1. Introduction

Fatigue can be defined as the process of permanent, progressive and localised structural change which occurs to a material point subjected to strains and stresses of variable amplitudes which produces cracks leading to total failure after a certain number of cycles. progressive loss of the material strength occurs as a function of the number of stress/strain cycles, reversion index, load amplitude, etc. Loss of strength may be interpreted as micro-cracking followed by crack coalescence leading to the final collapse of structural parts. Failure typically takes place for stress levels well below the strength limit of the material obtained in static tests.

Structural materials are, in general and at less from a microscopic point of view, non-homogeneous materials. In the case of metals, they include discontinuities such as grain boundaries, microscopic pores, and other particles as impurities and carbon. All of them are commonly considered as defects. This does not denote that those materials are defective and consequently useless. Defects are a normal feature of metallic material microstructure. However, defects are also source of local stress concentrations inducing local plasticity and/or damage and therefore they can turn into fatigue crack initiation sites.

According to their location, defects can be classified as internal or external. External or surface defects can be introduced through component assembly, impact of foreign objects or material processing. They can...