Issue 43

B. Saadouki et alii, Frattura ed Integrità Strutturale, 43 (2018) 133-145; DOI: 10.3221/IGF-ESIS.43.10 141 Figure 11 : Variation of the total energy to failure as function of fatigue lifetime. Fatigue resistance Basquin and Manson-Coffin defined experimentally laws characterizing low cycle fatigue life, between strain variations ∆ ε and the fatigue life. - Manson-Coffin relation:       ' 2 2 c R N (10) - Basquin relation:       ' 2 2 f b el R N E (11) SICLANIC ® cyclic tests were conducted at imposed stress, achievement of fatigue resistance curves ε – N cannot be direct. We used prediction formulas. These formulas have been proposed in principle to limit the number of tests and to avoid even realize one fatigue test. The resistance relations fatigues are predicted by two methods, whose coordinates come from monotonic characteristic of the alloy. a) Four correlation point method This method developed by Manson and Halford [25], allows the plot of elastic and plastic lines by knowing some monotonic characteristics. b) Universal slopes method A second estimation method for resistance curves is proposed [27]. Manson, after many tests on various materials, made the followings hypotheses: elastic and plastic lines have respectively mean slopes of -0.12 and -0.6. The points corresponding to the total strain variations ∆ ε t are placed on an asymptotic curve to the elastic and plastic lines. For low cycle fatigue, plastic strain predominates. The curve representing the plastic strain variation is placed above that of elastic strain up to a certain number of cycles (Fig. 12). SICLANIC ® damage behavior, at low number of cycles, is accommodated by plastic cyclic strain. In approximating of the yield stress, plasticizing will be located at certain points of the alloy structure. The plastic strain curve presents an important slope; this reflects the high gradient of this parameter. This gradient is a sign of cracks priming. Approximation methods appear in agreement at the strain values. However, this agreement is not verified for the number of cycles in transition point P Tr . The universal slopes method seems most pertinent. The elastic line expressing SICLANIC ® resistance is located in the same order as the monotonic characteristics UTS and YS 0.2% . For the plastic line, the capacity of plastic cyclical strain is dependent on the ductility; it operates to the same effect as this last. In terms of total strain, the SICLANIC ® resistance is closely linked to materiel ductility at low cycle