Growth and retardation of physically short fatigue cracks in an aircraft Al-alloy after shot peening

Abstract

The paper contains results of an investigation of an effect of shot peening on initiation and growth of physically short fatigue cracks in an aircraft V-95 Al-alloy, which is of a similar type as 7075 alloy. The first part deals with an adaptation and verification of direct current potential drop (DCPD) method for detection and measurement of short crack initiation and growth in flat specimens of specific shape and dimensions. The possibilities of the method in terms of sensitivity and resolution as well as the limits are discussed. The material properties and quite large dimensions of flat specimens with side necking of a low stress concentration factor had to be considered when position of electrodes was specified. The specimen type and dimensions were proposed taking account of the investigation of shot peening effects. Physically 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 loading of a constant nominal stress amplitude 160 MPa. Specimens with existing short fatigue cracks were shot peened using two different groups of parameters. Development of crack growth after shot peening was measured and compared with crack growth in specimens without shot peening. Retardation of crack growth was significant particularly for cracks shorter than 2 mm. For the specific stress amplitude, evaluated results enable to estimate threshold length of defects, which after the application of the shot peening will be reliably arrested.