| Laser peening (LP) is a new surface modification technology, during the process of LP, high amplitude shock wave pressure induced by high energy and short pulsed laser can effectively improve the stress distribution and micro-structures of metal materials, thus effectively delay the fatigue crack growth (FCG) rate and improve the fatigue life of metal parts. At present, the study of life-extending technology mainly focus on experiments, but the basic theory of FCG laws and life extension mechanism under residual stress field induced by LP are still pending.The aim of this paper was to provide some foundational researches on the macro-properties, micro-structure evolution and fatigue properties of6061-T6aluminum(Al) alloy subjected to LP. Base on the theoretical discussion of FCG properties, the LP processes on typical notched and compact tension (CT) samples were studied, and the influence of surface integrity on fatigue properties was analyzed. Combined with the macroscopic FCG performance and microscopic fatigue fracture morphology, thorough descriptions of life extension mechanism induced by LP were performed at the different fracture stages, meanwhile, numerical simulation of FCG properties under residual stress field induced by LP was carried out. The main contents of this paper include the following aspects,(1) The estimation formulas for FCG life of the untreated and LPed samples were put forward. Based on the fracture mechanics theory, the effects of LP process on FCG properties, including the FCG threshold, stress intensity factor (SIF), FCG rate, crack tip opening displacement(CTOD) and plastic deformation at the crack front were explored. With the comprehensive consideration of the applied load and the residual stress induced by LP, estimation formulas for FCG life of the untreated and LPed samples were obtained based on the Paris formula. Based on the metal physics theory, reverse calculation method for fatigue stress and fatigue life of samples subjected to LP was analyzed by the quantitative description of fatigue fracture morphology. It provided theoretical basis for revealing the enhancement mechanism of FCG properties and fatigue life of metal materials subjected to LP.(2) The surface integrity (nano-hardness, elastic modulus, surface roughness, surface profile, residual stress and micro-structure) of6061-T6Al alloy subjected to different LP process parameters was investigated systematically, and the influence of surface integrity on fatigue properties was researched. Nano-indentation test was performed, and the influence of different LP impact times and laser energy on contact depth and contact area of nano-indentation was studied. The improve mechanism of nano hardness and elastic modulus induced by LP was explored. Surface morphology and surface roughness were tested, and the effects of line roughness and surface roughness on fatigue performance were analyzed. Residual stress distribution of CT samples subjected to LP was investigated, and the effects of LP process parameters on the residual stress along surface and depth direction were explored. Micro-structures under different laser energy were observed, and microscopic strengthening mechanism of aluminum samples subjected to multiple LP treatment was obtained.(3) The effects of LP process on the residual stress and fatigue performance were investigated. The relations between LP impact times, peening paths, laser energy and fatigue life were obtained according to the relation curves of axial load, axial displacement and fatigue life on the notched samples. LP process of pre-crack CT samples was studied, and the FCG properties under different peening paths and laser energy were investigated by the curves of da/dN-ΔK and a-N. The relation curves of CTOD and FCG rate as well as fatigue life were analyzed, in order to verify the feasibility of using CTOD criterion to characterize the FCG performance. The above research can provide experimental datas for optimizing the process parameters and formulating technical criterion during LP.(4) The fatigue fracture morphology after LP was analyzed, and the fracture quantitative prediction of FCG life was achieved. The macro and micro fatigue fracture morphology and microscopic strengthening mechanism of the notched and CT samples during fatigue crack initiation period, the earlier and medium FCG period, as well as the instantaneous fracture period were systematically analyzed under different LP impact times, peening paths and laser energy. The micro fatigue fracture morphology of the stop-continue crack tip during FCG was investigated. According to the fatigue striations at different crack lengths, the comparison of macro and micro FCG rate was performed, and trapezoidal method was used to predict FCG life based on the quantitative analysis of fatigue fracture. The reconstruction of three dimensional morphology and the analysis of roughness on typical fatigue fracture of CT samples were carried out, and the variation tendency of altitude difference, line roughness and surface roughness were also investigated.(5) Based on the software platform of ABAUQS-MSC.Fatigue, a digital analysis method of FCG properties was established. Special loading module of laser shock wave was established in ABAQUS/CAE, and the residual stresses of the notched and CT6061-T6Al alloy samples subjected to different laser energy and coverage areas were simulated. Combined with MSC.Fatigue, the FCG performance under residual stress field was simulated, and the curves of da/dN-ΔK and a-N were obtained. The FCG inhibition induced by LP along the driving force direction during different FCG stages was investigated. It indicated that the simulation results were consistent with the experimental results, therefore, an effective method for predicting the FCG properties under the residual stress field induced by LP was provided. |
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