Abstract
The paper contains results of an investigation of effects of shot peening on growth of physically short fatigue cracks in a 2.4 mm thick sheet of an aircraft V-95 Al-alloy (a type of a 7075 alloy), clad with a 7072 Al-alloy (Al-Zn1). Using an improved and verified experimental methodology, particularly direct current potential drop (DCPD) method, short fatigue cracks of the length from 0.2 mm to more than 3 mm, most of them between 0.8 - 1.5 mm, were prepared under high cycle fatigue repeated tensile loading of the constant nominal stress range 160 MPa, load asymmetry R 0. Edges of the specimens with already existing short fatigue cracks were then shot peened using two different groups of parameters. Some reference specimens without cracks were shot peened with the same parameters. Microhardness measurement and informative measurement of residual stresses near the peened edges were carried out. Significant retardation of crack growth was observed particularly in case of cracks shorter than 2 mm. The work was completed by a unique measurement of actual crack growth rate throughout the peened area with residual stresses starting with crack length less than 0.2 mm. Different stages of the growth are discussed.
Highlights
Shot peening is one of surface treatment methods applied to various metallic materials either separately or in a combination with other surface treatments like plating, hard anodising etc., in order to locally improve the performance of mechanical components under fatigue loads [1, 2]
The experimental programme contained the following steps: G preparation of series of a sufficient, representative number of specimens with physically short fatigue cracks of different lengths from 0.35 mm to 3.6 mm, G shot peening of the specimens with short cracks, using two groups of parameters, G characterisation of bulk material and shot peened layers in terms of deformation of the microstructure, microhardness and informative assessment of residual stresses, G evluation of residual fatigue life of the shot peened specimens with pre-existing short fatigue cracks, G evalution of fatigue life of specimens with no pre-cracking, both shot peened and without peening and, eventually, G evaluation of actual growth rate of a short crack growing throughout the peened area with residual stresses
It can be anticipated that the first progressive retardation with the minimum growth rate at the length 0.54 mm (ΔK ϭ 7.4 MPa m1/2) is caused more by the short crack phenomenon than by compressive residual stresses – it corresponds to the transient period between crystallographic and KI mode of growth
Summary
Shot peening is one of surface treatment methods applied to various metallic materials either separately or in a combination with other surface treatments like plating, hard anodising etc., in order to locally improve the performance of mechanical components under fatigue loads [1, 2]. This treatment creates compressive residual stresses that usually increase the operating life of the component. Complex crack growth measurement including subsurface profile, which is an essential condition of application of fracture mechanics approaches, is not an easy task, if specimens are of quite big dimensions. Results and analyses of an extensive experimental programme aimed at an evaluation of effects of shot peening applied to clad V-95 Al-alloy (7075 Al-alloy) sheets on growth and retardation of existing short fatigue cracks are presented and discussed
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