A comparison between the plastic work energy criterion and the J-Integral fracture criterion in fatigue crack propagation
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This thesis attempts to show the correlation between the plastic work required for a unit area of fatigue crack propagation and the J—Integral method of analysis. Fatigue crack propagation rate equations are given for 2024—T351 aluminum as defined by the plastic work parameter,(U), and for Rice’s path independent J—Integral, (AJ). The value of AJ was determined from areas within the load-displacement hysteresis loops using a double cantilever clip-on extensometer. The value of U was obtained from areas within hysteresis loops using small foil strain gages that were attached at various positions with respect to the advancing crack tip. Due to the path independence of the J—Integral, it is felt that a definite fatigue crack propagation rate equation could be established for the elastic-plastic range. It is hoped that conclusions drawn from this research will provide further insights into the applicability of the plastic work parameter using the J-Integral concept in determining the fatigue crack rate equations for various metals in the elastic and plastic ranges. Previous research has been done with the J—Integral in determining propagation rate equations based on the assumption that the J-Integral properly describes the behavior of the crack tip. This has been assumed correct, even though the method is based upon data obtained from the load line, which is far from the propagating crack tip. On the other hand, the plastic work parameter is based upon data obtained directly from the crack tip. Since the two methods are shown to be theoretically equal (except for some constant) it is hoped that the plastic work parameter and the J-Integral are essentially quantitatively equal.
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