International Journal of Scientific Engineering and Technology ISSN:2277-1581 Volume No.6 Issue No.2, pp: 56-60 01 Feb. 2017 IJSET@2017 Page 56 Prediction of Fatigue Life of 8090 Al-Alloy Using Beta Model J. R. Mohanty Department of Mechanical Engineering Veer Surendra Sai University of Terchnology, Burla, Sambalpur 768018, India, Email -, guddy95@gmail.com Abstract : In the present investigation, fatigue crack growth tests of 8090 Al-alloy have been conducted using compact tension (CT) specimen under constant amplitude loading. The experimental data have been smoothened by using exponential model. A new method called ‘Beta model’ has been devised and proposed to predict the fatigue life of the alloy from predicted crack lengths obtained from the model results. It has been observed that the overall growth rate of the model reasonably matches with the experimental values. Keywords: Fatigue crack growth rate; fatigue life; Beta model, stress intensity factor, maximum stress intensity factor, compact tension. 1. Introduction Fatigue is an important mode of failure in almost all engineering components. It results mainly due to variable loading or more precisely because of cyclic variations in the applied loading or induced stresses. Hence, fatigue life prediction is important in order to enhance the residual life of the structures/components from both economic and safety point of view. Conventional life prediction procedures are generally based on the safe-life approach. In this approach, components of a structure are replaced when the probability of failure reaches a prescribed level, even though some of them may have a significant residual life. Hence, it is a highly conservative approach coupled with a penalty on economy. To avoid this, the damage-tolerant approach is often a suitable alternative for life predictions. In this approach, the fatigue specimens are undergone several coupon tests by using fracture mechanics principle until complete fracture of the specimens take place in order to know the growth rate. Once the nature of fatigue crack growth rate is known, the schedule inspection intervals are set for suitable repair/replacement of parts in order to avoid catastrophic failure. It not only minimizes the cost but can save human life as well. However, the fatigue crack growth tests are very costly and time consuming. For example, most high strength steels take months together to complete a fatigue test. Hence, several prediction models have been suggested to predict the fatigue life of materials under various loading conditions. Among them the most primitive models are Paris model [1], Forman model [2], and Walker model [3]. However, each model has its own merits and demerits. The most important demerit in almost all deterministic models is that it involves robust numerical integration scheme in order to determine the fatigue life from crack growth rate equation. To avoid this, different soft-computing methods have been applied to predict the fatigue life. In the present work, a new method called ‘Beta model’ has been proposed to predict the fatigue life of 8090 Al-alloy under constant amplitude loading. It has been observed that the proposed model predicts the fatigue life with reasonable accuracy. 2. Material and Brief Experimental Procedure In the present study, the material used for fatigue crack growth rate (FCGR) study is 8090 Al-alloy with T651 heat treated condition. The compact tension (CT) specimens with a V-starter notch have been prepared from 12.5 mm thick plates in the longitudinal transverse (LT) direction as shown in Fig. 1. Fig. 1 – Compact tension (CT) specimen geometry Table 1: Composition (in % wt.) of 8090 T651 Al alloy Zn Mg Cu Fe Si Mn Ti Li Al 5.7 0.55 1.14 0.05 0.03 0.1 0.03 2.34 Bal. Table 2: Mechanical properties of 8090 T651 Al-alloy Young’s modulus (GPa) Yield strength (MPa) Tensile strength (MPa) Elongation (%) 81.2 430.0 480.0 13.0 The chemical composition and the mechanical properties of the alloy have been given in Table 1 and 2 respectively. Both the sides of the specimen surfaces have been mirror-polished by different grades of emery papers and