Low Cycle Fatigue Analysis of Aluminum Alloy and Metal Matrix Composites

Abstract

The exceptional increase in strength obtained by precipitation hardening, the Al–Mg–Si (6xxx) alloys are mostly used in extruded aluminum products, construction, turbine blades and automotive purposes. In low cycle fatigue the applied strain have a significant plastic component and resultant lives falls between 10 to 105. Strain based approach is suitable for low cycle fatigue analysis as strain can be measured and has been shown to be an excellent quantity for correlating with low cycle fatigue. For example gas turbines operate at fairly steady stresses, during starting or stopping they are subjected to a very high stress range. The local stress can be well above the yield stress, and the stresses are more difficult to measure or estimate than the strains. The aim of the present thesis is to study the effect of heat treatment temperature and soaking time on the low cycle fatigue performance and monotonic tensile properties of AA6063 alloy as well as effect of reinforcement particle volume fraction on low cycle performance and monotonic tensile properties of AA6063/SiCp MMC. In addition, present thesis also investigates the effect of strain amplitude on the low cycle fatigue performance studied by simply support experiment. The low cycle fatigue parameters obtained experimentally are also verified by theoretical and numerical methods. The experimental values of low cycle fatigue parameters viz. cyclic strain exponent and cyclic strength coefficient are in good agreement with that obtained by theoretical method. The rotating cantilever low cycle fatigue analysis is also performed by finite element analysis using ANSYS software. The elastic and plastic strain on the surface of the specimen along the cross section of failure obtained by finite element analysis are in good agreement with the experimental values for all cases. Transition fatigue life increases considerably with the increase in heat treatment temperature for constant soaking time and with the increase in volume fraction of reinforcement particle in MMC while it initially increases considerably with the increase in soaking time of heat treatment at constant temperature but after certain time decreases considerably. Fatigue ductility exponent has increasing tendency with the increase in heat treatment temperature for constant soaking time as well as with soaking time of heat treatment at constant temperature whereas initially increases with volume fraction of reinforcement particle in MMC but after certain volume fraction decreases. Fatigue ductility coefficient or cyclic plastic strain behavior is opposite to that of fatigue ductility exponent. Fatigue strength exponent and cyclic strength coefficient have decreasing tendency with the increase in heat treatment temperature for constant soaking time but have increasing tendency with soaking time of heat treatment at constant temperature whereas initially increase with volume fraction of reinforcement particle in MMC but after certain volume fraction decrease. Fatigue strength coefficient has decreasing tendency with the increase in heat treatment temperature for constant soaking time but initially decreases with soaking time of heat treatment at constant temperature but after certain time increases whereas initially increases with volume fraction of reinforcement particle in MMC but after certain volume fraction increases. Cyclic strain exponent has opposite behavior than fatigue strength coefficient with respect to heat treatment temperature for constant soaking time and soaking time of heat treatment at constant temperature except volume fraction of reinforcement particle in MMC. The energy dissipation per cycle of loading for low cycle fatigue analysis using simply support specimen increases with increase in strain amplitude. Out of six cases of strain amplitude two cases show strain hardening behavior which is evident from the fact of increase of stress with number of cycles. This observation is in contrary to the criteria of softening behavior of σUTS/σYS < 1.2. This implies that cyclic softening or hardening behavior during low cycle fatigue analysis using simply support specimen is strongly related to strain amplitude applied during the experiment. Crystallite size initially increases with increase in heat treatment temperature at constant soaking time but decreases after certain temperature and with increase in volume fraction of reinforcement particle in MMC while it has decreasing tendency with increase in soaking time at constant heat treatment temperature. Observations of SEM pictures of fracture surfaces show low cycle fatigue features like crack initiation, ratchet marks, fatigue striations, fatigue cleavages, beach marks, progression marks and overload zones.