Short fatigue crack growth in an aircraft Al-alloy of a 7075 type after shot peening

Abstract

An effect of shot peening with two groups of parameters representing limits for practical use in service on growth of physically short fatigue cracks in a 2.4mm thick sheet of an aircraft V-95 Al-alloy (a type of a 7075 alloy), clad with a 7072 Al-alloy (Al–Zn1) was investigated. The first part of the work deals with measurement techniques of short cracks, with an adaptation and verification of direct current potential drop (DCPD) method for detection of short crack initiation and early stages of growth. The possibilities of the method in terms of sensitivity and resolution as well as the limits are discussed. Using the improved experimental methodology, physically short fatigue cracks of the length from 0.2mm to more than 3mm, most of them between 0.8 and 1.5mm, were prepared under high cycle fatigue repeated tensile loading of the constant nominal stress range 160MPa, load asymmetry R=0. Specimens with existing short fatigue cracks together with some specimens with no cracks were then shot peened using two different groups of parameters. Microhardness measurement and informative measurement of residual stresses near the peened edges were carried out. Development of crack growth after shot peening were measured and compared with crack growth in specimens without shot peening. Retardation of crack growth was significant particularly for cracks shorter than 2mm. For the specific stress range, evaluated results enabled to estimate threshold length of defects, which will be reliably arrested after the application of the shot peening.