SPECIMEN CONFIGURATION
The specimens alloy has volume percentages of 70% and 30% respectively for the ferrite and pearlite phases. The mixture rule concludes average values for \(\sigma_{y}\) equal to 275.9 MPa,\(=\sigma_{y1}\), for the ferrite grains and 689.7 MPa, \({=\sigma}_{y2}\), for the pearlite grains. Search in the literature shows that values up to\(\sigma_{y1}=\ \)312 MPa and \(\sigma_{y2}=\ \)1020 MPa can be found. Thus, standard deviations of 35.9 MPa and 331 MPa are invoked for\(\sigma_{\text{y\ }}\)of the ferrite and pearlite grains respectively. Variations in \(\sigma_{y}\) of the grains of each phase are assumed randomly distributed between the minimum and maximum values of the corresponding phase. The cyclic yield stress of steel is approximately equal to 0.6 of its monotonic yield stress 11, 12, 5, 53-55. Following that, a grain is assumed having a value of 0.6 for the ratio of its cyclic and monotonic yield stresses.
A mean grain size of 36 µm is assumed for both phases of the alloy with 13 µm standard deviation. This accommodates surface 445 ferrite grains and 191 pearlite grains arranged along the circumference of the specimen minimum cross section.
The specimen surface roughness resembles micro cracks of different sizes and locations. The model assumes the existence of multi coplanar micro cracks along the specimen minimum circumference. The starting maximum crack length is 0.1 µm and the crack lengths follow a normal distribution.
Different virtual specimens were randomly prepared with the above configuration. A virtual test starts with one of those specimens. A specimen is subjected to a cyclic axial load with the parameters listed in Table1.