Issue 43

B. Saadouki et alii, Frattura ed Integrità Strutturale, 43 (2018) 133-145; DOI: 10.3221/IGF-ESIS.43.10 138 Figure 6 : Fracture surface of the specimen. Figure 7 : Fatigue failure initiation site on cleavage planes of global dimension highlighted in the white box have approximately 426 µm ×284 µm. Low cycle fatigue behavior Cyclical behavior law The comparison between the cyclic behavior curve and monotonic behavior curve reveals a softening during the cyclic loading (Fig. 8). The conventional yield strength falling of 19% in the cyclic tensile. Hardening or softening phenomena depend on the materials and their heat treatments. So, the evolution of fatigue mechanical characteristics depends on the initial state of the solid solution [21]. Softening is generally observed for steels whose structures are naturally work hardened by quenching and tempering, this softening corresponds to a decrease of the yield strength which can reach 30%. Similarly, SICLANIC ® structural hardening by heat treatment of quenching and tempering leads to a stable phase represents a cyclical softening. The softening of SICLANIC ® is similar to the others Cu-Ni-Si alloys behavior in low cycle fatigue, such behavior is usually attributed to the formation of intense bands of slip in which the precipitation hardening effect has been reduced [2]. In structural analysis, design of parts is based on yield strength YS , however, for SICLANIC ® , which has a cyclic softening in fatigue solicitation ( YS’ 0.2 < YS 0. 2 ), even a cycle level of σ a < YS 0.2 , could result a deformation higher than ε max and placing the piece out of service. The following equation describes the cyclic behavior:      ' ' n K (6)