Issue 48

L. Reis et alii, Frattura ed Integrità Strutturale, 48 (2019) 318-331; DOI: 10.3221/IGF-ESIS.48.31 320 The SSF equivalent shear stress is given by Eq. (3). ௘௤௩ ൌ ௔ ൅ ሺ , ௔ ሻ ∙ ௔ (3) To estimate fatigue life, the criterion uses the maximum equivalent shear stress as the damage parameter and uses the S-N trend line in pure shear loading conditions, as shown in Eq. (4). ൫ ௘௤௩ ൯ ൌ ൫ ௙ ൯ ௕ (4) where A and b are the trend line parameters of the S-N curve, obtained in pure shear loading conditions. For variable amplitude loading conditions, Anes et al. proposed a block extraction method. In this method, illustrated in Fig. 1, every time a τ eqv peak greater than the previous peak is identified, a block is defined and a new block starts. This new maximum value of τ eqv is the reference equivalent shear stress for the next loading block. The method is applied until all blocks have been identified. Figure 1: Block extraction method [15]. The number of cycles to failure, for each extracted block, is then calculated, as formulated in Eq. (5). ൫ ௘௤௩ ൯ ௠௔௫,௕௟௢௖௞ ൌ ൫ ௙,௕௟௢௖௞ ൯ ௕ (5) To calculate fatigue life, a cycle counting method must be used to count cycles in each extracted block. Then, the damage for each reversal can be determined by the ratio ௙ ⁄ and summed with a damage accumulation rule. Critical plane models Findley Findley [16] proposed a stress-based critical plane criterion that determines the critical plane as the one with the maximum damage parameter. Its formulation is given by Eq. (6). min ఏ ൫ ௔ ൅ ௔,௠௔௫ ൯ (6) where, for each plane θ, τ a is the shear stress amplitude, σ a,max is the maximum normal stress amplitude, and k is a stress scale factor, that must be determined experimentally. The k parameter varies between 0.2 and 0.3 for ductile materials. Brown-Miller After an extensive review Brown and Miller (B-M) concluded that both the octahedral shear strain and the maximum shear strain criteria fail to correctly describe the low-cycle fatigue behaviour. After combined tension and torsion fatigue testing,