A Study On The Multiaxial Ratcheting And Fatigue Properties Of 316L Stainless Steel Thin Wire

316L austenitic stainless steel,owing to its excellent biocompatibility and corrosion resistance,high strength and ductility,has become one of the most widely used biomedical implant materials for coronary stents.The coronary stent is a critical medical device for restoring the stenotic arterial lumen.The stent usually has a strut only in the micron scale.The mechanical behaviors in this scale could significantly differ from those of the bulk materials and display certain size effects.Since the coronary stents are subjected to multiaxial cyclic loadings during service,the multiaxial cyclic behavior of the small-scale stent material is significant in the optimal design and reliability evaluation of the biomedical devices.In this paper,the uniaxial tensile and multiaxial ratcheting fatigue properties of the 316 L stainless steel material in the micron scale were investigated using the cold-drawn 316 L thin wires with two different diameters and four grain sizes that were achieved by different heat-treatment.The uniaxial tensile tests were first conducted on thin wires with various diameters and grain sizes.Multiaxial ratcheting tests were then performed under six different axial stress-shear strain loading paths.Scanning electron microscopy(SEM)and electron backscattered diffraction(EBSD)were used to characterize the surface morphologies and microstructures after deformation,respectively.The multiaxial fatigue behaviors of the thin wires were finally studied under symmetric cyclic torsion with different constant axial stresses.Based on the cyclic mechanical behaviors and the SEM observation of failure mode,a new model was proposed for fatigue life prediction for the thin wires.The conclusions can be reached as follows:(1)Both the diameter and grain sizes of the 316 L stainless steel thin wires influenced the uniaxial tensile properties.The size effects of “thinner wire is stronger” and “smaller grain size is stronger” were observed on the yield strength.(2)Under the tension-torsional cyclic loading,the ratcheting strains were suppressed in the thinner wire,and with decreasing diameter,the shear stress hardening became higher.(3)The ratcheting strain evolution of the thin wires were independent of the multiaxial loading paths.(4)Symmetric cyclic torsion led to unsaturated cyclic hardening until failure.The cyclic hardening rate increased with the shear strain amplitude.Under symmetrical torsion with constant axial stress,316 L stainless steel thin wire presented an initial rapid hardening following by gradually softening until failure.The initial hardening caused by axial stress greatly influenced the fatigue life.A new multiaxial fatigue life model considering this effect was proposed.And good agreement was obtained between the experimental data and the prediction.