Issue 36

L. C. H. Ricardo et alii, Frattura ed Integrità Strutturale, 36 (2016) 201-215; DOI: 10.3221/IGF-ESIS.36.20 202 effect generally occurs when the life is dominated by the initiation and growth of small cracks (linear elastic fracture mechanics). Large cyclic strains (elasto-plastic fracture mechanics), which might occur locally at stress raisers due to overload, may pre- damage the material and lower its resistance to fatigue. The experimentally derived crack growth equations are independent of the loading sequence and depend only on the stress intensity range and the number of cycles for that portion of the loading sequence. The central problem in the successful utilization of fracture mechanic techniques applied to the fatigue spectrum is to obtain a clear understanding of the influence of loading sequences on fatigue crack growth [2]. Investigations covering the effects of particular interest, after high overload, in the study of crack growth under variable-amplitude loading in the growth rate region, called crack growth retardation, seem to have little interest nowadays. Stouffer & Williams [3] and other researchers show a number of attempts to model this phenomenon through manipulation of the constants and stress intensity factors used in the Paris-Erdogan equation however little appears to have been done in the effort to develop a completely rational analysis of the problem. Probably, the only one reason that the existing models of retarded crack growth are not satisfactory is that these models are deterministic whereas the fatigue crack growth phenomenon shows strong random features. In addition, most of the reported theoretical descriptions of the retardation are based on data fitting techniques, which tend to hide the behavior of the phenomenon. If the retarding effect of a peak overload on the crack growth is neglected, the prediction of the material lifetime is usually very conservative [4]. Accurate predictions of the fatigue life will hardly become possible before the physics of the peak overload mechanisms is better clarified. According to the existing findings, the retardation is a physically very complicated phenomenon which is affected by a wide range of variables associated with loading, metallurgical properties, environment, etc., and it is difficult to separate the contribution of each of these variables [5]. Figure 1 : Fatigue crack growth da/dN versus ΔK stress intensity factor [8]: (a) Threshold range ΔKth; (b) Intermediate region following a power equation; (c) Unstable. C RACK PROPAGATION CONCEPTS aris & Erdogan [6] conducted a revision on the crack propagation approach from Head [7] and others and discussed the similarity of these theories and the differences of results between them, isolated and in group tests. Paris suggested that, for a cyclical load variation, the stress field in the crack tip for a cycle can be characterized by a variation of the stress intensity factor, Eq. (2.1), P