Fatigue life estimation after crack repair in 6005 A-T6 aluminium alloy using the cold expansion hole technique

In this paper, the hole drilling (IID) and the cold expansion (CE) processe s, which were used as a technique for crack repair, were investigated in or der to estimate the beneficial effects on fatigue crack initiation (FCI). T he FCI life is defined as the number of cycles to initiate a new crack of 0 .2 mm on the surface of the specimen. Three hole radii and three degrees of cold expansion (DCE%) values were tested after a crack propagation period. Crack retardation after the CE process was observed. This phenomenon is du e to two mechanisms: retardation owing to both geometric and mechanical eff ects, which is produced by the stress concentration at the drilled hole, an d the large strain-induced compressive residual stresses around the hole. I n this report, the influence of the loading conditions was studied. For hig h values of the stress intensity factor range DeltaK(rho) around the hole ( based on the pseudo crack length a+rho), the number of cycles corresponding to crack initiation Ni is low. At the edge of the hole, the maximum stress range can be approximated by the following formula: Delta sigma (max) = 2 DeltaK(rho)/root pi rho, where rho is the hole radius and DeltaK(rho) is th e related stress intensity factor range. The FCI life extension, defined by the number of cycles corresponding to crack re-initiation N-i, is related to the relative maximum stress range ratio R-sigma = [(Delta sigma (max))/( Delta sigma (max))(th)] where (Delta sigma (max))(th) is the value of the t hreshold maximum stress range obtained when N-i = 2 x 10(6) cycles. The rel ationship between N-i and R-sigma may be written as a power function.