ORAL/POSTER REFERENCE:ICF100510PR A VARIABLE-AMPLITUDE FATIGUE LIFE PREDICTION TAKING INTO ACCOUNT CRACK RETARDATION Z.Perovic Department of Mechanical Engineering, University of Montenegro, 81000 Podgorica, Yugoslavia ABSTRACT A bimodal concept for predicting a high-cycle fatigue life of the structural details subjected to a variableamplitude loading is considered in this paper. The total fatigue life was separated into two phases: crack initiation and crack propagation. The portion of life spent in crack initiation was estimated by using S-N data obtained on smooth specimens. A fracture mechanics concept was used to calculate the portion of life spent in crack propagation. A modified Gray-Gallagher model was used to predict fatigue crack retardation following multiple overloads in a block spectrum. An original Gray-Gallagher model was proposed to predict fatigue crack retardation following a single overload, not taking into account either a delayed retardation or effect of multiple overloads. Both of these effects were incorporated in a modified Gray-Gallagher model used in this work. A computer program based on this model was applied to a welding joint subjected to a block spectrum loading and the results were compared with the experimental data reported in the literature. KEY WORDS Fatigue life prediction, crack initiation and propagation, retardation model, welded joint. INTRODUCTION Based on the previously determined stress distribution at the critical location, the fatigue life is calculated for each ‘microelement’ [Figure 1(a)] along the potential crack path, by using S-N data obtained on smooth specimens, as illustrated in Figure 1(b). The reciprocal derivative of this curve represents a rate of crack initiation in terms of dx/dN (x = distance from notch root; N = number of cycles), as shown in Figure 1(c). This rate may be interpreted as the rate at which the short crack grows due to the breaking of the microelements. It was proved [1] that the fatigue crack growth rate can be predicted on the basis of the smooth specimen data. A more consistent, but at the same time more complex, use of that approach would require a re-determination of the increased stress in each block when the crack tip reaches it and by taking into account the previous damage accumulation. It can be concluded that this method [1] should give a greater crack growth rate than the initiation rate obtained by the bimodal concept. However, an opposite effect is present during short crack growth, i.e. a gradual increase of crack closure level (decrease of ∆Keff) which causes a decrease in crack growth rate. In this work, it is assumed, as a first approximation, that these two opposite effects are equal and so the estimated rate of crack initiation (or short crack growth) can be considered a reasonably accurate solution. The macrocrack growth rate da/dN (a = crack size; N = number of cycles) obtained by using fracture mechanics approach, is also shown in Figure 1(c). At some distance ai , from the notch root, the rate of the crack initiation equals the rate of crack propagation. The upper zones of these curves, before and after this distance, indicate which of the mechanisms is more damaging. This distance ai, when initiation is finished, may be regarded as the crack initiation size [2]. Determination of the crack initiation life Ni is based on the stress range at distance
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