Crack length measuring and calculating method based on curvature correction principle

Abstract

The invention relates to a crack length measuring and calculating method based on the curvature correction principle. The method comprises the following steps: (1) prefabricating a crack on a notched sample, and coating the tip position of the crack with a coloring agent prepared by mixing anhydrous alcohol and a dye penetrant from the side surface of the sample; (2) carrying out fatigue loading on the sample for several times to make the cracks circularly opened on the premise of not generating fatigue propagation, and completing marking of the prefabricated initial cracks after the coloring agent is dried; (3) continuing to perform the fatigue crack growth rate test, and after the test is finished, measuring the marked initial crack length and final crack length under an optical microscope; and (4) performing curvature correction on the crack length measured in real time by the flexibility method according to an equal proportion method. The deviation between the crack length measured by the flexibility method at any moment and the actual crack length in the test process can be reduced, so that the accuracy of the test result of the fatigue crack propagation rate test is improved, and the fatigue crack propagation resistance of the material is reflected more truly.

Description

technical field [0001] The invention belongs to the technical field of detection of fracture mechanical properties of metal materials, and in particular relates to a method for measuring and calculating crack length based on the principle of curvature correction. Background technique [0002] Metal materials are mostly subjected to alternating loads in the actual service process, and after a certain number of cycles, it is easy to experience undetectable and dangerous fatigue damage. The total life of the loaded part is composed of two stages: crack initiation and crack propagation. The premise of the modern fatigue design method is: engineering components originally have cracks (such as hypothetical cracks such as material defects, inclusions, welding defects, etc.), and the service life That is, the time required for a hypothetical crack to grow to a certain critical size. Therefore, it is of great scientific significance to predict and evaluate the service life of materi...

AI Technical Summary

Problems solved by technology

[0011] According to the current test standard (ASTM E647 or GB / T 6398), the fatigue crack growth rate test has the following three problems: 1. Before the formal test, it is necessary to prefabricate the fatigue crack of the notched sample, so that the stress state of the crack tip before the test Not affected by the notch of the sample, because the prefabricated crack and the fatigue crack are connected together, the initial crack length cannot be identified and measured on the crack propagation surface by conventional means (such as optical microscope, electron microscope)

2. During the test, the real crack length at any moment cannot be accurately measured

3. The national standard GB / T 6398-2017 and the American standard ASTME647-15 stipulate that "if the change of the stress intensity factor K caused by the correction of the crack curvature exceeds 5%, the curvature correction should be performed during the analysis of the test data", but it is not pointed out that the calculation The specific method of correcting the crack length

After reviewing relevant test standards, patents, and articles, it was found that none of the existing materials mentioned the marking identification method of fatigue cracks

And only the article "Realization of Crack Length Curvature Correction after Fracture Mechanics Test" mentions the algorithm of interpolating the deviation of crack length to any cycle for length correction according to the average cycle times, but in the actual test, due to the difference between the crack length and The linear relationship between cycles is poor, and the correction results of this method are not consistent with the actual situation to a certain extent

Images