Improvement of very high cycle fatigue properties in an AA7075 friction stir welded joint by ultrasonic peening treatment

Abstract

The present paper aims to investigate the effect of ultrasonic peening treatment on the very high cycle fatigue resistance of an AA7075 friction stir welded joint. Microscopy observation, microhardness and X‐ray diffraction measurements were carried out to characterize the treated surface of peened specimens. Fatigue crack initiation sites were investigated through scanning electron microscope, and the role of enhanced surface on fatigue resistance was analyzed. The results indicate that a sensible fatigue strength improvement can be obtained through application of ultrasonic peening treatment and that fatigue cracks can initiate from the interior of the specimen. To clarify the fatigue failure mechanism, we analyzed the microstructure characteristics, compressive residual stress profile and intermetallic inclusion distribution in the surface layers, and we discussed the capability of ultrasonic peening treatment to hinder the surface crack initiation.

Interior failure mechanism and life prediction of surface‐treated 17Cr‐Ni steel under high and very high cycle fatigue

“…The reason is that fatigue crack mostly initiates from free surfaces of cyclically loaded metals. To avoid these impacts, a number of surface hardening technologies such as carburizing, nitriding, and shot peening are widely used to improve the fatigue resistance of structural materials . However, with the concept of very high cycle fatigue (VHCF) being put forward, studies show that surface harden sometimes cannot improve the fatigue properties of structural materials .…”

Very High Cycle Fatigue Crack Initiation Mechanism in Nugget Zone of AA 7075 Friction Stir Welded Joint

“…It revealed that the fatigue crack originated at the advancing side due to the effects of microstructural heterogeneity. Our previous work [12,13] also found that fatigue crack initiation sites were localized at the thermomechanically affected zone on the advancing side below 10 8 cycles. Nevertheless, the failure was prone to occur at the nugget zone in the VHCF regime.…”

Interior induced fatigue of surface‐strengthened steel under constant and variable loading: Failure mechanism and damage modeling

“…Recently, the occurrence of very high cycle fatigue (VHCF), which is associated with unexpected failures in the life regime beyond 10 7 cycles under the applied stresses below the conventional fatigue limit, means that the traditional criteria for fatigue assessment evolved from infinite life to damage tolerance are sometimes not appropriate. Moreover, a number of fundamental VHCF problems such as duplex or stepwise S‐N curve characteristics, competing failure mechanisms, life prediction approach, etc., are not yet well understood . Therefore, in order to keep the long‐term safety of mechanical structure or components, the VHCF mechanisms of structural materials and the relevant fatigue assessment approach still need to be further studied.…”

“…For alloy steels, the interior failure is mainly caused from the interior nonmetallic inclusions or inhomogeneous microstructures . Generally, a fisheye‐shaped crack occurs on the fracture surface, in which a peculiar area, named “fine granular area (FGA)” by coauthor Sakai, sometimes can be observed in the vicinity of crack nucleus, but sometimes it does not occur even in the life regime beyond 10 8 cycles . Some ideas or models such as “Polygonization and debonding,” “Depressive decohesion of spherical carbide,” “Continuous grain refinement,” “Numerous cyclic pressing,” “Hydrogen embrittlement‐assisted cracking,” “Repeating contact of crack surfaces,” and “Stage I‐like crystallographic crack propagation” have been proposed to explain the formation mechanism of the FGA from the various viewpoints.…”

“…ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi area FGA;ni p , FGA size corresponding to the σ i level with cycles n i ; ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi area FGA;Ni p , the final size of FGA at the σ i level with life N i ; ffiffiffiffiffiffiffiffiffi area p , size of crack; d inc , depth of inclusion causing failure; N f , fatigue life; n eq , equivalent number of cycles; n i , applied number of cycles; q, fitting parameter; R, stress ratio; R ti , the maximum height of the profile; R z , roughness; x, number of measured points; ΔK, SIF ranges; ΔK eq , equivalent SIF ranges; ζ, distribution of hardness; σ b , tensile strength; σ i , a given stress amplitude; σ max , maximum stress; σ s , yield strength; 3D, three-dimensional; CA, constant amplitude; EDS, energy dispersive X-ray spectrometer; FGA, fine granular area; LEFM, linear elastic fracture mechanics; OM, optical microscopy; SEM, scanning electron microscope; SIF, stress intensity factor; S-N, stress-number of cycles; VA, variable amplitude; VHCF, very high cycle fatigue failure mechanisms, life prediction approach, etc., are not yet well understood. [1][2][3][4][5] Therefore, in order to keep the long-term safety of mechanical structure or components, the VHCF mechanisms of structural materials and the relevant fatigue assessment approach still need to be further studied.…”