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

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Abstract
In this paper, the hole drilling (HD) and the cold expansion (CE) processes, which were used as a technique for crack repair, were investigated in order to estimate the beneficial effects on fatigue crack initiation (FCI). The 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 due to two mechanisms: retardation owing to both geometric and mechanical effects, which is produced by the stress concentration at the drilled hole, and the large strain‐induced compressive residual stresses around the hole. In this report, the influence of the loading conditions was studied. For high values of the stress intensity factor range ΔKρ around the hole (based on the pseudo crack length a + ρ), 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: Δσmax = 2ΔKρ /√πρ , where ρ is the hole radius and ΔKρ is the related stress intensity factor range.The FCI life extension, defined by the number of cycles corresponding to crack re‐initiation Ni , is related to the relative maximum stress range ratio Rσ = [(Δσmax )/(Δσmax )th ] where (Δσmax )th is the value of the threshold maximum stress range obtained when Ni = 2 × 106 cycles. The relationship between Ni and Rσ may be written as a power function.

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