Low-cycle Fatigue Behavior And Life Prediction Of Gradient Structured 316 Stainless Steel

The 316 stainless steels,with their good comprehensive mechanical properties,are ones of the most widely used materials in key components of many mechanical systems.As the reliability of machine constructions under cyclic operating conditions has nowadays becoming a matter of great importance to engineering industries,it has thereby motivated the fabrication of a new type of material called the gradient structure.The gradient structure,which possesses dual property of high strength and comparable ductility,can not only inhibit crack initiation but also retard crack growth and thus,improving the fatigue performance of structural components.In the present work,the low-cycle fatigue test under strain-control mode is carried out on 316 stainless steels subjected to shot peening process.The present work focuses on a few aspects:firstly,investigating the effect of shot peening on low-cycle fatigue behavior and fatigue life at various strain amplitudes;and then,establishing fatigue life prediction model using the hysteresis-energy based model to reveal the intrinsic factors that determine the fatigue property.The main findings of the present work are as follows:The results show that the cyclic deformation responses of both the un-peened and peened specimens reveal a three stage behavior.First initial cyclic hardening?stage ??,then cyclic stabilization?stage II?and finally secondary cyclic hardening?stage ??.The presence of the gradient structured surface layer can,on the one hand,lower the number of cycles associated with the transition from stage ? to stage ?(N_?/?),which can correspond to the limiting ability of cyclic hardening;and on the other hand,raise the number of cycles associated with the transition from stage ? to stage ?(N_?/?),which can be related with the increasing uniformity of plastic deformation.The analyses are further made by employing the low cycle fatigue life prediction model using the hysteresis energy-based model(W_p=W'?Nf?^-1/?,with w'f is intrinsic fatigue toughness and ? is damage transition exponent)to reveal the dominant intrinsic factor that determine the fundamental evolutionary character of the gradient structured 316 stainless steels.It is indicated that the lower values of N_?/? can be attributed to the reduced fatigue damage capacity,as is described by the decreased 'f value;while the raised values of N_?/? can be ascribed to the improved ability of damage defusing,as is characterized by the increased ? value.These findings accordingly emphasize that the increased ability of defusing fatigue damage has been acting as the main contributors to the enhanced fatigue behavior of 316 stainless steels with gradient structured surface layer.In the present work,efforts have been given on the assessment of the effect of shot peening on fatigue behavior and fatigue life of 316 stainless steels under low-cycle fatigue conditions at different strain amplitudes.The results obtained hereby can thereby be used as the design guideline for engineering industries.