The influence of the environment on the plastic zone of a propagating fatigue crack tip in aluminum and steel
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Crack closure measurements were used in an effort to measure changes in bulk plasticity due to changes in environment at the tip of a propagating fatigue crack. The study was done to determine if there are changes in the residual stresses in the plastically deformed region as a function of the two environments which were vacuum and humid bottled air. The materials investigated were 2024-T351 aluminum alloy and 4130AQ steel. Crack closure measurements were taken with a surface mounted displacement gauge located at the middle of a center cracked panel. The environment clearly has an influence on the fatigue crack growth rate. In vacuum, crack growth rate is slower than in air. However crack opening stress levels did not reveal changes with respect to various environments for the steel and the aluminum materials. Thus the results of this research suggest that no changes occur in the plastic zone size with respect to the influence of the environment which in turn implies that the yield strength of the plastic zone material has not been influenced by environmental effects. Therefore, the observation seen in this work demonstrates no environmental influence on the fatigue crack tip plasticity in either the 2024-T551 material or the 4130AQ steel.
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