The steady-state work density gradient: A new parameter and strategies for characterizing crack propagation in thin ductile sheets
Lanning, Wade Richard
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This dissertation presents a new parameter for characterizing the crack growth resistance of thin ductile sheets: the steady-state work density gradient. The steady-state work density gradient describes the stress necessary to drive crack propagation in the late stages of mode I crack growth as the crack tip approaches a specimen edge and crack propagation is at steady-state. This parameter was discovered by comparison of different specimen types (edge notch and middle notch) cut from thin (25.4, 50.8, and 127 µm thick) annealed aluminum sheet specimens using plots of nominal stress vs. normalized crack length. The steady-state work density gradient may also be used in conjunction with digital image correlation and tracking (DICT)-based strain increment maps to spatially resolve the work density along contours of constant strain which run from the crack tip to the specimen boundaries. Thus, the spatial distribution of energy absorption around a steady-state crack tip may be experimentally measured, and the magnitude of energy absorbed can be compared between different specimens. This dissertation demonstrates the application of the steady-state work density gradient to thin sheet systems, including the aforementioned aluminum sheets, 120 nm thick gold sheets, thin tin sheets, and thin copper sheets. The dissertation also explores other fracture toughness approaches such as the linear-elastic K parameter, the elastic-plastic J concept, and essential work of fracture (EWF) to give context to the steady-state work density gradient and its applicability to a variety of thin ductile systems.