Issue 43

B. Saadouki et alii, Frattura ed Integrità Strutturale, 43 (2018) 133-145; DOI: 10.3221/IGF-ESIS.43.10 136 Figure 3 : Tensile repeated solicitation on fatigue test. E (GPa) YS 0.2% (MPa) UTS (MPa) A (%) n K (MPa) 121 498 615 11.62 0.034 591 Table 1 : Monotonic tensile characteristics of the SICLANIC ® . Furthermore, the remarkable tensile strength of 615 MPa enhanced by the presence of silicon, the SICLANIC ® has an elongation at fracture of higher than 10%. This ductility is mainly due to the high temperature of the solution treatment- quench phase during the hardening treatment. The progressive passage, in the stress-strain curve, from linear elasticity to plastic deformation requires the definition of the conventional yield strength YS 0.2% which is of the order of 498 MPa. From this stress value, we realize a low permanent elongation which can be measured accurately. Mechanical behavior ceases to be elastic to become plastic. S-N curve SICLANIC ® fatigue results show significant dispersion. This dispersion is now considered as one of physical fatigue aspects [19]. Indeed, for the same material, and the same load level, the lifetime may be different. Iso-probabilistic S-N curve As it is, the fatigue curve gives a tendency on the material behavior, but it is of limited use: we know that for a given stress level, half the specimens break for cycle number less than N(σ) ,the other half breaks for cycle number more than N(σ) . For given stress level, the ratio between maximum and minimum value of failure cycle can exceed 10. In a probabilistic concept, Wöhler curve represents the border separating the area where failure is less likely (left of the curve) to the area where failure is most likely (right part of the curve). The fatigue life is generally described by the statistical longevity curve called iso-probabilistic Wöhler curve or median curve (N50, that to say 50% of survival specimens) [20]. For the SICLANIC ® , fatigue curve median is illustrated in Fig. 4. Probabilistic S-N curve The confidence interval method using the probability of Student's law is also frequent to characterize fatigue behavior. It does not estimate directly the number of cycle to failure, but the estimations have percentage chance to containing the N f .Thus, it is interesting to work in confidence interval of 80% that will give proper framing 80 times out of 100 on average, which meaning, if we could repeat the same tests a large number of times, affirming each time than N f is in this interval, we would be wrong 20 times out of 100 on average. Fig. 5 shows SICLANIC ® probabilistic Wöhler curve corresponding at 80% of survival, such a curve is sometimes called “curve design”.