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Fatigue is an important parameter in studying the behaviour of materials subjected to constant and variable amplitude loading [1]. These material which are regularly subjected to cyclic loading are prone to fatigue damage, which often start at the surface due to localized stress concentrations caused by machining marks, exposed inclusions or the contrasting movement of dislocations. Fatigue prediction is an important way to improve service performance and control failure particularly where safety is paramount. Components of machines, vehicles and structures are frequently subjected to cyclic loads, which in some cases may lead to their failure due to fatigue [2]. According to Bannantine et al. [3] fatigue failure involves a multi-stage processes that begins with crack initiation, followed by a progressive crack growth across the part with continued cyclic loading, and finally the sudden fracture of the component or specimen. Crack initiation and propagation, in most cases, can be attributed to surface integrity produced by machining [4]. Fatemi and Yang [2] reported that there are three commonly recognized forms of fatigue namely; high cycle fatigue (HCF), low cycle fatigue (LCF) and thermal mechanical fatigue (TMF). The average surface roughness (Ra) value might vary from 0.2 microns for a good polish to 8 microns for a rough turned finish [5]. Fatigue cracks initiate predominantly at the free surface of a material, and the condition of the surface can be assumed to be critical with regard to fatigue crack initiation [6]. Since the surface finish of the materials can be controlled during machining, it is often considered by manufacturers in order to increase the life of the products [7]. Various kinds of surface effects can be of great importance to the fatigue life. Surface effects include all conditions capable of reducing or enhancing the crack initiation period [8]. Bayoumi [9] studied the effect of surface finish on fatigue strength. Surface roughness of each group of specimens was measured and the quality of surface evaluated from the profile graph. Low-cycle fatigue tests subject specimens to repeated stress or strain until failure occurs at a relatively small number of cycles [10]. The upper limit in low-cycle life has generally been selected arbitrarily by individual investigators to lie in the range of 10 2 to 10 5 cycles. A study on effect of low cycle fatigue on surface finish was conducted by Kuroda and Marrow [11]. Nitriding is a thermo chemical surface treatment in which nitrogen is transferred from an ammonia atmosphere into the surface of steels at temperatures within the ferrite and carbide phase region [12, 13]. The fatigue life prediction of components have drawn more interests in many fields, in recent times. Reliable life prediction method Abstract: The effects of surface roughness and nitriding treatment on fatigue failure of SAE1035 steel were investigated. After nitriding heat treatments, the fatigue strength of the various specimen having different finishes further improved. For the three average surface roughness (Ra) considered, three equations which describe the S-N curves were established. The application of these equations to specimens tested under cumulative fatigue damage shows that the roughness parameter must not be ignored. Hence a model considering this parameter is formulated. From the model, the fatigue strength predictions were in good agreement with the experimental results.

A Fatigue Strength Prediction Model for Surface Roughness and Nitriding Effectson SAE 1035 Steel