Study On Crack Growth Path Of 2A12-T4 Aluminum Alloy Under Multiaxial Fatigue Load

Engineering structures are often subjected to complex multiaxial fatigue loads during service,especially in the field of aeronautics and astronautics,such as airframe skins and wing skins are subjected.The rapid development of aviation technology requires increasingly lighter and safer fuselage structures.The failure prediction methods that simplifies complex load states by relying on single-axis loads are no longer applicable.Therefore,accurately predicting the multiaxial fatigue failure behavior of engineering structures under complex loads is of great significance for improving structural safety and achieving lightweight structures.This article is based on the actual engineering background of a large number of structures in aircraft structures withstanding multiaxial high-cycle fatigue.In this study,multiaxial fatigue tests under different stress amplitude ratios with fixed von Mises stress amplitude were carried out by using 2A12-T4 aluminum alloy solid round bar specimens.Metallographic microscope was used to record the surface crack initiation and propagation path of the test section of the specimen,and to explore the effect of stress amplitude ratio on the multiaxial fatigue crack initiation and propagation behavior of 2A12-T4 aluminum alloy.Combining theoretical analysis and experimental study,a new multiaxial fatigue failure model was established.The main contributions of this study are described in the following three aspects:(1)Under the same von Mises stress amplitude,multiaxial fatigue tests with different stress amplitude ratios were conducted,a scheme for extracting surface cracks of specimens was designed.The variation of the maximum principal stresses and the maximum shear stresses under different stress amplitude ratios were analyzed.(2)The discontinuous microscopic observation method was used to observe and record the surface cracks of the specimen.The effects of stress amplitude ratios on the initiation direction of multiaxial fatigue cracks,the growth phase I and phase II stage directions were studied.The complete process of multiaxial high-cycle fatigue failure under different stress amplitude ratios was obtained,and the main stress parameters affecting high-cycle fatigue failure were determined.The variation of the crack propagation length with life,under different stress amplitude ratios,was explored.(3)By test research and theoretical analysis on the multiaxial high-cycle fatigue,and discussions on the surface crack initiation and propagation paths under different stress amplitude ratios,the multi-axial high-cycle fatigue failure mechanism and controlling parameters affecting fatigue failure under tensile and torsional loading were obtained.On the basis of multiaxial fatigue crack initiation and propagation behavior,a multi-axis high-cycle fatigue failure model based on the critical plane method was established.The model was used to predict the test results of various materials.In conclusion,based on theoretical analysis and experimental investigation,the initiation and propagation law of high-cycle fatigue crack in 2A12-T4 aluminum alloy with different stress amplitude ratios were systematically analyzed.A prediction model of multiaxial high-cycle fatigue failure based on the critical plane was established,which provides a reference for the prediction of the multiaxial fatigue life of aluminum alloy materials affected by different loading parameters.