CRACK PROPAGATION STUDIES ON Al 7475 ON THE BASIS OF CONSTANT AMPLITUDE AND SELECTIVE VARIABLE AMPLITUDE LOADING HISTORIES

Abstract

Fatigue crack propagation behaviour under variable amplitude loading is significantly influenced by load history induced sequence effects. Because the understanding of the causes of such sequence effects is still rather limited, crack propagation predictions become difficult. In the present investigation an exploratory test program with simple variable amplitude load sequences was performed, which was designed to improve the general understanding of sequence effects and to offer a basis both for an evaluation of existing crack propagation prediction models and for the development of improved methods.The investigations were performed on center cracked 8 mm thick A1 7475-T7351 specimens. In the first part of the study constant amplitude tests were performed over a wide range of R-ratios. The test results and the transition of the fracture mode during tests could be represented on the basis of the effective AK-value.In the second part of the paper tests with simple systematically varied sequences of high loads, low loads and underloads were performed and the crack propagation behaviour was measured. In order to obtain detailed information on the crack propagation increments per cycle, fractographic analyses were also performed. It was found that also under variable amplitude loading the crack propagation approximately followed the effective AK-values, except in one case.A linear (non-interaction) prediction model could not describe the observed crack propagation behaviour. The results of the application of a model assuming a constant crack opening level are also given. Finally, some general viewpoints regarding further developments of models are outlined. NOMENCLATURE a = crack length, mm a, = crack length before fracture mode transition a2 = crack length after fracture mode transition C = constant in crack propagation equations CA = constant amplitude loading history d a / d N = crack propagation rate, mm/cycle dK/da = variation of the stress intensity factor due to an increase in crack length K = stress intensity factor, MNm-3'2 AK = range in stress intensity factor between the maximum load and the minimum load in a cycle, MNm-3'2 K, , = maximum stress intensity in a cycle, MNm-3'2 Kmi, = minimum stress intensity in a cycle, MNm-3'2 Kop = stress intensity factor at opening of a fatigue crack, MNrn-"' AK8 = K,, -KO9 = stress intensity range between the maximum load in a cycle and crack opening, MNm-3'2 n = exponent in crack propagation equations N = cycle number N,= cycle number to failure N4 = cycle number up to a crack length of a = 4 mm R = stress ratio (= Smin/Smax) N,in,,,c = prediction after a linear calculation *Visiting scientist from the Institute of Aeronautical Materials, Beijing, China. 315 316 S. ZHANC et al. S = stress, N/mmz S,,, = maximum stress in a cycle, N/mm2 S,,, = minimum stress in a cycle, N/mmz Sap = crack opening stress, N/mm2 So,, = 0.2% offset yield strength, N/mm2 S, = ultimate tensile strength, N/mm2 6 =strain, % t = time U = AK,,/AK w = specimen width, mm a = geometry factor

Fatigue crack growth in thin aluminium alloy sheets under loading sequences with periodic overloads

“…Initially, a retardation period identical to that induced by a single peak overload [3][4][5] is observed for less frequently applied overloads. For overloads applied each 10 baseline cycles this initial stage is similar to the one generally observed after an underload [10,11]. The subsequent crack growth rate increases in a stable manner.…”

“…In contrast, other variable amplitude loading sequences have not yet been exhaustively investigated. Among them, intermittent single or block overloads [8][9][10][11][12] are experienced by a large number of engineering components.…”

Variable amplitude fatigue crack growth behavior — a short overview

“…In the literature, the separation between periodic single overloads is almost invariably reported in terms of baseline cycles separating consecutive overloads, ΔN s [58,89,90,92,95,101,103], with few expressing this in terms of separation distances, Δas [14, 107,120]. ΔN s has been a favored choice particularly due to the ease at which overload periodicity can be controlled during fatigue experiments, rather than using Δa s as the controlling parameter, which requires accurate monitoring of crack length.…”

Strip yield modelling of fatigue crack under variable amplitude loading

“…Numerous attempts have been made to understand fatigue behavior under simple single overload transients [2,[4][5][6][7][8][9][10]. For an understanding on fatigue crack response during realistic service loads, the effects of periodic overloads (dual or multiple, isolated or block) have been investigated [3,[11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. When successive (or periodic) overloads are applied between periods of constant amplitude fatigue loading, there arises the potential for overload interaction [21], for example, observed an apparent dependence of interactions between repeated overloads on the interval between the overloads.…”