Composite material plate strain measurement device and method based on DIC
By designing a strain measurement device for composite material plates based on DIC, and combining DIC and strain gauge measurement methods, the instability problem of bending strain measurement of carbon fiber composite material plates was solved, and high-precision strain data acquisition was achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-02-02
- Publication Date
- 2026-04-03
AI Technical Summary
Existing technologies are difficult to accurately measure the bending strain of carbon fiber composite plates, especially the strain in the non-principal axis direction of anisotropic materials. Furthermore, the DIC strain measurement method is unstable, cannot observe the deformation of the bending surface in real time, and the strain gauge measurement results are greatly affected by the environment.
A strain measurement device for composite material plates based on DIC was designed, including an inflatable frame, a pressure sensor, a composite material plate, a strain rosette, and a camera. Combining DIC and strain gauge measurement methods, data is acquired through the strain rosette and camera, and the measurement accuracy is improved by using a correction formula.
It enables simultaneous adaptation of DIC and strain gauge measurement methods under the same working conditions, corrects DIC measurement errors, and obtains higher-precision full-field strain data of composite materials.
Smart Images
Figure CN121783679A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of material physics simulation test methods, specifically relating to a strain measurement device and method for composite material plates based on DIC. Background Technology
[0002] In the military and aerospace fields, carbon fiber composite materials occupy an important position in the lightweighting research of rocket launchers, missile launchers, aircraft, etc. due to their advantages such as high specific strength, light weight, high specific modulus, and corrosion resistance.
[0003] The physical properties of these carbon fiber composite panels are often affected by factors such as fiber thickness, layup direction, resin content, and construction process during manufacturing. The actual physical properties of the finished panels vary and need to be confirmed through testing.
[0004] The bending strain of sheet metal is crucial experimental data for establishing accurate material constitutive models. Therefore, accurately obtaining the bending strain of sheet metal is essential for establishing accurate material constitutive models, improving the accuracy of finite element simulations, and ultimately obtaining the most reasonable structural model.
[0005] Structural deformation is an important indicator that directly reflects the stress state of a structure and is a necessary parameter for structural analysis. Common strain measurement methods include strain gauge measurement and DIC (digital image correlation) strain measurement.
[0006] Currently, the national standard GB / T228.1-2021 for tensile testing of metallic materials only specifies methods for obtaining tensile strain in sheet metal. Carbon fiber composites are anisotropic materials, and tensile tests cannot measure the flexural strength of anisotropic materials. Strain gauge measurements are significantly affected by environmental factors such as temperature, and it is difficult to measure strain in non-principal axis directions; therefore, the DIC strain measurement method is often used.
[0007] In the testing of physical properties of materials, the three-point bending test method is often used to test the mechanical properties of materials that are difficult to tensile test. However, when measuring the strain of the material under test, due to the large volume of the test platform and the fixed orientation of the material, this method is only suitable for strain gauge measurement and cannot be adapted to DIC strain measurement. It also cannot observe the deformation of the bending surface in real time.
[0008] The DIC strain measurement method is a relatively new method developed in recent years. However, the measurement method is not mature and the measurement results are unstable. Therefore, it is necessary to verify and correct the results by using strain gauges. Summary of the Invention
[0009] The purpose of this invention is to provide a strain measurement device and method for composite material plates based on DIC.
[0010] The technical solution to achieve the purpose of this invention is: a composite material plate strain measurement device based on DIC, including an inflatable frame, a pressure sensor, a first composite material plate, a second composite material plate, an air compressor, three triaxial strain rosettes and two cameras.
[0011] The inflatable frame is rectangular in shape, with a first composite material plate and a second composite material plate detachably and connectedly installed on two opposite sides;
[0012] The other two sides of the inflatable frame are the air inlet and the air outlet, respectively. The top surface of the inflatable frame is connected to a pressure sensor, and the bottom surface of the inflatable frame is fixed to the workbench. The air inlet of the inflatable frame is connected to an air compressor.
[0013] Two cameras are mounted on the side of the first composite material plate, and three strain gauges are attached to the second composite material plate. One strain gauge is located at the center of the composite material plate, the second strain gauge is located directly above the first strain gauge, and the third strain gauge is located on both sides of the second strain gauge. The distance between the first strain gauge and the second strain gauge, as well as between the second strain gauge and the third strain gauge, is one-quarter of the side length of the inflatable frame. The three strain gauges are arranged in an isosceles right triangle.
[0014] Furthermore, the inflatable frame has flange mounting surfaces on both sides of the composite material plate. The flange mounting surfaces have annular sealing grooves for placing rubber rings. The fixing rings and mounting surfaces are detachably connected by screws and nuts, and the composite material plate is fixed by the fixing rings.
[0015] Furthermore, the retaining ring is made of 6061 aluminum alloy.
[0016] Furthermore, it also includes an intake shut-off valve, an exhaust shut-off valve, an intake hose, and an exhaust hose;
[0017] The inflatable frame is connected to the outlet end of the inflatable shut-off valve via threads, and the inflatable frame is also connected to the inlet end of the outlet shut-off valve via threads. The inlet end of the inflatable shut-off valve is connected to the air compressor via an inflatable hose, and the outlet end of the outlet shut-off valve is connected to an exhaust hose.
[0018] Furthermore, the inflatable frame is fixed to the steel flat workbench by a bottom magnetic strip, and the top is connected to the pressure sensor by a thread.
[0019] A method for measuring the strain of a composite material plate using the above-described apparatus includes the following steps:
[0020] Step (1): Spray speckle evenly on the outer surface of the composite material plate near the camera; attach three strain gauges to the outer surface of another composite material plate, with the first gauge at the center of the composite material plate, the second gauge directly above the first gauge, and the third gauge directly to the right of the second gauge, and the distance between two adjacent strain gauges being one-quarter of the side length of the frame; connect the strain gauges to the data acquisition device, and measure the strain of the plate under test using the strain gauge measurement method during the inflation process;
[0021] Step (2): Fix the inflatable frame to the steel plane using the bottom magnetic strip, and adjust the camera bracket and 3D gimbal so that the optical axes of the two cameras are aligned with the center of the composite material plate.
[0022] Step (3): Close the outlet shut-off valve, start the air compressor, adjust the inlet shut-off valve, and gradually increase the air pressure in the inflation frame to the specified requirements to obtain the strain cloud diagram measured by the DIC method and the strain curve measured by the strain gauge method. Correct the strain data measured by DIC according to the strain curve.
[0023] Step (4): Turn off the air compressor, open the inlet shut-off valve and the outlet shut-off valve, and discharge the internal compressed air through the exhaust hose to end the experiment.
[0024] Furthermore, in step (2), three to five turns of PTFE tape are tightly wrapped around the external threads of the inlet shut-off valve, outlet shut-off valve and pressure sensor in a clockwise direction.
[0025] Furthermore, in step (2), two cameras are placed in front of a composite material plate, so that the optical axis of the two camera lenses is at the same height as the center of the corresponding side of the composite material plate and is horizontal. The two lenses are aligned with the center of the composite material plate and are at the same distance from the center of the composite material plate. The angle between the two lenses is 15°.
[0026] Further, in step (2), the camera aperture, focal length, and distance from the center of the composite material plate are adjusted to make the image clear and the speckle size 6 to 10 pixels.
[0027] Furthermore, the corrective formula used in step (3) is as follows:
[0028] ,
[0029] in, For strain in any direction measured at any DIC location, For the strain after correction, Precision coefficient, ,in To correspond to the strain measured by DIC at the strain gauge points, The strain was measured for three strain gauges.
[0030] Compared with the prior art, the significant advantages of this invention are:
[0031] The strain measurement method and apparatus for composite material plates based on DIC described in this invention are easy to carry and transport. They can be adapted to both DIC measurement method and strain gauge measurement method under the same working conditions to achieve better comparison results. They can also verify and correct DIC strain measurement errors to obtain higher precision full-field strain data of composite materials. Attached Figure Description
[0032] Figure 1 Schematic diagram of a strain measurement device for composite material plates based on DIC.
[0033] Figure 2 Schematic diagram of the inflatable frame structure.
[0034] Figure 3 Schematic diagram of strain gauge mounting positions.
[0035] Figure 4 Diagram showing camera placement.
[0036] Figure 5 A schematic diagram of the experimental process of a DIC-based composite material plate strain measurement device.
[0037] Explanation of reference numerals in the attached figures:
[0038] 10-Inflatable frame, 20-Pressure sensor, 30-Inlet shut-off valve, 31-Outlet shut-off valve, 40-First rubber ring, 41-Second rubber ring, 50-First fixing ring, 51-Second fixing ring, 60-First composite material plate, 61-Second composite material plate, 70-Inlet hose, 71-Exhaust hose, 80-Air compressor. Detailed Implementation
[0039] The present invention will now be described in further detail with reference to the accompanying drawings.
[0040] A strain measurement device for composite material plates based on DIC includes an inflatable frame 10, a pressure sensor 20, an inlet shut-off valve 30, an outlet shut-off valve 31, a first rubber ring 40, a second rubber ring 41, a first fixing ring 50, a second fixing ring 51, a first composite material plate 60, a second composite material plate 61, an inlet hose 70, an outlet hose 71, and an air compressor 80, etc. Figure 1 As shown.
[0041] The structure of the inflatable frame 10 is as follows Figure 2The first rubber ring 40 is fixed to the steel surface by a bottom magnetic strip, and is connected to the pressure sensor 20 by threads on the top. The left side is connected to the outlet end of the intake shut-off valve 30 by threads, and the right side is connected to the inlet end of the exhaust shut-off valve 31 by threads. The first rubber ring 40 is placed in the front groove of the inflation frame 10, and the first fixing ring 50 presses the first composite material plate 60 against the front of the inflation frame 10 and is fixed to the inflation frame 10 by screws and nuts. The second rubber ring 41 is placed in the rear groove of the inflation frame 10, and the second fixing ring 51 presses the second composite material plate 61 against the rear of the inflation frame 10 and is fixed to the inflation frame 10 by screws and nuts. The inlet end of the intake shut-off valve 30 is connected to the air compressor 80 via an intake hose 70, and the outlet end of the exhaust shut-off valve 31 is connected to the exhaust hose 71.
[0042] The inflatable frame 10 is a square box that runs from front to back. The front and rear sides of the frame have mounting surfaces, which are rounded rectangular flanges with annular grooves. Eight circular through holes are evenly distributed around the perimeter of the annular grooves, and a magnetic strip is located at the bottom of the flange. The left and right sides of the frame are hollow circular tubes with internal threads at the tube outlets. Circular threaded holes are located on the top of the frame. The frame is made of 304 stainless steel, and the magnetic material at the bottom is neodymium iron boron.
[0043] The first fixing ring 50 and the second fixing ring 51 are flange rings, with eight circular through holes evenly distributed on the rings, and the material is 6061 aluminum alloy.
[0044] The first rubber ring 40 is placed in the annular groove on the front mounting surface of the inflatable frame 10. The first fixing ring 50 presses the first composite material plate 60 against the front of the inflatable frame 10 and is fixed to the inflatable frame 10 with screws and nuts. The second rubber ring 41 is placed in the annular groove on the rear mounting surface of the inflatable frame 10. The second fixing ring 51 presses the carbon fiber composite material plate 61 against the rear of the inflatable frame 10 and is fixed to the inflatable frame 10 with screws and nuts.
[0045] The inflatable frame 10 is fixed to the steel surface by a bottom magnetic strip. It is connected to the pressure sensor 20 by threads on the top, to the outlet end of the inlet shut-off valve 30 by threads on the left, and to the inlet end of the outlet shut-off valve 31 by threads on the right. The inlet end of the inlet shut-off valve 30 is connected to the air compressor 80 through an inlet hose 70, and the outlet end of the outlet shut-off valve 31 is connected to the exhaust hose 71.
[0046] A method for measuring strain in DIC-based composite material plates, using the aforementioned measuring device for quasi-static bending strain of plates, includes the following steps:
[0047] Before the test, the outer surface of the first composite material plate 60 was uniformly sprayed with speckle. During the inflation process, the strain of the plate under test was measured using three-dimensional full-field DIC.
[0048] Before the test, three strain gauges were attached to the outer surface of the second composite material plate 61. The first strain gauge was positioned at the center of the composite material plate, the second directly above the first strain gauge, and the third directly to the right of the second strain gauge. The distance between any two adjacent strain gauges was one-quarter of the side length of the frame. The strain gauges were then connected to a data acquisition device, and the strain of the plate under test was measured using the strain gauge method during the inflation process.
[0049] To prevent air leakage, tightly wrap 3 to 5 turns of PTFE tape clockwise around the external threads of the inlet shut-off valve 30, outlet shut-off valve 31, and pressure sensor 20.
[0050] The inflatable frame 10 is fixed to the steel plane by the bottom magnetic strip. The camera bracket and 3D gimbal are adjusted so that the optical axes of the two cameras are aligned with the center of the carbon fiber composite plate 90.
[0051] Connect the data acquisition device and its network cable end to the same router as the camera's network cable end, and then connect the network cable to the computer.
[0052] At the start of the experiment, the outlet shut-off valve 31 is closed, the air compressor 80 is started, and the inlet shut-off valve 30 is adjusted to gradually increase the air pressure in the inflation frame 10 to the specified requirements. The strain cloud diagram measured by the DIC method and the strain curve measured by the strain gauge method are obtained. The accuracy of the strain data measured by DIC is compared and corrected.
[0053] Finally, turn off the air compressor 80, open the intake shut-off valve 30 and the exhaust shut-off valve 31, and discharge the internal compressed air through the exhaust hose 71 to end the experiment.
[0054] A camera is positioned in front of the first composite material plate 60, with the optical axis of the two camera lenses at the same height and horizontal to the center of the first composite material plate 60. The two lenses are aligned with the center of the first composite material plate 60 and at the same distance from the center of the first composite material plate 60, with an included angle of 15°.
[0055] Adjust the camera's aperture, focal length, and distance from the center of the first composite plate 60 to make the image clear with a speckle size of 6 to 10 pixels.
[0056] The formula for calculating accuracy is: ,in To measure strain at the corresponding strain gauge points using DIC, The strain is measured using three strain gauges. The correction formula is: ,in For strain in any direction measured at any DIC location, This refers to the strain after correction.
[0057] The experimental steps involved in this invention are as follows:
[0058] Before the test, the outer surface of the first composite material plate 60 was uniformly sprayed with speckle. During the inflation process, the strain of the plate under test was measured using three-dimensional full-field DIC.
[0059] Before the test, strain gauges were attached to designated positions on the outer surface of the second composite material plate 61 and connected to a data acquisition device. During the inflation process, the strain of the plate under test was measured using the strain gauge measurement method.
[0060] To prevent air leakage, tightly wrap 3 to 5 turns of PTFE tape clockwise around the external threads of the shut-off valves 30 and 31 and the pressure sensor 20.
[0061] The first rubber ring 40 is placed in the annular groove on the front mounting surface of the inflatable frame 10. The first fixing ring 50 presses the first composite material plate 60 against the front of the inflatable frame 10 and is fixed to the inflatable frame 10 with screws and nuts. The second rubber ring 41 is placed in the annular groove on the rear mounting surface of the inflatable frame 10. The second fixing ring 51 presses the carbon fiber composite material plate 61 against the rear of the inflatable frame 10 and is fixed to the inflatable frame 10 with screws and nuts.
[0062] Place the rubber ring 41 into the annular groove at the rear of the inflatable frame 10 and press it down with the carbon fiber composite plate 61. Press the fixing ring 51 onto the carbon fiber composite plate 61 at the outermost edge. Use 8 screws and nuts to press the fixing ring 51, rubber ring 41 and carbon fiber composite plate 61 onto the inflatable frame 10.
[0063] The inflatable frame 10 is fixed to the steel plane by a bottom magnetic strip. The left side is connected to the outlet end of the air inlet shut-off valve 30 by a thread, and the right side is connected to the inlet end of the air outlet shut-off valve 31 by a thread.
[0064] The inlet end of the intake shut-off valve 30 is connected to the air compressor 80 via the intake hose 70, and the outlet end of the exhaust shut-off valve 31 is connected to the exhaust hose 71.
[0065] The pressure sensor 20 is connected to the inflatable frame 10 by a thread, and the other end is connected to the data acquisition device.
[0066] Adjust the camera mount and 3D gimbal so that the optical axes of the two cameras are aligned with the center of the first composite material plate 60.
[0067] Connect the data acquisition device and its network cable end to the same router as the camera's network cable end, and then connect the network cable to the computer.
[0068] At the start of the experiment, the air shut-off valve 31 is closed, the air compressor 80 is started, and the air inlet shut-off valve 30 is adjusted to gradually increase the air pressure in the inflation frame 10 to the specified requirements. The strain cloud diagram measured by the DIC method and the strain curve measured by the strain gauge method are obtained. The accuracy of the strain data measured by DIC is compared and corrected.
[0069] Finally, turn off the air compressor 80, open the intake shut-off valve 30 and the exhaust shut-off valve 31, and discharge the internal compressed air through the exhaust hose 71 to end the experiment.
Claims
1. A strain measurement device for composite material plates based on DIC, characterized in that, It includes an inflatable frame (10), a pressure sensor (20), a first composite material plate (60), a second composite material plate (61), an air compressor (80), three triaxial strain gauges, and two cameras; The inflatable frame (10) is rectangular in shape, and the first composite material plate (60) and the second composite material plate (61) are detachably and connectedly installed on the two opposite sides. The other side of the inflatable frame (10) is the air inlet and the air outlet respectively. The top surface of the inflatable frame (10) is connected to the pressure sensor (20), and the bottom surface of the inflatable frame (10) is fixed to the workbench. The air inlet of the inflatable frame (10) is connected to the air compressor (80). Two cameras are mounted on the side of the first composite material plate (60), and three strain flowers are pasted on the second composite material plate (61). One strain flower is located at the center of the composite material plate, the second strain flower is located directly above the first strain flower, and the third strain flower is located on both sides of the second strain flower. The distance between the first strain flower and the second strain flower, as well as between the second strain flower and the third strain flower, is one-quarter of the side length of the inflatable frame. The three strain flowers are arranged in an isosceles right triangle.
2. The apparatus according to claim 1, characterized in that, The inflatable frame (10) has flange mounting surfaces on both sides of the composite material plate. The flange mounting surfaces have annular sealing grooves for placing rubber rings. The fixing ring and the mounting surface are detachably connected by screws and nuts, and the composite material plate is fixed by the fixing ring.
3. The apparatus according to claim 2, characterized in that, The retaining ring is made of 6061 aluminum alloy.
4. The apparatus according to claim 1, characterized in that, It also includes an intake shut-off valve (30), an exhaust shut-off valve (31), an intake hose (70), and an exhaust hose (71). The inflatable frame (10) is connected to the outlet end of the inlet shut-off valve (30) by a thread, and the inflatable frame (10) is connected to the inlet end of the outlet shut-off valve (31) by a thread; the inlet end of the inlet shut-off valve (30) is connected to the air compressor (80) through the inlet hose (70), and the outlet end of the outlet shut-off valve (31) is connected to the exhaust hose (71).
5. The apparatus according to claim 1, characterized in that, The inflatable frame (10) is fixed to the steel flat workbench by the bottom magnetic strip and the top is connected to the pressure sensor (20) by the thread.
6. A method for measuring the strain of a composite material plate using the apparatus described in claims 1-5, characterized in that, Includes the following steps: Step (1): Spray speckle evenly on the outer surface of the composite material plate near the camera; attach three strain gauges to the outer surface of another composite material plate, with the first gauge at the center of the composite material plate, the second gauge directly above the first gauge, and the third gauge directly to the right of the second gauge, and the distance between two adjacent strain gauges being one-quarter of the side length of the frame; connect the strain gauges to the data acquisition device, and measure the strain of the plate under test using the strain gauge measurement method during the inflation process; Step (2): Fix the inflatable frame to the steel plane using the bottom magnetic strip, and adjust the camera bracket and 3D gimbal so that the optical axes of the two cameras are aligned with the center of the composite material plate. Step (3): Close the outlet shut-off valve, start the air compressor, adjust the inlet shut-off valve, and gradually increase the air pressure in the inflation frame to the specified requirements to obtain the strain cloud diagram measured by the DIC method and the strain curve measured by the strain gauge method. Correct the strain data measured by DIC according to the strain curve. Step (4): Turn off the air compressor, open the inlet shut-off valve and the outlet shut-off valve, and discharge the internal compressed air through the exhaust hose to end the experiment.
7. The method according to claim 6, characterized in that, In step (2), wrap three to five turns of PTFE tape tightly around the external threads of the inlet shut-off valve, outlet shut-off valve and pressure sensor in a clockwise direction.
8. The method according to claim 7, characterized in that, In step (2), two cameras are placed in front of a composite material plate, so that the optical axis of the two camera lenses is at the same height as the center of the corresponding side of the composite material plate and is horizontal. The two lenses are aligned with the center of the composite material plate and are at the same distance from the center of the composite material plate. The angle between the two lenses is 15°.
9. The method according to claim 8, characterized in that, Step (2) Adjust the camera's aperture, focal length, and distance from the center of the composite material plate to make the image clear and the speckle size 6 to 10 pixels.
10. The method according to claim 6, characterized in that, The corrective formula for step (3) is as follows: ， in, For strain in any direction measured at any DIC location, For the strain after correction, Precision coefficient, ,in To correspond to the strain measured by DIC at the strain gauge points, The strain was measured for three strain gauges.