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A Method for Determination of Type II Cracking Threshold and S-n Curve of Composite Materials
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A composite material and measurement method technology, applied in the field of fatigue fracture test of composite materials, can solve problems such as difficult to obtain interlayer delamination expansion rate of composite materials
Active Publication Date: 2016-06-29
CHINA HELICOPTER RES & DEV INST
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[0016] The invention provides a method for measuring the type II cracking threshold value of composite materials. For the delamination defects of composite materials, the method overcomes the complicated operation of the current G-dropping method and the difficulty in obtaining the performance parameter of the expansion rate of interlayer delamination of composite materials. Technical Difficulties
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[0106] Step 1: 1.1 Randomly select 30 samples and number them;
[0107] 1.2 Measurement of sample geometric dimensions. Use a vernier caliper to measure and read the thickness values at the four corners of the sample, and take the average thickness at the four corners as the nominal thickness of the sample. In this example, the dimensional accuracy of the length and width of the sample is guaranteed during processing, so there is no need to measure again during the test;
[0108] 1.3 Sample surface treatment. The skin sample is black and opaque, and it is difficult to observe the cracks directly, so it is necessary to treat the crack observation surface. The treatment method is as follows: first, use diamond abrasive paste and fine sandpaper to grind the surface of both sides of the skin sample to improve the smoothness of the crack observation surface; secondly, the g...
[0114] Step 1: 1.1 Randomly select 30 pieces of fiberglass girder belt samples and number them;
[0115] 1.2 Measurement of sample geometric dimensions. Use a vernier caliper to measure and read the thickness values at the four corners of the sample, and take the average thickness at the four corners as the nominal thickness of the sample. In this example, the dimensional accuracy of the length and width of the sample is guaranteed during processing, so there is no need to measure again during the test;
[0116] 1.3 Sample surface treatment. The material of the girder belt sample is translucent, and the cracks can be observed directly under the microscope without surface treatment as in Example 1;
[0117] 1.4 The sample crack is pre-cracked. Carry out artificial type II pre-cracking on the sample, the specific method is: firstly, draw a line 45mm from the pre-cr...
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Abstract
The invention relates to a determination method of composite material fatigue performance, and particularly relates to determination of a composite material II-type cracking threshold value. The method adopts an end-notched flexure (ENF) sample under constant-load and increasing-K control, and is carried out at several stress levels. In the test process, the racking length a, the sample deflection value [delta]max, and the corresponding load cycle number N are recorded at regular intervals. An a-N curve of the sample is obtained by double-growth exponential function fitting, and da / dN-[delta]GII scatter diagrams are further obtained; da / dN-[delta]GII scatter diagrams at different stress levels are superposed and fitted to obtain a full-range crack growth da / dN-[delta]GII curve, so as to obtain the cracking threshold value. Since the maximum stress between layers are constant at the same stress level group, the test can also be used for determining a fatigue S-N curve. When compared with G-decreasing methods under load control, the determination method is simple in operation, and obtains more information on material fatigue performance.
Description
technical field [0001] The invention belongs to the composite material fatigue fracture test technology, in particular to a composite material type II cracking threshold value and S-N curve determination method Background technique [0002] Compared with traditional metal materials, composite materials have higher specific strength and specific stiffness, so they are more and more widely used in structural lightweight design. However, the outstanding problem of composite material structure is the occurrence of interlaminar cracking under the action of fatigue load. One of the means to avoid the occurrence of fatigue cracking between layers of composite materials is to control the fatigue load below the material cracking threshold, which requires the experimental determination of the cracking threshold in advance. [0003] The currently known method for determining the threshold value of fatigue cracking between layers of composite materials generally adopts the method of fa...
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