Issue 41

A. S. Cruces et alii, Frattura ed Integrità Strutturale, 41 (2017) 54-61; DOI: 10.3221/IGF-ESIS.41.08 57                                           ' ' ' ' ' ' 2 2 1 2 1 2 1 2 b c f f f f y b b c f f f e f p f f N N G k N N N E (2) Sample ɛ a ɤ a σ a (MPa) τ a (MPa) N f IP1 0.0015 0.0032 198 156 36,147 IP2 0.0015 0.0028 238 151 141,938 IP3 0.0015 0.0028 234 151 103,138 IP4 0.0015 0.0026 238 148 162,119 IP5 0.0011 0.0032 177 176 179,628 IP6 0.0011 0.0032 180 185 72,011 IP7 0.0011 0.0028 183 165 179,446 IP8 0.0011 0.0028 178 163 268,051 IP9 0.0011 0.0026 185 154 662,706 IP10 0.0009 0.0032 146 184 248,009 IP11 0.0009 0.0032 143 183 188,219 IP12 0.0009 0.0028 151 172 624,521 IP13 0.0009 0.0026 152 162 870,886 Table 4 : Strain and stress conditions for in-phase strain conditions and the obtained fatigue life in number of cycles. Suman & Kallmeyer damage parameter (SKDP) The importance of interaction of normal and shear stress on the critical plane has recently been investigated by Suman & Kallmeyer [19]. They have used the product of normal and shear stress at the critical plane to model this interaction. The product term in Suman & Kallmeyer model represents the maximum value of the product of normal and shear stress at the critical plane. By considering this product term, Suman & Kallmeyer were able to overcome the ambiguity caused by the non-proportional loading where both normal and shear stress peaks do not occur at the same time point. This product term can model the interaction effects for wide range of in-phase and out of phase fatigue data. Apart from that, this formulation (Eq 3) also has significantly less number of material dependent parameter in comparison to the model previously developed by the same group of researchers, and provides excellent correlation between experimental and predicted fatigue lives of the steel and titanium specimen. Suman & Kallmeyer fatigue model also captures the effect of strain hardening due to LCF loading and the mean shear stress at the critical plane.                      w 1 w max max 2 0 σ τ DP G γ τ 1 k σ (3) R ESULTS n this study, most of the load path used are proportional and sinusoidal. Due to the proportionality, both shear and normal stress peaks in these tests happened at the same time point. The critical plane stresses for the test IP1 (Tab. 4) are presented in Fig 2 and Fig 4. The peaks of normal and shear stresses are presented by solid and dashed lines respectively, and the damage parameter values are presented by the green solid line (damage parameter is obtained with the maximum load). I