Comparison of retardation behaviour of 2024‐T3 and 7075‐T6 Al alloys

Abstract

A B S T R A C T Retardation in fatigue crack growth rate following the application of single and periodic tensile overloads was studied for 2024-T3 and 7075-T6 aluminium alloys. Tests were performed at constant stress and at constant stress intensity factor ranges, at a load ratio of R = 0.1, at a baseline K in the 10-20 MPa √ m range which corresponds to the Paris regime. Overload ratios of 1.3-1.65 were studied with overload spacing, n, varying from 20 to 10 000 cycles. 2024-T3 displayed an order of magnitude higher retardation, N d , due to single tensile overloads compared to 7075-T6. Periodic overloads induced maximum retardation when n/N d ≈ 0.5 for both alloys, the magnitude being only 15% higher for 2024-T3. a = crack length CA = constant amplitude da/dN = crack growth rate K = stress intensity factor K min , K max = minimum and maximum stress intensity factor for base cycling K ol = stress intensity factor corresponding to overload peak M(T) = middle cracked tension specimen n = number of cycles between overloads N = number of cycles N d = number of retarded (delayed) cycles N CA = number of constant amplitude cycles for the crack to grow by an amount equal to the overload-induced plastic zone size OL = overload OLR = overload ratio (K ol /K max ) POL = periodic overload R = stress or load ratio (P min /P max ) S = nominal stress, load/cross-sectional area 2r ol y = plane strain plastic zone size associated with the overload S y = yield strength β = ratio of the constant amplitude crack growth rate to the rate during periodic overloads-retardation factor K = stress intensity factor range corresponding to base cycling, K max -K min K ol = K ol -K min N d = transient portion of the retarded growth ( Fig. 1) S = stress range (= load range/cross-sectional area)

Fatigue crack propagation in API 5L X-70 pipeline steel longitudinal welded joints under constant and variable amplitudes

“…reported that overloads applied too close lead to crack acceleration because crack jumps at each overload exceed the retardation in the subsequent few baseline cycles. According to Celik et al , 19 the severity of retardation is a function of the overload peak proximity: overloads applied closely and separately sufficiently are responsible for crack growth acceleration and retardation, respectively. Several models have been proposed to explain the FCP delay due the OLs, including models based on residual stresses, 5,20 strain hardening, 5 plasticity induced crack closure 20–21 and crack tip blunting 5,21 …”

Fatigue Behavior of Al 7075-T6 Plates Repaired with Composite Patch under the Effect of Overload

“…It has been shown in the literature that there is a threshold overload rate below which the retardation effect will not exist [7]. In plane conditions, especially plane stress, higher plasticity around the crack front leads to a blunting of the crack tip, reported by several authors who studied overloaded specimens made of aeronautical aluminum alloys, such as 2024-T3 or 7075-T6 [8][9][10]. Bichler and Rippan [11] showed that the crack blunting increases the effective stress intensity factor (SIF), which causes an initial acceleration in the crack propagation, but the closure level increases rapidly as the crack grows because of the wedging action of the overload.…”