Abstract:In variable amplitude fatigue of high strength low alloy (HSLA) steel components, overloads can severely retard subsequent crack propagation for a number of cycles. In order to be able to predict fatigue crack propagation with a reduced degree of conservatism, retardation has to be taken into account. Of all numerical models that have been developed over time, crack tip plasticity models are selected based on the need for a detailed and fast cycle-by-cycle simulation of high cycle. After introducing the load interaction zone concept, common to all crack tip plasticity models, the Wheeler and Willenborg models are discussed, implemented and compared to experimental data. It is concluded that the Modified Wheeler model provides the most promising results, whereas the main limitation of Willenborg models is the need for extensive experimental data. Keywords INTRODUCTIONThe effects of variable amplitude loading on fatigue crack propagation were first observed by the airplane industry. It was found that linear cumulative damage evolution (as predicted with Miner's rule) was often ultraconservative. Experiments revealed that after applying a single overload cycle in between constant amplitude loading cycles, crack propagation was slower than for constant amplitude fatigue [1]. This effect was called retardation. In an attempt to lower the degree of conservatism of a fatigue design -and thus indirectly the safety factor, material usage and cost -researchers tried to account for the retardation effect. Most studies have focused on the behavior of aluminum alloys, which were frequently used in that industry at the time [1][2][3][4][5].Lately, there has been a lot of interest in the offshore industry for simulating variable amplitude fatigue crack propagation [6,7]. Offshore constructions are continuously subjected to variable loading conditions, due to various influences from sea and wind current amongst others. As this problem concerns high cycle fatigue, a first requirement for a suitable cycle-by-cycle fatigue crack propagation model is a reasonable total computation time. Additionally, the model should be able to yield satisfactory results without requiring extensive experimental material characterization. As most models have been developed for aluminum, good correlation for offshore steel grades is not guaranteed.

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