Issue 30

A. Carofalo et alii, Frattura ed Integrità Strutturale, 30 (2014) 349-359; DOI: 10.3221/IGF-ESIS.30.42 355 (a) (b) Figure 3: Correlation between resulting stress range and hysteresis area (a) and hysteresis area trend against fatigue life (b) . Fig. 4 reports an example of an hysteresis cycle, while the change of peak stress for two tests carried out at a strain range Δε/ Δε max = 0.44 is showed in Fig. 5. Material plastic behaviour is very limited and the stress range is practically constant for the whole life of the specimen, with the exclusion of the last cycles, when a fatigue crack initiation process is already started. Similar trends are obtained for TIG welded specimen and at high temperature also. Change of tangent modulus E T and hysteresis area H is generally considered an important indicator of the progress of the damage process in the material. Crack initiation has the effect to reduce specimen stiffness and a measurable reduction of the tangent modulus is showed. Hysteresis area represents the work associated to plastic deformation in each load cycle [20], which is partially stored in the material as a potential damage and partially dissipated as heating [21]. Therefore, these two parameters have been calculated starting from the whole data cycle and correlated to number of cycles for each specimen. An example of tangent modulus E T trend is reported in Fig. 6. Observing this last graph, it is possible to show that the decay of tangent modulus is always measurable only at the end of life, when a crack is already present. Consequently, this parameter has a limited interest to follow the damage evolution in the material. Moreover, the trends of hysteresis area H (Fig. 3b) have no practical utility, due to its low values. From a quantitative point of view, fatigue behaviour of base and TIG welded material is synthetically expressed by the fatigue curves in terms of applied strain range (Fig. 7) or in terms of measured stress range (Fig. 8). In all the conditions, TIG welded material shows a lower fatigue curve than base material. The reduction of fatigue strength can be quantified interpolating the experimental data to calculate Basquin’s law and finally the strain range Δε A and the stress range Δσ A corresponding to a reference life N ref (Tab. 6). A careful analysis of these data reveals that the fatigue strength expressed in terms of strain range Δε A is increased when temperature test changes from RT to 538°C. On the contrary, fatigue stress in terms of stress range Δσ A is reduced in the same condition. Figure 4: Example of stabilized hysteresis cycle.