Advanced introduction to fatigue

1. INTRODUCTION

Fatigue is commonly referred to as a process in which damage is accumulated in a material undergoing fluctuating loading, eventually resulting in failure even if the maximum load is well below the elastic limit of the material. Fatigue is a process of local strength reduction that occurs in engineering materials such as metallic alloys, polymers, and composites, e.g. concrete and fibre reinforced plastics. Although the phenomenological details of the process may differ from one material to another, the following definition given by ASTM [1] encompasses fatigue failures in all materials:

"Fatigue - the process of progressive localised permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations."

In EN 1993-1-9- Eurocode 3: “Design of steel structures” - Part 1-9: Fatigue [3], fatigue is defined as “the process of initiation and propagation of cracks through a structural part due to action of fluctuating stress.”

The important features of the process relevant to fatigue in metallic materials are indicated by the bolded words in the definition above. Fatigue is a progressive process in which the damage develops slowly in the early stages and accelerates very quickly towards the end. Thus, the first stage consists of a crack initiation phase, which for smooth and mildly notched parts that are subjected to small loads cycles may occupy more than 90 per cent of the life. In most case cases, the initiation process is confined to a small area, usually of high local stress, where the damage accumulates during stressing. In adjacent parts of the components, with only slightly lower stresses, no fatigue damage may occur, and these parts thus have an infinite fatigue life. The initiation process usually results in a number of micro-cracks that may grow more or less independently until one crack becomes dominant through a coalescence process as the microcracks start to interact. Under steady fatigue loading, this crack grows slowly, but starts to accelerate when the reduction of the cross section increases the local stress field near the crack front. Final failure occurs as an unstable fracture when the remaining area is too small to support the load. These stages in the fatigue process can in many cases be related to distinctive features of the fracture surface of components that have failed under fluctuating loads. The presence of these features can therefore be used to identify fatigue as the probable cause of failure.