Proceedings of the National Conference on Trends and Advances in Mechanical Engineering, YMCA Institute of Engineering, Faridabad, Haryana.., Dec 9-10, 2006. ANALYSIS OF MIXED MODE CRACK INITIATION ANGLE AND FATIGUE CRACK GROWTH D.K. Rao 1 , Dr Anadi Misra 2 , V. K. Singh 3 1,2,3 Department of Mechanical Engineering, College of Technology, G. B. Pant University of Agriculture & Technology, Pantnagar -263145 3 E-mail: vks2319@yahoo.co.in Abstract Fracture mechanics has been established as an important principle dealing with the growth of fatigue crack. During recent years, the fracture mechanics has obtained a considerable importance for studding the crack growth behavior under static and fatigue loading. The largest number of in-service failures results from the scenario involving fluctuating, cyclic or repeated loading, usually combination with static load. Typically, a defect located near a stress raiser enlarges under repeated tensile stresses into a crack that continues to grow stably a small amount during each load cycle until the crack either penetrates the structure or attains critical length, after which unstable crack propagation occurs. The present investigation has been under taken keeping in mind some of the problems concerning the crack propagation direction and growth under constant amplitude load in an inclined crack geometry. The present studies mainly focus on to develop and modified the crack growth criterion to account of the effect of biaxial factor, shearing loading and number of stress terms for different criterion. The effect of one, two and three solution on crack initiation angle determined on the basis of DET-criterion & MTS criterion. 1.0 Introduction The majority of the research done in the area of fracture mechanics was accomplished in the opening mode only. Hence the practical application of the fracture mechanics is limited. The mixed mode crack growth problem has drawn comparably little attention, but is actually more realistic and possibly more dangerous than mode-I crack. The mixed mode growth occurs when a fatigue crack is nucleated along the inclusions or welded defects located making an angle with the axis of the applied load. The fatigue crack growth under multi axial stress is to be considering as mixed mode. The mixed mode cracks generally propagate in a non-self similar manner. Hence, in case of mixed mode loading condition, the study of the crack initiation angle and crack growth rate is of equal importance. The predictions of mixed mode crack growth direction, different criterion have been proposed. Using the stress as parameter, MTS- criterion proposed by Erdigan & Sih (1963) is based on the assumption that the material behaves ideally brittle. This assumption is not supported by all kind of material. SED-criterion of Sih (1974) is based on the minimum strain energy density principle. This criterion corresponds to maximum potential energy, which is regarded as the integral of field variable over some domain rather than a point and is insensitive to sign of the loading. T-criterion proposed by Theocaries et. al. (1982) is also based on the SED-criterion, which is modified by using a variable radius for core region. Papadopoulos (1987) proposed the DET-criterion is based on determinants of the stress tensor. The predicted crack initiation angles by DET-criterion are found to be much higher relative to the one obtained by the MTS or the SED criterion. The theoretical results obtained for different loading cases are shown and discussed by Khan et. al. (2000). They have shown that the result for MTS-criterion and T- criterion match exactly for all loading cases, although both have been derived on different theoretical basis. Prais (1963) co-relate the rate of crack growth to the stress intensity factor for simple opening (mode-I). Robert and Kibla (1971) indicated in their result that the fatigue crack under mixed loading grew in a manner which did not reduce the mode-II component of load to zero and that the growth rate was accelerating by mode-II components of the stress applied simultaneously. A considerable amount of the work has been done to define governing mixed mode loading and crack growth behavior. 1