A method for measuring residual stress of composite material curing based on wire deformation

Abstract

The present invention belongs to the field of residual stress measurement of composite materials, and provides a method for measuring residual stress of composite material curing based on metal wire deformation. First, lay the prepreg to the required thickness; draw a cross center line to mark the insertion position of the metal wire; punch holes in the laid prepreg with a steel needle, and insert the low-rigidity metal wire into the punched hole, and ensure that the metal wire is not bent and fits tightly with the hole wall during the insertion process; use CT to obtain the original geometric shape of the metal wire inserted in the uncured prepreg; then select a molding process to cure the prepreg; then use industrial CT again to obtain the geometric deformation data of the inserted metal wire after curing; finally, input the deformation data into the established simulation model to obtain the distribution law of the residual stress of the composite material along the thickness direction. The above method is used to obtain the distribution law of residual stress at different positions along the thickness direction, thereby establishing an initial residual stress distribution model for a multi-layer heterogeneous composite flat plate test piece.

Description

Technical Field

[0001] The invention belongs to the field of residual stress measurement of composite materials, and relates to a composite material curing residual stress measurement method based on metal wire deformation. Background Art

[0002] Composite materials will produce obvious residual stress during the molding and cooling process. The generation of residual stress will cause the plate to warp and deform. The accumulation of these defects and deformations has an adverse effect on processing and assembly, and is related to the molding quality of the parts. Therefore, in order to reduce the deformation caused by residual stress, it is necessary to measure the residual stress of composite panels. Currently, the more commonly used residual stress measurement methods are mainly divided into two categories: non-destructive testing and destructive testing. Non-destructive testing includes, for example, X-ray methods. However, this type of method is very expensive and requires a high level of equipment and personnel operation, which is not conducive to practical application. Destructive testing is very common in practical applications, mainly including drilling and cutting methods.

[0003] However, the traditional destructive measurement method requires the use of tools such as CNC machine tools to destroy the composite material after curing, so as to measure the residual stress distribution of the composite plate. This will inevitably produce cutting stress, resulting in inaccurate measured residual stress distribution of the plate.

[0004] In order to solve the above problems, the present invention proposes a method for measuring the residual stress of composite materials along the thickness direction based on industrial CT, low-rigidity metal wire, and finite element model. First, after the prepreg is laid, a small hole is drilled along the thickness direction of the workpiece with a steel needle and a low-rigidity metal wire is inserted to ensure that the metal wire is not bent and fits tightly with the hole wall. The test piece before curing is scanned by industrial CT to obtain the initial geometric shape of the metal wire. After that, after the test piece is prepared by the curing molding process, the test piece is scanned again by industrial CT to obtain the geometric shape of the metal wire after curing. Computer image processing technology is used to compare the geometric shape of the metal wire before and after curing to obtain the geometric deformation of the metal wire caused by the curing residual stress, and the finite element analysis method is used to obtain the distribution law of the residual stress along the thickness direction of the workpiece based on the geometric deformation of the metal wire. The above method is used to obtain the distribution law of residual stress along the thickness direction at different positions, thereby establishing an initial residual stress distribution model of a multi-layer heterogeneous composite flat plate test piece. Summary of the invention

[0005] In order to solve the above problems, the purpose of the present invention is to provide a method for accurately measuring and calculating the residual stress distribution of composite material plates after curing.

[0006] The technical solution of the present invention is:

[0007] A method for measuring residual stress of composite material curing based on wire deformation, the steps are as follows:

[0008] Step 1: Cutting and laying out the prepreg

[0009] Cut the prepreg according to the sample and lay the prepreg to the required thickness;

[0010] Step 2: Determine the wire insertion position in the prepreg

[0011] Divide the laid prepreg material into axial symmetry, arrange the metal wire insertion positions at equal intervals on the axial symmetry line, and select one of the divided parts to arrange the metal wire insertion positions at equal intervals in the horizontal direction; the insertion positions of two adjacent metal wires are 20 mm apart, and finally draw a cross center line at the insertion position as a mark;

[0012] Step 3: Punch holes in the laid prepreg with a steel needle

[0013] Wipe the surface of the fixture clean, clamp the laid prepreg with the fixture, and make sure the clamping is flat and firm. At the same time, pay attention to the clamping method of the workpiece to prevent the prepreg from being deformed due to clamping. Drill a hole with a steel needle along the thickness direction of the prepreg, and then insert the metal wire into the drilled hole.

[0014] Step 4: Use industrial CT to obtain the geometry of the metal wire inserted in the prepreg before curing

[0015] The uncured sheet is scanned using industrial CT to obtain the initial geometry of the metal wire inserted into the prepreg before curing;

[0016] Step 5: Curing the prepreg using the molding process

[0017] Select a molding process to solidify the scanned prepreg, causing the inserted wire to deform;

[0018] Step 6: Use industrial CT to obtain the geometry of the inserted wire after curing

[0019] The cured composite material is scanned again using industrial CT to obtain geometric deformation data of the metal wire inserted into the composite material after curing;

[0020] Step 7: Data Analysis

[0021] (1) The deformation data of the metal wire obtained by the two industrial CT scans before and after are processed and analyzed by computer image processing technology to obtain the geometric deformation of the metal wire before and after solidification, and to calculate the average deformation value of each layer of the metal wire;

[0022] (2) After data processing, the deformation value of the metal wire along the thickness direction of the composite material is obtained, which is expressed by the formula:

[0023] S(a i )=Si

[0024] Among them, a i is the depth of the i-th layer, i is the number of prepreg layers, i = 1, 2..., n, S(a i ) indicates the depth is a i The deformation value measured at i represents the deformation value of the i-th layer of prepreg;

[0025] (3) A composite material finite element model was established in the commercial finite element software ABAQUS. The geometric deformation obtained by industrial CT scanning was input into the finite element model as the load. The stress value of each layer was output in the stress cloud map, which was the residual stress of the layer, so as to obtain the distribution law of the residual stress along the depth direction.

[0026] (4) The above method is used to obtain the distribution law of residual stress at different positions along the thickness direction, thereby establishing an initial residual stress distribution model for multilayer heterogeneous composite flat plate specimens.

[0027] Beneficial effects of the present invention:

[0028] 1. After the curing is completed, CNC machine tools are no longer used for drilling. While reducing the workload, it can also greatly reduce the influence of cutting stress on the residual stress distribution of composite materials during the measurement process, making the final residual stress distribution closer to the initial state of the residual stress, improving the accuracy of the measurement, and having obvious engineering application significance.

[0029] 2. The present invention can more accurately measure the original residual stress distribution and improve the accuracy of measurement by reasonably dividing the area and reasonably determining the insertion position of the metal wire. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a flow chart of residual stress measurement and calculation of composite material plate of the present invention.

[0031] Figure 2 It is a schematic diagram of dividing a composite material plate into several parts according to the symmetry of the composite material plate and the positions where the metal wires are inserted.

[0032] Figure 3a Schematic diagram of low stiffness metal wire inserted into uncured prepreg.

[0033] Figure 3b Schematic diagram of low stiffness metal wire inserted into the composite material after curing. DETAILED DESCRIPTION

[0034] The specific implementation of the present invention is described in detail below in conjunction with the technical scheme and the accompanying drawings.

[0035] Figure 1 It is a flow chart of residual stress measurement of composite material plate of the present invention.

[0036] (1) Cut the prepreg with protective film according to the sample. Pay attention to the fiber direction when cutting. After cutting, use a vernier caliper to measure the cross-sectional dimensions of the sample. If it is qualified, remove the protective film from the cut prepreg.

[0037] (2) Use acetone to clean the surface of the prepreg, wipe off the impurities on the surface, and lay the prepreg to the required thickness.

[0038] (3) Figure 2 As shown in the figure, determine the position for inserting the low-rigidity metal wire into the prepreg, use a high-precision ruler to draw a cross center line to mark the insertion position, and the line must be clear and accurate.

[0039] (4) Wipe the surface of the fixture clean and use the fixture to clamp the prepreg after the layering is completed. The clamping should be flat and secure.

[0040] (5) Drill holes along the thickness direction of the prepreg with the prepared steel needle, and finally insert the low-rigidity metal wire into the drilled hole. Note that the prepreg after the hole is drilled should be firmly taped with transparent tape to prevent the prepreg from moving relative to each other after the metal wire is inserted, causing deformation of the metal wire. Finally, cut off the excess metal wire to avoid accidental contact with the metal wire during movement, causing deformation. The final schematic diagram is as follows Figure 3a shown.

[0041] (6) Use industrial CT scanning to obtain the original geometric state of the inserted metal, and pay attention to radiation safety protection and monitoring during the operation.

[0042] (7) After measuring the initial state of the low-rigidity metal wire, the prepreg is cured using a curing molding process.

[0043] (8) After curing, use sandpaper to simply trim the burrs on the edges of the composite material to meet the design requirements.

[0044] (9) Industrial CT scanning is used to obtain the geometric deformation state of the low-stiffness metal wire after curing.

[0045] (10) Computer image processing technology is used to compare the geometric shapes of the metal wire before and after curing to obtain the geometric deformation of the metal wire caused by the curing residual stress, and the average deformation of each layer of metal wire is calculated.

[0046] (11) The finite element model of the composite material was established using the commercial finite element software ABAQUS. The elastic modulus and density were input, the displacement value obtained was input into the load, and finally the stress value of the layer was obtained in the stress cloud diagram. The stress value of each layer was obtained in turn to obtain the stress distribution along the thickness direction.

[0047] (12) The above method is used to obtain the distribution law of residual stress at different positions along the thickness direction, thereby establishing an initial residual stress distribution model for multilayer heterogeneous composite flat plate specimens.

Claims

1. A method for measuring residual stress of composite material curing based on metal wire deformation, characterized in that: Here are the steps: Step 1: Cut and lay out the prepreg; Cut the prepreg according to the sample and lay the prepreg to the required thickness; Step 2: Determine the wire insertion position in the prepreg; Divide the laid prepreg material into axial symmetry, arrange the metal wire insertion positions at equal intervals on the axial symmetry line, and select one of the divided parts to arrange the metal wire insertion positions at equal intervals in the horizontal direction; the insertion positions of two adjacent metal wires are 20 mm apart, and finally draw a cross center line at the insertion position as a mark; Step 3: Punch holes in the laid prepreg with a steel needle; Wipe the surface of the fixture clean, clamp the laid prepreg with the fixture, and make sure the clamping is flat and firm. At the same time, pay attention to the clamping method of the workpiece to prevent the prepreg from being deformed due to clamping. Drill a hole with a steel needle along the thickness direction of the prepreg, and then insert the metal wire into the drilled hole. Step 4: Use industrial CT to obtain the geometry of the metal wire inserted in the prepreg before curing; The uncured sheet is scanned using industrial CT to obtain the initial geometry of the metal wire inserted into the prepreg before curing; Step 5: Curing the prepreg using a molding process; Select a molding process to solidify the scanned prepreg, causing the inserted wire to deform; Step 6: Use industrial CT to obtain the geometry of the inserted wire after curing; The cured composite material is scanned again using industrial CT to obtain geometric deformation data of the metal wire inserted into the composite material after curing; Step 7: Data analysis; (1) The deformation data of the metal wire obtained by the two industrial CT scans before and after are processed and analyzed by computer image processing technology to obtain the geometric deformation of the metal wire before and after solidification, and to calculate the average deformation value of each layer of the metal wire; (2) After data processing, the deformation value of the metal wire along the thickness direction of the composite material is obtained, which is expressed by the formula: S(a i )=S i Among them, a i is the depth of the i-th layer, i is the number of prepreg layers, i = 1, 2..., n, S(a i ) indicates the depth is a i The deformation value measured at i represents the deformation value of the i-th layer of prepreg; (3) Establish a finite element model of the composite material, input the geometric deformation obtained by industrial CT scanning into the finite element model as the load, output the stress value of each layer in the stress cloud map, that is, the residual stress of the layer, and thus obtain the distribution law of the residual stress along the depth direction; (4) The above method is used to obtain the distribution law of residual stress at different positions along the thickness direction, thereby establishing an initial residual stress distribution model for multilayer heterogeneous composite flat plate specimens.

2. The method for measuring residual stress of composite material curing based on wire deformation according to claim 1, characterized in that: Make sure the metal wire fits tightly against the hole wall.

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