Investigations On Microstructural Features In Crack Initiation Region Of Very-high-cycle Fatigue For Metallic Materials

Very-high-cycle fatigue(VHCF)refers to the phenomenon that fatigue failure beyond 107 cycles of metallic materials subjected to cyclic stress far lower than yield strength.In the VHCF stage,the cracks are prone to originate from the interior of materials,and the unique fine granular area(FGA)or rough area(RA)are formed around crack origins,which are regarded as characteristic region of crack initiation.The existed research results show that crack initiation process consumes more than 95% of the total fatigue life.Therefore,it is of great significance to study VHCF for the understanding of the formation mechanism of crack initiation region.In this paper,the formation mechanism is explored by examining the detailed microstructural features of metallic materials beneath crack initiation region.For this,the focused ion beam technique was used to prepare the sectional samples in the crack initiation regions of failed specimens(high-strength steels and titanium alloys).The microstructure morphology near the surface of these sectional samples was observed by transmission electron microscopy with selected area electron diffraction method.Experimental results further show that the surface of FGA and RA of failed specimens under negative stress ratios is a nanograin layer,but the surface of crack initiation region of failed specimens under positive stress ratios is still coarse grains,which indicates that the plastic deformation at the crack tip can only cause certain extent of microstructure refinement,but is insufficient to produce nanograins.For the purpose of revealing the reasons of nanograin formation,a normalized quantity is defined to quantitatively describe the distributions of grain size on the surface layers of FGA and RA regions of failed specimens under negative stress ratios.The results show that for high-strength steel samples,the grain size gradually increases along the crack growth path.In the meantime,the distribution of contact stress between crack surfaces under different loading conditions was analyzed by using the finite element method.The results show that the contact stress between the crack surfaces gradually decreases along the crack growth path,which is consistent with above mentioned experimental results.The thickness of the nanograin layer for titanium alloys with duplex microstructure is large,and grain size increases in the depth direction,but there is no obvious variation trend along the crack growth path,which is related to the failure mode that the crack origin is formed by the cleavage of multiple equiaxed ? grains.For the titanium alloys with equiaxed microstructure,the thicknesses of the nanograin layers are small and discontinuously distributed due to the small size of the equiaxed ? grains in the original microstructure.The experimental and numerical results show that the nanograin formation at the crack initiation region is closely related to the contacting actions between crack surfaces,that is,higher compressive stress and longer loading cycles will promote microstructure refinement and nanograin formation.Consequently,the corresponding results show that the “numerous cyclic pressing” process dominates the formation of nanograin layer at the crack initiation region.