A complex stress loading device and processing method based on DIC technology

Abstract

The application discloses a complex stress loading device and processing method based on DIC technology, and belongs to the technical field of material complex loading mechanical property testing, and comprises a Hopkinson pressure bar system and a pair of connecting pieces, two connecting pieces are connected with an incident rod and a transmission rod of the Hopkinson pressure bar system through a screw rod and a cushion block, a mounting part is horizontally arranged on the connecting piece, a rectangular accommodating groove is arranged on the mounting part, the accommodating groove is through the upper and lower end faces of the mounting part, a plurality of clamping grooves are communicated around the accommodating groove, a fastener is placed in the accommodating groove, matched threaded holes are arranged on the two fasteners, and a threaded test sample gauge section is threadedly connected in the two threaded holes. The application obtains real-time full-field strain of the threaded test sample gauge section through digital image technology, solves the problems that the threaded test sample gauge section has Poisson effect and rotates due to deviation from ideal constraint, and provides important technical support for completing material mechanical property testing under different working conditions.

Description

Technical Field

[0001] This invention relates to the field of testing technology for the mechanical properties of materials under complex loading, and in particular to a complex stress loading device and processing method based on DIC technology. Background Technology

[0002] Engineering materials often operate under complex stress states during service, and in fields such as aerospace and defense engineering, they are frequently subjected to strong dynamic loads. Their mechanical response in dynamic loading experiments differs significantly from that under quasi-static conditions. Because the response time is on the order of microseconds, it is difficult to accurately capture their precise stress-strain response. Currently available composite loading devices cannot achieve accurate decomposition of stress and strain along the loading path, leading to errors in the material's mechanical analysis. While digital image correlation (DIC) technology has been used to measure the full-field deformation of the gauge length of threaded test specimens, this technique only obtains data on the material's non-uniform strain field and has not been applied to the stress-strain response under complex loading conditions.

[0003] The gauge length of the test thread specimen under dynamic loading suffers from the Poisson effect and rotation due to deviation from the ideal constraint, making it impossible to accurately obtain the real-time stress-strain state of the gauge length of the test thread specimen. Summary of the Invention

[0004] To address the problem of failing to accurately capture stress-strain decomposition in materials under dynamic loading paths, this invention provides a complex stress loading device and method based on DIC technology. Utilizing DIC technology combined with a Hopkinson bar system (SHPB) and a composite fixture, dynamic loading with composite compression and shear at different angles is achieved. The deflection angle of the gauge length of the test threaded specimen is tracked and calculated in real time, establishing an accurate stress-strain decomposition relationship for the gauge length of the test threaded specimen under dynamic composite compression and shear at different angles. This improves the accuracy of analyzing complex stress states in materials and enables accurate stress-strain decomposition of the gauge length of the test threaded specimen under dynamic composite loading conditions, which is of great significance for the accurate dynamic mechanical analysis of materials.

[0005] To achieve the objectives of this invention, a complex stress loading device and processing method based on DIC technology is provided, comprising a Hopkinson pressure bar system and a pair of connectors used in conjunction with the Hopkinson pressure bar system. The two connectors are respectively connected to the incident rod and the transmission rod of the Hopkinson pressure bar system via screws and pads. The screws and pads are made of the same material as the incident rod and the transmission rod. Specifically, one side of the connector is bonded to the incident rod / transmission rod of the Hopkinson pressure bar system with ergo adhesive, thereby enabling dynamic test loading according to testing requirements. To ensure that the stress wave can travel undamagedly from the incident rod to the connector during dynamic testing, the cross-sectional area of ​​one side of the connector is determined based on the matching value of the wave impedance ρcA (ρ: density, c: elastic wave velocity, A: cross-sectional area) between the incident rod and the connector surface. One side of the connector is connected to the Hopkinson pressure bar system. The connector has a horizontally extending mounting part with a rectangular receiving groove. The receiving groove is connected to the upper and lower end faces of the mounting part. Multiple locking grooves are connected around the receiving groove. Fasteners are placed in the receiving groove. Two fasteners have matching threaded holes, and test thread specimen gauge sections are threadedly connected in the two threaded holes.

[0006] The Hopkinson pressure bar system in this technical solution includes a drive unit, an incident rod, and a transmission rod. The right drive shaft of the drive unit is connected to an impact rod. A pulse shaper is provided at the left end of the incident rod. An absorption rod is provided on the right side of the transmission rod. The connector and the gauge length of the test thread specimen are located between the incident rod and the transmission rod. Strain gauges are provided in the middle of both the incident rod and the transmission rod. The strain gauges are connected to a hyperdynamic strain gauge, which is connected to a data acquisition system and a computer.

[0007] The fasteners are bolted to the centrally symmetrical fasteners via a gauge section of a test thread specimen. The gauge section of the test thread specimen can be designed according to material testing standards. The gauge section of the test thread specimen is connected to both ends by an arc transition, and both ends of the gauge section of the test thread specimen are threaded to the threaded holes of the fasteners. t To test the total length of the gauge length of the threaded specimen, L c To test the parallel length of the gauge length segment of the threaded specimen, Lo is the original gauge length of the gauge length segment of the threaded specimen, and d is the parallel length of the gauge length segment. o The original diameter of the parallel length of the gauge section of the test thread specimen is given by r, which is the arc transition value of the gauge section of the test thread specimen.

[0008] The stress-strain decomposition method for dynamic composite compression and shear at different angles using DIC technology includes the following steps:

[0009] Step 1: Apply matte black and white paint evenly to the gauge length of the test thread specimen, ensuring the grayscale values ​​of the gauge length follow a standard normal distribution. Then, using two high-speed cameras based on binocular vision with DIC technology, establish a spatial coordinate system with the center of the gauge length as the origin. A Hopkinson bar system performs a dynamic compression-shear experiment on the gauge length. The DIC image acquisition system captures speckle images of the gauge length throughout the deformation process. By correlating pixel subsets or positional changes in the speckle images from the original and deformed images, and using different interpolation curves and shape functions, the displacement and strain changes of the gauge length are determined. The axial strain ε is then calculated using different strain measurement methods. xx and normal strain ε yy ;

[0010] Step 2: Acquire speckle images of the gauge length of the test thread specimen throughout the deformation process using an image acquisition system. Calculate the rotation angle α of the gauge length of the test thread specimen from the initial coordinate axis. The actual stress components can be solved using the initial axial-tangential coordinate system (ε... xx ,ε yy ) to the deflection axis-tangential coordinate system (ε xx ',ε yy The transformation matrix is:

[0011]

[0012] The axial strain ε at each equidistant moment is obtained from the image acquisition system. xx 'and normal strain ε yy The actual value of ' is represented as:

[0013]

[0014] As a further improvement to the above scheme, the stress components of the gauge length section of the test threaded specimen are determined by calculating the forces at both ends of the gauge length section of the test threaded specimen using the strain gauge signals collected on the incident and transmission rods of the Hopkinson pressure bar system via a three-wave formula.

[0015]

[0016] In the formula, ε i (t) and ε r (t) represents the incident and reflected strain pulses collected on the incident rod, respectively, ε t (t) represents the transmitted strain pulse collected on the transmission rod. The pulse is collected by measuring the strain gauges on the incident rod and the transmission rod. A0 is the cross-sectional area of ​​the incident rod / transmission rod, and E0 is the elastic modulus of the incident rod / transmission rod.

[0017] The load is decomposed based on the threaded hole angle θ inside the fastener to obtain the normal force F of the gauge length section of the test thread specimen. n and tangential force F s During loading, the gauge length of the test thread specimen is in force equilibrium along the F0 direction, as shown in:

[0018]

[0019] The rotation angle α of the gauge length of the test thread specimen from the initial coordinate axis is calculated based on the image acquisition system, and the normal force F at each equal interval time t is... n 'and tangential force F s The actual value of ' is:

[0020]

[0021] Necking occurs during the plastic deformation of metallic materials, and the engineering stress-strain ratio contains errors in describing the true state of the gauge length of the test threaded specimen. The true stress is obtained by dividing the true strain obtained using DIC (Discrete Incompressible Coefficient) based on the assumption of the incompressibility of metallic materials and the load data from the Hopkinson bar system by the instantaneous cross-sectional area of ​​the gauge length of the test threaded specimen. This method accurately reflects the stress-strain relationship of the material. The expression for the instantaneous cross-sectional area of ​​the gauge length of the test threaded specimen is as follows:

[0022] A sample =A0·A factor (5)

[0023] A factor =exp(ε yy ') / exp(ε xx '+ε yy ') (6)

[0024] In the formula, A0 and A factor These are the initial cross-sectional area and instantaneous cross-sectional area factor of the gauge length section of the test thread specimen, respectively.

[0025] Stress state of the gauge length section of the threaded specimen under combined compression and shear test, and normal stress σ at the time of equal interval t. x (t) and shear stress τ xy The method for calculating the (t) component is as follows:

[0026]

[0027] The beneficial effects of this invention are:

[0028] Compared with existing technologies, the present invention provides a complex stress loading device and processing method based on DIC technology. By using digital image processing technology (DIC) to process experimental data, it solves the problems of Poisson effect and rotation due to deviation from ideal constraints in the gauge length of the test threaded specimen, so as to obtain accurate real-time strain of the gauge length of the test threaded specimen. The present invention has a reasonable and reliable structural design, is easy to process, can be assembled with the Hopkinson bar system, facilitates disassembly and replacement of fasteners with threaded holes at different angles, and realizes dynamic loading test of composite compression and shear at any angle. Attached Figure Description

[0029] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings, wherein:

[0030] Figure 1 This is a schematic diagram of the overall structure of the complex stress loading device of the present invention;

[0031] Figure 2 A two-dimensional schematic diagram of the pressure shearing device of the present invention;

[0032] Figure 3 Schematic diagram of the compression shearing device of the present invention;

[0033] Figure 4 This is a schematic diagram of the upper and lower connecting parts structure;

[0034] Figure 5 Schematic diagrams of upper and lower fastener structures with threaded holes at different angles, where (a) the threaded hole is 20°; (b) the threaded hole is 30°; (c) the threaded hole is 40°; (d) the threaded hole is 45°; (e) the threaded hole is 60°; and (f) the threaded hole is 90°.

[0035] Figure 6 A schematic diagram showing the gauge length dimensions of a test thread specimen;

[0036] Figure 7 This is a schematic diagram of the deflection angle of the strain coordinate axis during dynamic testing.

[0037] Figure 8 The diagram shows the geometry and stress state of a point within the gauge length of a test thread specimen under dynamic testing. (a) shows the geometry of a point within the gauge length of the test thread specimen under dynamic testing; (b) shows the stress state of the shaded area in (a).

[0038] Figure 9 This is a schematic diagram of the deflection angle of the force coordinate axis during dynamic testing. Detailed Implementation

[0039] like Figure 1-9As shown, the overall structure of a complex stress loading device based on DIC technology according to the present invention is a dynamic testing device for composite compression and shear at any angle. It includes a Hopkinson bar system and a pair of connectors 1 used in conjunction with the Hopkinson bar system. The two connectors 1 are connected to the incident rod 4 and the transmission rod 5 of the Hopkinson bar system respectively through screws 2 and pads 3. The screws 2 and pads 3 are made of the same material as the incident rod 4 and the transmission rod 5. Specifically, one side of the connecting surface of the connector 1 is bonded to the incident rod 4 / transmission rod 5 of the Hopkinson bar system with ergo glue, thereby realizing dynamic test loading according to the test requirements. In order to ensure that the stress wave can travel from the incident rod 4 to the connector 1 without damage during dynamic testing, the cross-sectional area of ​​the connecting surface of the connector 1 is determined according to the matching value of the wave impedance ρcA (ρ: density, c: elastic wave velocity, A: cross-sectional area) between the incident rod 4 and the connecting surface. The connecting surface of the connector 1 is connected to the Hopkinson bar system. A mounting part 6 extends laterally on the connector 1. A rectangular receiving groove 7 is provided on the mounting part 6. The upper and lower end faces of the receiving groove 7 are connected to the upper and lower end faces of the mounting part 6. Multiple locking grooves 8 are connected around the receiving groove 7. Fasteners 9 are placed in the receiving groove 7. Matching threaded holes 10 are provided on the two fasteners 9. Test thread specimen gauge length 11 is threadedly connected in the two threaded holes 10. The test thread specimen gauge length 11 is selected according to the relevant material testing standards.

[0040] The Hopkinson pressure bar system in this technical solution includes a drive unit 20, an incident rod 4, and a transmission rod 5. The right drive shaft of the drive unit 20 is connected to an impact rod 14. A pulse shaper 15 is provided at the left end of the incident rod 4. An absorption rod 16 is provided on the right side of the transmission rod 5. The connector 1 and the gauge length section 11 of the test thread sample are located between the incident rod 4 and the transmission rod 5. Strain gauges 13 are provided in the middle of both the incident rod 4 and the transmission rod 5. The strain gauges 13 are connected to the ultra-dynamic strain gauge 17. The ultra-dynamic strain gauge 17 is connected to the data acquisition system 18 and the computer 19.

[0041] Two centrally symmetrical fasteners 9 are bolted together via a gauge section 11 of the test thread specimen. The dimensions of the gauge section 11 can be designed according to material testing standards. The two ends of the gauge section 11 connecting to the fasteners 9 are connected by an arc transition, and both ends of the gauge section 11 are threaded into the threaded holes 10 of the fasteners 9. t To test the total length of gauge section 11 of the threaded specimen, L c To test the parallel length of gauge section 11 of the threaded specimen, L o To test the original gauge length of the gauge section 11 of the threaded specimen, d o The original diameter of the parallel length of the gauge section 11 of the test thread specimen is given by r, which is the arc transition value of the gauge section 11 of the test thread specimen.

[0042] This embodiment adopts a preferred scheme, where the cross-sectional area of ​​the connecting surface at the tail end of the connector 1 is determined based on the matching of the wave impedance pcA of the incident rod 4 and the connecting surface.

[0043] Taking the fastener 9 with a 45° tilt angle as an example, the specific implementation process is as follows: First, determine the cross-sectional area of ​​the connecting surface at the tail end of the connector 1 based on the matching of the wave impedance pcA of the incident rod 4 and the connecting surface. Then, bond the pad 3 and the incident rod 4 coaxially with ergo glue, and screw in the screw 2 at the same time. Then tighten the connector 1 to the tail end of the screw 2. The fastener 9 is connected to the locking groove 8 by a tenon and tenon joint through the rectangular protrusion 12. The connection process of the centrally symmetrical connector 1 and fastener 9 with the transmission rod 5 is the same as the above process.

[0044] like Figure 6 As shown, black and white matte paint is evenly sprayed onto the gauge section 11 of the test thread specimen. One end of the gauge section 11 is threadedly connected to the threaded hole 10 of the fastener 9, and the other end is threadedly connected to the threaded hole 10 of the second fastener 9, which is centrally symmetrical. The two sets of tenon and mortise structures remain coaxial. This is used to calibrate the high-definition camera, as shown in the figure. Figure 7 As shown, the photographing position corresponds to the gauge length segment 11 of the test threaded specimen. Based on 3D-DIC digital image processing technology and the strain gauge 13 signals recorded on the incident rod 4 and the transmission rod 5, the real-time data of the gauge length segment 11 of the test threaded specimen is calculated and analyzed.

[0045] The specific implementation of the processing method is as follows: using an image acquisition system and speckle images of the gauge length section 11 of the test thread specimen throughout the entire deformation process, speckle regions are divided on the surface of the gauge length section 11 of the test thread specimen. Based on the speckle images, the axial strain ε is calculated. xx and normal strain ε yy Simultaneously, the rotation angle α of the gauge length segment 11 of the test thread specimen deviating from the coordinate axis is calculated. The actual strain components can be solved by using the initial axial-tangential coordinate system (ε). xx ,ε yy ) to the deflection axis-tangential coordinate system (ε xx ',ε yy Transformation of '), such as Figure 8 As shown, the transformation matrix Q is:

[0046]

[0047] The axial strain ε at each equally spaced time t is obtained from the image acquisition system. xx 'and normal strain ε yy The actual value of ' is:

[0048]

[0049] When performing dynamic testing of the mechanical properties of composite compression-shear materials at arbitrary angles, the forces acting on both ends of the gauge length 11 of the test threaded specimen are calculated using the three-wave formula based on the signals collected by the strain gauges 13 on the incident rod 4 and transmission rod 5 of the Hopkinson pressure bar system. The expression is as follows:

[0050]

[0051] In the formula, ε i (t) and ε r (t) represents the incident and reflected strain pulses collected on the incident rod 4, respectively, ε t (t) is the transmitted strain pulse collected on the transmission rod 5, which is measured by the strain gauge 13 on the incident rod 4 and the transmission rod 5. A0 is the cross-sectional area of ​​the incident rod 4 / transmission rod 5, and E0 is the elastic modulus of the incident rod 4 / transmission rod 5.

[0052] The load is decomposed based on the 45° angle of the threaded hole 10 inside the fastener 9, such as... Figure 9 As shown, the normal force F of gauge length 11 of the test thread specimen is obtained. n and tangential force F s During the loading process, the gauge length 11 of the test thread specimen is in force equilibrium along the F0 direction, as shown in:

[0053]

[0054] The rotation angle α of the gauge segment 11 of the test thread specimen deviating from the coordinate axis is calculated based on the image acquisition system, such as... Figure 9 As shown, the normal force F at each equally spaced time t n 'and tangential force F s The actual value of ' is:

[0055]

[0056] Necking occurs during the plastic deformation of metallic materials, and the engineering stress-strain ratio contains errors in describing the true state of gauge length 11 of the test threaded specimen. The true stress is obtained by dividing the true strain obtained using DIC (Discrete Incompressible Coefficient) based on the assumption of plastic incompressibility of metallic materials and the load data from the Hopkinson bar system by the instantaneous cross-sectional area of ​​gauge length 11 of the test threaded specimen. This method accurately reflects the stress-strain relationship of the material. The expression for the instantaneous cross-sectional area of ​​gauge length 11 of the test threaded specimen is as follows:

[0057] A sample =A0·A factor

[0058] A factor =exp(ε yy ') / exp(ε xx '+ε yy')

[0059] In the formula, A0 and A factor These are the initial cross-sectional area and instantaneous cross-sectional area factor of the gauge length segment 11 of the test thread specimen, respectively;

[0060] The stress state of gauge segment 11 of the threaded specimen under combined compression and shear test, and the normal stress σ at the time of equal interval t. x (t) and shear stress τ xy The method for calculating the (t) component is as follows:

[0061]

[0062] The scope of protection claimed by this invention is not limited to the specific embodiments described above. Moreover, for those skilled in the art, this invention can have various modifications and alterations. Any modifications, improvements, and equivalent substitutions made within the concept and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A complex stress treatment method based on DIC technology, characterized in that: Based on a complex stress loading device; The complex stress loading device includes a Hopkinson pressure bar system and a pair of connectors used in conjunction with the Hopkinson pressure bar system. The two connectors are respectively connected to the incident rod and the transmission rod of the Hopkinson pressure bar system via screws and pads. The screws and pads are made of the same material as the incident rod and the transmission rod. A mounting part is provided on the connector with a lateral extension. The mounting part is provided with a rectangular receiving groove. The receiving groove is connected to the upper and lower end faces of the mounting part. Multiple locking grooves are connected around the receiving groove. Fasteners are placed in the receiving groove. Two fasteners are provided with matching threaded holes. Test thread specimen gauge sections are threadedly connected to the two threaded holes. The steps include the following: Step 1: Apply matte black and white paint evenly to the gauge length of the test thread specimen, ensuring the grayscale values ​​of the gauge length follow a standard normal distribution. Then, using two high-speed cameras based on binocular vision with DIC technology, establish a spatial coordinate system with the center of the gauge length as the origin. A Hopkinson bar system performs a dynamic compression-shear experiment on the gauge length. The DIC image acquisition system captures speckle images of the gauge length throughout the deformation process. By correlating pixel subsets or positional changes in the speckle images from the original and deformed images, and using different interpolation curves and shape functions, the displacement and strain changes of the gauge length are determined. The axial strain is then calculated using different strain measurement methods. and normal strain ; Step 2: Acquire speckle images of the gauge length section of the test thread specimen throughout the entire deformation process using an image acquisition system, and calculate the rotation angle of the gauge length section of the test thread specimen from the initial coordinate axis. The actual stress components are solved by using the initial axial-tangential coordinate system. From the mid-to-tangential coordinate system The transformation matrix is: The axial strain at each equidistant moment is obtained based on the image acquisition system. and normal strain The actual value is expressed as: (1)； The stress components of the gauge length of the test thread specimen are determined by using strain gauge signals collected from the incident and transmission rods of the Hopkinson bar system, and then calculating the forces at both ends of the gauge length of the test thread specimen using the three-wave formula. (2)； In the formula, and These are the incident and reflected strain pulses collected on the incident rod, respectively. The transmitted strain pulses are collected on the transmission rod. The pulse acquisition is obtained by measuring the strain gauges on the incident rod and the transmission rod. Let be the cross-sectional area of ​​the incident rod / transmitting rod. The elastic modulus of the incident / transmitting rod; The threaded hole angle is set according to the fastener. The load is decomposed to obtain the normal force of the gauge length section of the test thread specimen. and tangential force During the loading process, the gauge length of the test thread specimen along... Forces in equilibrium are expressed as follows: (3)； The rotation angle of the gauge segment of the test thread specimen from the initial coordinate axis, calculated by the image acquisition system, is: Each equally spaced moment normal force and tangential force The actual value is: (4)； Necking occurs during the plastic deformation of metallic materials, and the engineering stress-strain ratio contains errors in describing the true state of the gauge length of the test threaded specimen. To accurately reflect the stress-strain relationship, we use the true strain obtained through DIC (Discrete Incompressible Coefficient) based on the assumption of the incompressibility of the metallic material, combined with the load data from the Hopkinson bar system, divided by the instantaneous cross-sectional area of ​​the gauge length of the test threaded specimen. The expression for the instantaneous cross-sectional area of ​​the gauge length of the test threaded specimen is as follows: (5)； (6)； In the formula, and These are the initial cross-sectional area and instantaneous cross-sectional area factor of the gauge length section of the test thread specimen, respectively. Stress state and time interval of gauge length section of threaded specimen under combined compression and shear test Normal stress With shear stress The calculation method for the components is as follows: (7)； (8)。

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