Issue 30

A. Carofalo et alii, Frattura ed Integrità Strutturale, 30 (2014) 349-359; DOI: 10.3221/IGF-ESIS.30.42 356 (a) (b) Figure 5: Peak stress trend of a base (a) and TIG welded specimen (b) . Figure 6: Example of tangent modulus E T trend This behaviour is clearly confirmed by the experimental data both for base and TIG welded material. However, not only this observation is not reported in any experimental work existing in literature, but also it is not verified for other experimental data obtained using standard circular specimen. It is very difficult to indicate a clear explanation, but some hypothesis can be formulated. A possibility is to invoke a complex interaction between fatigue and creep damage, which become relevant when applied strain is so low that the load cycle can be considered fully elastic and the test duration is over 24 hours. Another possibility is to consider that the specimens have been obtained by a laminated plate rather than round bars, as usual: the different influence of the residual stress field existing in the laminated plate could shows its detrimental effect only at Room Temperature, while at higher temperature relaxation phenomena could mitigate the effects. However, no quantitative data or qualitative observations are available to support one hypothesis with respect to others and this constitutes an open problem. Creep test. Creep strength of base and TIG welded material has been expressed in terms of time to reach a fixed inelastic strain or failure (Tab. 7). Also in this case data reported in Tab. 3 are normalized with respect to the maximum measured life and represents the mean values over 3 specimens. It is interesting to observe that TIG specimens show a lower time to reach a fixed strain or failure but only if the lower stress level is considered. At a stress level of 475 N/mm 2 , creep strength is