A fracture mechanics numerical model is developed to simulate the collective behavior of growing short fatigue cracks originating from the surface of unnotched round specimens made of a two-phase alloy. The specimen surface roughness is considered resembling microcracks of different sizes and locations along the minimum specimen circumference. Material grains of different phases, sizes, and strengths are randomly distributed over that circumference.Variations in mechanical and microstructural features of grains are randomly distributed. Possible activities of surface cracks are predicted against loading cycles till either fracture occurs or all existing cracks become nonpropagating.The material's S-N curve and fatigue limit can, thus, be assessed. Published experimental data on ferritic-pearlitic steel specimens in push-pull constant amplitude loading (CAL) were utilized. Different specimens were randomly configured and virtually tested. Comparison of experimental results and corresponding predictions validates the model, which, further, recognizes the effect of surface roughness, specimen size, and mean stress on lives. K E Y W O R D Sendurance curve, ferritic-pearlitic steel, fracture mechanics, push-pull loading, short fatigue cracks, surface roughness

The present work is an extension of a previously developed fracture mechanics cracking damage model and highlights the ability of that model to predict the fatigue lifetime of un‐notched round specimens made of a ferrite–pearlite 0.4C‐70/30 carbon steel 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. This model numerically simulates the collective behavior of growing short fatigue cracks originating from the specimen surface. The surface roughness is assumed to resemble microcracks of different sizes and locations along the minimum specimen circumference. Material grains of different phases, sizes, and strengths are randomly distributed over that circumference. Possible activities of surface cracks are predicted against loading cycles till a fracture occurs. Published experimental data on ferritic‐pearlitic steel specimens in fully reversed variable amplitude loading are utilized. Different specimen tests are randomly configured and simulated. The present predictions are in fair agreement with the corresponding experimental results.

This work presents a cracking damage model to assess fatigue lives of un-notched round specimens in axial stresses due to constant and variable amplitude loading. The model numerically simulates collective behavior of growing short fatigue surface cracks originating from the surface roughness of specimens made of a two-phase alloy. The specimen roughness is assumed resembling micro cracks of different sizes and locations along the minimum specimen circumference. Material grains of different phases, sizes and strengths are randomly distributed over that circumference. To verify the model, this work utilized published experimental data on round specimens made of ferritic-pearlitic steel and tested in push-pull constant and variable amplitude loading with (a) two-step high-low and low-high sequences, (b) repeated two-step loading blocks. To simulate laboratory testing, different specimens were randomly configured and virtually tested. Comparison of the experimental results and corresponding predictions shows the validity of the model.