb model and the original lower limb model.

[0034] Furthermore, the adjustment of the simplified lower limb model includes adjusting connection parameters and adjusting 1D elements.

[0035] Furthermore, the adjustment of 1D elements includes adjusting the type of 1D elements, adjusting the material parameters of 1D elements, and introducing a non-linear material model for specific tissues. Brief Description of the Drawings

[0036] Figure 1 It is a flowchart of a method for simplifying the lower limbs of a human finite element model;

[0037] Figure 2 It is a schematic diagram of the enlarged mesh of the femur and tibia;

[0038] Figure 3 It is a schematic diagram of re-meshing the subcutaneous tissue of the tetrahedral elements at the knee and ankle;

[0039] Figure 4 It is a schematic diagram of rigidifying the right femur. Detailed Description of the Specific Embodiment

[0040] The following is a further detailed description through specific embodiments:

[0041] Example 1

[0042] This embodiment provides a method for simplifying the lower limbs of a human finite element model, as Figure 1 shown, including the following steps:

[0043] S1. Analyze the original lower limb model and formulate a simplification strategy:

[0044] This step mainly analyzes the geometric characteristics of the original lower limb model and formulates different simplification strategies for each part of the original lower limb model.

[0045] The simplification strategy includes magnifying the mesh size of the bones and subcutaneous soft tissues in the original lower limb model, shelling the bones of the original lower limb model and stiffening the materials, and optimizing the ligament connections at the joints.

[0046] At the same time, the simplification strategy also needs to consider that the simplified lower limb model can accurately reflect the mechanical behavior of the lower limb model, and avoid introducing too large errors in the simplified lower limb model resulting in distorted test results. Through segment verification, ensure that the simplified lower limb model can maintain the same motion trend and mechanical response as the original lower limb model.

[0047] S2. Reconstruct and modify the mesh of the original lower limb model, including the following steps:

[0048] S21. Confirm the total number of solid meshes of the subcutaneous soft tissues of the original lower limb model. When the total number exceeds the preset threshold, mesh reconstruction is required and S22 is executed; if the total number of meshes does not exceed the preset threshold, bone shelling is performed.

[0049] The calculation range of the subcutaneous soft tissues of the lower limbs includes parts such as the buttocks, thighs, calves, knees, ankles, and feet. The preset threshold for the total number of meshes is determined according to the performance of the solver and the time progress requirements of the calculation task. In this embodiment, the preset threshold is set to 150,000 and can be adjusted according to actual computing resources and time requirements.

[0050] S22. Increase the mesh size of the subcutaneous soft tissues.

[0051] Sparsify the overall mesh of the subcutaneous soft tissues and magnify the mesh to reduce the total number of meshes.

[0052] S23. Perform corresponding mesh magnification on the meshes in the middle sections of the femur and tibia.

[0053] Since the femur and tibia are required to share nodes with the subcutaneous soft tissues, the meshes in the middle sections of the femur and tibia also need to be magnified accordingly, as Figure 2As shown. The mesh magnification factor in the middle sections of the femur and tibia should be consistent with the mesh magnification factor in S22. To ensure the accurate outer contour of both ends of the bone, only the meshes in the middle sections of the femur and tibia are magnified.

[0054] S24. Retain the meshes at both ends of the femur and tibia and perform co - node transition processing:

[0055] Since the meshes in the middle sections of the bone were magnified in S23, and the meshes at both ends of the bone were not processed to ensure the accurate outer contour. To accurately simulate the contact interface between the bone and soft tissue, it is necessary to perform magnification transition between the meshes in the middle section of the bone and those at both ends, especially local mesh magnification, to ensure that the meshes of the bone and soft tissue have the same node distribution in the contact area. Through reasonable mesh generation, co - node constraints, and mesh quality inspection, the computational cost can be reduced and the reliability of the model can be improved while ensuring the accuracy.

[0056] The specific steps of the co - node transition processing include:

[0057] Data preparation: After obtaining the original mesh data of the femur and tibia, denoise and smooth the meshes, segment the femur and tibia separately and mark the boundaries;

[0058] Retain the meshes at both ends of the femur and tibia: This can be accomplished by common mesh processing tools, including Meshlab, Gmsh, ParaView, Blender, etc. Import the original mesh data into the mesh processing tool, select the areas to be retained (i.e., both ends of the femur and tibia), and crop the parts that do not need to be retained;

[0059] Perform co - node transition processing: Ensure that both ends and the middle sections of the femur and tibia share the same nodes, guaranteeing continuity and consistency in subsequent analyses. First, extract the boundaries of the retained parts of the femur and tibia in the mesh processing tool, align the boundaries of the femur and tibia, and create transition meshes between the boundaries. The transition meshes can be added by means such as manual addition, interpolation algorithms, and Boolean operations;

[0060] Post - processing and verification: Smooth the generated transition meshes to eliminate possible sharp edges and irregular shapes; check the mesh quality to ensure there are no problems such as overlapping patches and isolated nodes; finally, use visualization tools to visually verify the final meshes to ensure that the transition processing effect meets expectations.

[0061] S25. Repopulate the meshes of the subcutaneous tissue of the knee and ankle tetrahedral elements.

[0062] The knee and ankle contain various structures such as bones (e.g., femur, tibia, fibula, talus, etc.), joints, ligaments, muscles, tendons, and subcutaneous tissues. The geometric shape of the subcutaneous tissues in these areas is complex and irregular. Remeshing can more accurately capture the geometric features of these complex structures, ensuring that the finite element model can truly reflect the actual anatomical situation. At the same time, it can improve the mesh quality (such as element shape, size uniformity, etc.), construct a more accurate and reliable finite element model, and optimize the stability and accuracy of numerical calculations.

[0063] A schematic diagram of mesh remeshing is as Figure 3 shown and includes the following steps:

[0064] 1) Data preparation: Check and repair topological defects in the model to ensure that the surface model is smooth and conforms to anatomical features;

[0065] 2) Mesh generation: Set appropriate mesh size parameters and use finite element processing software to generate an initial tetrahedral mesh to ensure a moderate mesh density;

[0066] 3) Check the quality indicators of tetrahedral elements, including:

[0067] Jacobian Ratio: Used to evaluate the degree of element deformation;

[0068] Aspect Ratio: Used to measure the proportional relationship of element side lengths;

[0069] Minimum Angle: Used to avoid numerical instability caused by too small angles.

[0070] 4) Mesh remeshing:

[0071] For key areas of the knee and ankle (such as around joints, at the junction of bone and soft tissue), local mesh refinement is carried out.

[0072] While retaining the meshes at both ends of the femur and tibia, ensure that the subcutaneous soft tissues of the knee and ankle share nodes with the bone ends, and regenerate the meshes of the subcutaneous soft tissues of the knee and ankle with reference to the meshes at both ends of the femur and tibia.

[0073] 5) Verify the remeshed mesh:

[0074] This step includes geometric verification and physical verification. Geometric verification mainly checks whether the remeshed mesh accurately represents the geometric shape of the subcutaneous tissue and ensures that the mesh boundary fits well with the interfaces of bones and other tissues; Physical verification can perform a preliminary finite element analysis to verify whether the behavior of the mesh under stress is reasonable and check whether the results such as stress distribution and displacement field meet expectations.

[0075] S3. Perform shelling on the bone part:

[0076] This step mainly performs stiffening on bones such as the femur and tibia, and specifically includes the following steps:

[0077] S31. Determine whether the bone part to be simplified belongs to a rigid body. If it belongs to a rigid body, go to S32. If it does not belong to a rigid body, simplify the femur and tibia into a 1D rigid body, delete the cancellous bone at both ends of the bone, and replace it with a layer of 2D mesh to reduce the number of meshes. Figure 4 It is a schematic diagram of the stiffening process for the right femur.

[0078] S32. Perform 2D shelling on the whole bone:

[0079] 2D shelling can simplify the three-dimensional solid structure into two-dimensional shell elements, which can significantly reduce the computational complexity and resource consumption while maintaining sufficient accuracy. The 2D shelling specifically includes the following steps:

[0080] Geometric simplification: According to the characteristics of the human body structure, extract the mid-plane from the three-dimensional solid structure as the basis for 2D modeling. At the same time, for complex geometric shapes, key geometric features can be retained and unnecessary details can be removed;

[0081] Material definition: Select appropriate material models according to different parts, including plane stress model, plane strain model, isotropic model, anisotropic model, etc.

[0082] S4. Simplify the ligament part.

[0083] It includes the simplification of ligaments at joints and the simplification of foot ligaments, and specifically includes the following steps:

[0084] S41. Hinge the ligaments at joints:

[0085] If the joint is connected by ligaments, refer to the joint connection method of the dummy model to create a hinge connection for the lower limb of the human body model and define the stiffness and rotation range. Take the knee joint as an example. First, roughly calculate the position of the rotation axis of the hinge based on the kinematic response of the lower limb of the human finite element model. Then, delete the ligament connection and define the parameters of the hinge and its rotational stiffness, friction, etc.;

[0086] S42. 1D-ize the foot ligaments:

[0087] If 2D shell element ligaments are used to connect the bones of the foot, 1D processing is adopted to improve the stability of the model. The 1D processing of the foot ligaments refers to the process of simplifying a three-dimensional solid or two-dimensional structure into a one-dimensional linear element (such as a beam element or a rod element). It is mainly aimed at the 3D or 2D ligaments that may exist between the toes of the foot to reduce the computational complexity. The 1D processing of the foot ligament model needs to ensure that the simplified lower limb model can accurately reflect the actual mechanical behavior of the ligaments of the original model.

[0088] S5. Counterweight and verification for the simplified part:

[0089] After the partial simplification of the original lower limb model, due to the change of solid elements, the mass relationship of this part cannot be truly reflected. Therefore, counterweight processing needs to be carried out on this part. Specifically, it includes:

[0090] S51. Counterweight for the simplified part:

[0091] The counterweight is mainly for parts such as the femur and tibia that have undergone 2D shelling. Since 3D solid elements are deleted, it is necessary to calculate the material density of the 2D shell element rigid body equally to accurately compensate for the mass of this part. At the same time, for the 1D ligaments of the foot, the original mass needs to be added to the nearby bones. The purpose of this step is to ensure that the overall mass of the lower limb remains unchanged as much as possible.

[0092] S52. Compare and verify the kinematic responses of the simplified lower limb model and the original lower limb model:

[0093] Since 2D or 1D elements are used to replace complex three-dimensional solid structures in the simplified lower limb model, the connection definitions of the lower limb have been modified to a certain extent. Therefore, the motion responses of the simplified lower limb model may change too much. For example, the simplified lower limb model may not be able to fully capture the complex displacement and rotation patterns in the original lower limb model, and at the same time, the stress distribution in the contact surface or stress concentration area may not be as accurate as that of the original lower limb model. Eventually, it may lead to different injury test results of the simplified lower limb model from those of the original lower limb model during the collision test. Therefore, it is necessary to compare and verify the kinematic response data of the simplified lower limb model to make it as consistent with the original lower limb model as possible.

[0094] The comparison of the kinematic response data includes the comparison of the displacement results at key points and the stress comparison in key areas of the two models. When the change trends are the same, it can be considered that the dynamic responses of the simplified lower limb model and the original lower limb model are consistent; if there are obvious differences in the change trends or they exceed the threshold, the simplified lower limb model needs to be adjusted.

[0095] S53. Adjust the simplified lower limb model to improve its accuracy and ensure that its kinematic response matches that of the original lower limb model:

[0096] Adjustments to the simplified lower limb model included adjustments to connection parameters and 1D units.

[0097] For example, for rigidly connected parts, such as fixed joints between bones, a rigid connection can be set to ensure that there is no relative movement between the two parts, which is suitable for simulating inactive joints or fixed bone parts; for single-degree-of-freedom rotation joints, such as the flexion and extension of the knee joint, a hinge connection can be used to allow one degree of freedom to rotate, but limit the movement of other degrees of freedom; for multi-degree-of-freedom rotation joints, such as the hip joint, it can be set as a ball-and-socket joint, allowing three degrees of freedom to rotate, but limiting translational movement; for soft tissues such as ligaments and muscles, it can be set as a spring-damper system, and by setting appropriate spring stiffness and damping coefficients, the stretching and rebound behavior of ligaments, as well as the contraction and relaxation of muscles, can be simulated; for some joints that may have coupled motion, such as the flexion and extension of the knee joint, which may be accompanied by a certain amount of internal and external flipping. This more complex motion relationship can be simulated by introducing coupling constraints.

[0098] Adjusting the 1D cell involves the following steps:

[0099] 1) Choose the appropriate 1D cell type:

[0100] This includes using beam elements to simulate slender structures such as long bones and ligaments; using rod elements to simulate structures that can only withstand axial tension and compression but not bending or torsion, such as tendons; and using truss elements to simulate structures that only withstand axial loads.

[0101] 2) Adjust the material parameters of the 1D unit:

[0102] Adjustable material parameters include elastic modulus E, Poisson's ratio ν, density ρ, shear modulus G, etc.

[0103] Among them, by adjusting the elastic modulus, the degree of deformation of the structure can be controlled. The elastic modulus of bones is usually large, while the elastic modulus of ligaments and tendons is small;

[0104] By adjusting the Poisson's ratio, the relationship between the lateral strain and the longitudinal strain of the material when it is subjected to force is described. The Poisson's ratio is usually set between 0.2 and 0.5;

[0105] By adjusting the density parameter, the inertial force of the structure is described;

[0106] The torsional stiffness of the model is described by adjusting the shear modulus of the 1D element. Generally, the shear modulus of bones is larger, while the shear modulus of ligaments and tendons is smaller.

[0107] 3) Introducing nonlinear material models for specific tissues:

[0108] For soft tissues such as ligaments and tendons, hyperelastic material models (such as Mooney-Rivlin and Ogden models) can also be introduced to simulate the non-linear mechanical properties of soft tissues, so as to accurately capture the stress-strain relationship of soft tissues under large deformations;

[0109] For some soft tissues (such as skin and fat), viscoelastic material models (such as Maxwell and Kelvin-Voigt models) can be introduced to simulate the time-dependent behavior of the materials and better capture the hysteresis effect of soft tissues during loading and unloading.

[0110] The above are only embodiments of the present invention. The invention is not limited to the fields involved in this embodiment case, and common knowledge such as specific structures and characteristics well-known in the art are not described in detail here. It should be noted that for those skilled in the art, without departing from the structure of the present invention, several changes and improvements can still be made, which should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicality of the patent. The protection scope required by this application should be based on the content of its claims, and the specific implementation manners described in the specification can be used to explain the content of the claims.

Claims

1. A method for simplifying lower limbs of a human finite element model, characterized in that: The following steps are involved: S1. Analyze the original lower limb model and develop a simplification strategy; S2, reconstructing and modifying the mesh of the original lower limb model; S3, performing shell extraction on the skeleton part; S4, simplify the ligament part; S5. Weight and verify the simplified parts.

2. The method for simplifying lower limbs of a human finite element model according to claim 1, characterized in that: The simplification strategy includes enlarging the mesh size of the bones and subcutaneous soft tissues in the original lower limb model, shelling the bones of the original lower limb model and stiffening the materials, and optimizing the ligament connections at the joints. Segment verification is then used to ensure that the simplified lower limb model maintains consistent motion trends and mechanical responses with the original lower limb model.

3. The method for simplifying lower limbs of a human finite element model according to claim 1, characterized in that: The S2 comprises the following steps: S21, confirming the total number of entity meshes of the subcutaneous soft tissue of the original lower limb model, and executing S22 when the total number exceeds a preset threshold, and performing shell extraction on the bone part if the total number of meshes does not exceed the preset threshold; S22, increasing the mesh size of the subcutaneous soft tissue to reduce the total number of meshes to below a preset threshold; S23, enlarging the meshes of the femur and the mid-tibia accordingly; S24, retain the meshes at both ends of the femur and tibia and perform common node transition processing; S25. Re-mesh the subcutaneous tissue of the knee and ankle tetrahedral elements.

4. The method for simplifying lower limbs of a human finite element model according to claim 3, characterized in that: The preset threshold of the total number of entity grids is 150,000.

5. The method for simplifying lower limbs of a human finite element model according to claim 3, characterized in that: The S3 comprises the following steps: S31, judging whether the bone part to be simplified belongs to a rigid body, if it belongs to a rigid body, executing S32, if it does not belong to a rigid body, simplifying the femur and tibia into a 2D rigid body, deleting the cancellous bones at both ends of the bone, and replacing them with a layer of 2D mesh; S32. Perform 2D shelling on the entire skeleton.

6. The method for simplifying lower limbs of a human finite element model according to claim 5, characterized in that: The S4 comprises the following steps: S41, performing hinge processing on the ligaments at the joints, wherein the joints include hip joints, knee joints and ankle joints; S42. Perform 1D treatment on the foot ligaments.

7. The method for simplifying lower limbs of a human finite element model according to claim 6, characterized in that: The S5 comprises the following steps: S51, weighting the simplified part; S52. Compare and verify the kinematic response data of the simplified lower limb model and the original lower limb model; S53. Adjust the simplified lower limb model to improve its accuracy and ensure that its kinematic response matches the original lower limb model.

8. The method for simplifying lower limbs of a human finite element model according to claim 7, characterized in that: The S52 includes comparing the displacement results at key points and the stress in key areas of the simplified lower limb model with the original lower limb model.

9. The method for simplifying lower limbs of a human finite element model according to claim 7, characterized in that: The adjusting of the simplified lower limb model includes adjusting connection parameters and adjusting 1D units.

10. The method for simplifying lower limbs of a human finite element model according to claim 9, characterized in that: The adjusting of the 1D unit includes adjusting the type of the 1D unit, adjusting the material parameters of the 1D unit, and introducing a nonlinear material model for a specific tissue.

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