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A study was conducted to investigate the effects of surface microhardness on different phases of fatigue damage. This helps to estimate the evolution of the material resistance from microplastic distortions and gives pertinent data about cumulated fatigue damage. The objective of this work is to propose a damage criterion, associated with microstructural changes, to predict the fatigue life of steel structures submitted to cyclic loads before macroscopic cracking. Instrumented indentation tests (IIT) were conducted on test samples submitted to high cycle fatigue (HCF) loads. To evaluate the role of the microstructure initial state, the material was considered in two different conditions: as-received and annealed. It was observed that significant changes in the microhardness values happened at the surface and subsurface of the material, up to 2 µm of indentation depth, and around 21% and 7% of the fatigue life for as-received and annealed conditions, respectively. These percentages were identified as a critical period for microstructural changes, which was taken as a reference in a damage criterion to predict the number of cycles to fatigue failure (Nf) of a steel structure.

The hardness of a material shows its ability to resist to microplastic deformation caused by indentation or penetration and is closely related to the plastic slip capacity of the material. Therefore, it could be significant to study the resistance to microplastic deformations based on microhardness changes on the surface, and the associated accumulation of fatigue damage. The present work is part of a research study being carried out with the aim of proposing a new method based on microstructural changes, represented by a fatigue damage indicator, to predict fatigue life of steel structures submitted to cyclic loads, before macroscopic cracking. Here, Berkovich indentation tests were carried out in the samples previously submitted to high cycle fatigue (HCF) tests. It was observed that the major changes in the microhardness values occurred at the surface of the material below 3 μm of indentation depth, and around 20% of the fatigue life of the material, proving that microcracking is a surface phenomenon. So, the results obtained for the surface of the specimen and at the beginning of the fatigue life of the material will be considered in the proposal of a new method to estimate the fatigue life of metal structures.

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