Analysis of fatigue failures in components using the theory of critical distances

This paper explores the use of the theory of critical distances (TCD) in the prediction of high-cycle fatigue behaviour in engineering components. Theories of this type have been in use for over 50 years in various forms, and recently have been brought up to date by linking them with linear elastic fracture mechanics (LEFM). The TCD represents a major extension of LEFM, allowing it to be used for short cracks as well as for stress concentrations of arbitrary geometry, using the results of finite element analysis (FEA) or other computer-based methods. A number of different realisations of the TCD exist: several of these express the critical distance as a function of material fatigue limit (Δσ0) and crack propagation threshold (ΔKth). This paper illustrates the application of the TCD to a case study on the failure analysis of a large maritime vehicle component in three different designs. Unusually, the increasing of a fillet radius did not prevent fatigue failures from occurring: the TCD was able to explain this, due to its ability to accurately predict notch sensitivity effects. Further applications of the TCD, to problems not only in fatigue but also in brittle fracture in various materials, are also discussed.