Issue 37

N. Zuhair Faruq, Frattura ed Integrità Strutturale, 37 (2016) 382-394; DOI: 10.3221/IGF-ESIS.37.49 383 Stress or strain based approaches are used to assess fatigue damage in mechanical components. Stress based approaches are recommended to be used to perform the high-cycle fatigue assessment [2] since, under these circumstances, cyclic plastic deformations can be neglected with little loss of accuracy [3]. The accuracy and reliability of this design strategy has been validated by performing several experimental investigations [4, 5]. However, when cyclic plasticity cannot be disregarded, it is commonly accepted that strain based approaches are more accurate in predicting lifetime of components, with this holding true especially in the low -cycle fatigue regime [1, 6]. This explains why, nowadays, the strain based approach is considered as an irreplaceable tool that is daily used by structural engineers to assess fatigue damage in real engineering components [7, 8]. F UNDAMENTALS OF THE MODIFIED M ANSON -C OFFIN CURVE METHOD he Modified Manson-Coffin Curve Method (MMCCM) is a strain-based fatigue criterion that allows uniaxial/multiaxial fatigue damage in real mechanical components subjected to in-service time-variable load histories to be estimated accurately [9, 10]. According to the classical strain-based criterion, Manson-Coffin curve is defined by slopes c and b that links the maximum shear strain amplitude with the number of reversals to failure. From a physical point of view, the formalisation of the MCCM takes as its starting point the assumption that the material plane experiencing the maximum shear strain amplitude coincides with the Stage I plane [1] (Fig. 1). Figure 1 : Fatigue damage model [1]. According to the fatigue model depicted in Fig. 1, the following relationship can be defined [11]:     b c f a f f f N N G ' 2 ' 2      (1) where, ߛ a is the shear strain amplitude relative to the critical plane;  ’ f and  ’ f are the multiaxial fatigue strength coefficient and the multiaxial fatigue ductility coefficient, respectively; b and c are the multiaxial fatigue strength exponent and the multiaxial fatigue ductility exponent, respectively; N f is the number of cycles to failure. All these fatigue constants can be evaluated by running an appropriate experiment. The classic Manson and Coffin curve, as shown in Fig. 2a and defined by Eq. 1, is reformulated to deal with multiaxial fatigue stress/strain tensors by calibrating all functions in Eq. 1, as expressed in Eq. 2. By following a systematic validation exercise, the MMCCM is seen to be capable of accurately modelling not only the detrimental effect of non-zero mean stresses, but also the degree of multiaxiality and non-proportionality of the load history being assessed as shown in Fig.2b [11-13]. T