The effect of stress ratio, maximum stress, stress amplitude and yield strength of materials on fatigue crack closure of 2024 aluminum and 4130 steel
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Crack closure presented by Elber in 1968 was hoped to modify Paris’ Law in analyzing fatigue crack propagation. Validity of crack closure phenomenon was investigated by many researchers and conflicting results were presented in the literature. This experiment was conducted to investigate crack closure phenomenon more thoroughly and to identify the governing factors involved. Two materials were used, 2024 aluminum and 4130 steel, each at two heat treatment conditions. Tests were conducted under three loading conditions, constant stress ratio, constant maximum stress and constant stress amplitude. All specimens were loaded on the axis parallel to the rolling direction, sinusoidally at constant amplitude at room temperature and in laboratory air. Crack closure phenomenon was found to be present in both steel and aluminum alloy. It was also found that crack closure is dependent on fatigue crack length. Decreasing yield strength of the materials used increased the crack closure level. Increase in maximum stress increased crack closure load where altering the stress amplitude did not have a significant effect on crack closure. It can be concluded that crack closure is a function of plastic zone size at the tip of a growing fatigue crack. Crack closure is a plane stress phenomenon, but it also exists in plane strain condition.
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