Influence Of Corrosion Pitting On Multiaxial Fatigue Behavior Of304Stainless Steel

With the development of petroleum, chemical, marine engineering and aerospace industries, materials are more and more widely used in complex stress states and various harsh environments. The damage caused by the environment has been increasing. Although researches on the environment assisted failure of materials have made significant progress, the damage remains coming up with new forms. When complex environments are combined with stress, the issue has been challenging for decades. Aiming at the pitting corrosion fatigue mechanism of304austenitic stainless steel in chloride solution, this thesis studied the effect of multiaxial loads combined with corrosive environment on pitting mechanism. Experimental research, theoretical analysis and numerical simulation are conducted. The main progresses and conclusions are made as follows:The pitting and cracking behavior of304stainless steel under multiaxial corrosion fatigue conditions is studied with interruptive testing method. The pits geometry parameters are collected with3-D morphology instrument. The pit evolution and pit-to-crack transition processes under corrosive environment and multiaxial loading conditions are recorded. The effect of pitting on material failure mechanism under proportional and non-proportional loads has been examined.The statistics of pit growth parameters under proportional and non-proportional loading conditions are also carried out. It is found that the pit depth distribution accords with lognormal distribution. According to the experimental results, the pitting rates under different types of multiaxial loads are analyzed, and it is found that under proportional loading conditions the direction of maximum principal stress is fixed, the stress effect are more localized leading to fast rupture of the passive films and thus causing the accelerating pit evolution and initiation of cracks from pits.3-D finite element simulation is conducted on a series of semi-ellipsoidal shaped pits with different aspect ratios. The influence of aspect ratios on the stress concentration factors(SCF) under multiaxial loads is analyzed. Based on the simulation results, a formula is obtained describing the relation between the pit aspect ratio and SCF. The change of SCF is studied when local dissolution occurs. This process causes a sudden increase of SCF, being more dangerous to material failure. In the stress analysis of pitting area under multiaxial loads, the Mises stress at the bottom point presents different states, to some extent, having an explanation of the difference of material failure behavior.