The Study On The Growth Behavior Of 3D Short Fatigue Crack Under Complex Stress State

With the continuous development and advancement of the modern industrial technology,the metal materials are also widely used in various fields.Therefore,it has become one of the important research problems to explore the properties of metal materials and analyze the fatigue life of metal materials.With more than 100 years of study,scholars have made effort to the analysis of fatigue life,and made significant progress.Especially in recent decades,with the development of industrial CT scanning technology and computer technology,the analysis of the fatigue life problem is no longer limited to the two-dimensional plane,the research focus gradually transfered from the two-dimensional plane to the three-dimensional space,which provides a new way of thinking and methods for fatigue crack analysis.In this paper,the three-dimensional growth characteristics of fatigue crack of 2A12 aluminum alloy were studied by combining the low-cycle fatigue test with surface crack observation technique,CT scanning technique,image preprocessing technique and 3D reconstruction technology with 2A12 aluminum alloy as experimental material.Firstly,the low-cycle fatigue test of the aluminum alloy specimen pcoceed at room temperature.And different experimental strain-amplitude(0.30%,0.40%,0.50%)with different notch radii(R=3,R=4,R=5)were used in the experiment.The effects of different strain amplitude and notch radius on the fatigue life of aluminum alloy were investigated.The experimental results show that the fatigue life of aluminum alloy is significantly affected by the strain-amplitude,and the fatigue crack always occurs in the impurity phase(FeMn)Al6,the material defect and the grain boundary.In the earlier stage of fatigue life,the phenomenon of crack initiation occurs,and the number of short cracks increases gradually.In the late stage of crack propagation,with the occurrence of crack fit and interference,the single main crack plays a main role in the fatigue crack propagation of the material,and finally destroys the material.Secondly,based on the low-cycle fatigue test at room temperature,the CT scanning of the experimental specimens in different fatigue life stages was carried out in the experimental process.With the processing of contrast enhancement,denoising and binarization,the quality of the image will be improved.Among them,the contrast enhancement is used to enhance the visual effect of the image;filter denoising is used to improve the image quality;binarization is used to segment the target and the background,and ignore the details.Finally,the three-dimensional reconstruction of the fatigue crack is carried out by MATLAB program.According to the reconstructed three-dimensional fatigue crack model,the three-dimensional dimension of the reconstructed model is calculated.The model size is consistent with the actual crack size and the three-dimensional reconstruction effect is good.Finally,in order to facilitate the management of image preprocessing and 3D reconstruction program,the MATLAB program involved in the three-dimensional reconstruction of fatigue crack is integrated,and the GUI control subsystem is established for different pretreatment processes.Then the GUI subsystems are designed to be integrated,controlled by a single GUI main system.So that users can understand the premise of the program easily,and achieve the desired goal.