CONCLUSIONS
The present work is an extension of applying a previously developed fracture mechanics cracking damage model to assess the fatigue lifetime of un-notched round specimens made of a two phase ferrite-pearlite 0.4C-70/30 carbon steel alloy in the cases of (a) two-step fully reversed axial loading with low-to-high and high-to-low sequences and (b) repeated application of fully reversed two-step axial loading blocks. The model simulates collective behavior of growing short fatigue cracks originating from the specimen surface roughness. The model is applied on different specimens with surface grains randomly configured in terms of their phase, size, strength and included crack size. The model identifies the development of possible activities of surface short cracks against loading cycles due to the application of the analyzed constant and variable amplitude loading. Comparison of published experimental fatigue lifetimes on ferritic-pearlitic steel specimens in fully reversed variable amplitude loading and the corresponding prediction shows fair agreement.