Issue 49

S. Pereira et alii, Frattura ed Integrità Strutturale, 49 (2019) 450-462; DOI: 10.3221/IGF-ESIS.49.43 456 For the specimens with 0.25mm diameter, the fatigue tests were conducted with a strain ranging from 0.6% to 3%. The specimens with strain under 1%, that correspond to the austenitic region, show a large fatigue life, when compared with the rest of points with some specimens showing an infinite fatigue life. The specimens with strains equal to and above 1% show a decreasing fatigue life as the maximum imposed strain increases. The specimen with the shortest life time failed after 1989 cycles. Figure 6 : Uni-axial tension test with one hysteresis cycle, with the starting and final stress values for the forward and reverse transformations (the is not reached in this test, because it marks the beginning of the plastic deformation regions). Figure 7 : Surface Strain vs Number of Cycles. The surface strain is estimated using Eqn. (3). It is also possible to see from Fig. 7 that all the alloys present an infinite life only when in austenitic phase, but not when under all strains during this phase. For a finite life time, the alloys tend to increase the life time with a decreasing diameter. This life time increase can be explained as a result of the energy absorption experienced by the material during the deformation. The energy per volume is given, for a uniaxial tension test, by the Eqn. (4) and, as the material is under the same strain, the energy absorption will increase with the volume.