Definition Nature of Fatigue Fatigue under ambient and elevated temperature

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2.4 Fatigue

2.4.1 Definition

The process of progressive localized permanent structural damage occurring in a material subjected to conditions that produce fluctuating stress and strain at some points and that may culminate in cracks or complete fracture after sufficient number of fluctuations Liu 2005 Fatigue is one of the principle damage mechanisms for materials operating at elevated temperatures. Fatigue at elevated temperature produces larger strain deformation, crack initiation and growth for the material under elevated fatigue, there is serious influence on the properties and fatigue life of the material Mao Mahadevan 2000

2.4.2 Nature of Fatigue

Fatigue failure resulted from simultaneous action of cyclic stress, tensile stress and plastic strain. The crack will not initiate and propagates if there is none of these stress and strain. Fatigue can be stress based, strain based, and fracture mechanics based Liu 2005. Plastic strain initiates the cracks; while the tensile stress promotes crack growth propagation. Fatigue also behaves differently with elevated temperature, as high temperature exposure in services results in a progressive loss of fatigue strength due to the over-aging phenomenon with crack initiation and propagation Joyce, Styles Reed 2003 9

2.4.3 Fatigue under ambient and elevated temperature

When tested at ambient temperature and an elevated temperature of 190 C, aluminium tensile behavior increases as the temperature increases. As the temperature is increased the atomic bonds are weakened as the atoms are energized Leong 2008. This will result in the nucleation of micro inter-granular voids as shown in Figure 2. Figure 2 : Micro inter-granular void nucleation to failure process Anderson, 1995 10 At ambient temperature, fracture occurs in a brittle manner. The fine microscopic cracks grow into multiple macroscopic cracks which propagate in the direction parallel to the stress axis. At elevated temperatures, the fracture surface contains both trans granular and inter-granular failure. The quality of alloy aluminium that is tested will determine the results obtained. Ultimately, the dominant failure process is due to the bimodal failure of both trans granular and inter-granular failure in which is more susceptible at high temperature. 11 CHAPTER 3 METHODOLOGY

3.1 Introduction