Three-Dimensional Aspects of Fatigue Crack Closure in Surface Flaws in Polymethylmethacrylate Material

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Abstract
An experimental investigation using a Newton interference method with a transparent polymethylmethacrylate (PMMA) to measure crack opening displacement and closure on surface flaw crack growth is described. Fatigue crack growth tests are conducted on rectangular plates loaded in bending using optical interferometry measurements to map the crack surface opening displacement profiles and define crack opening loads. Results are presented for an experiment of constant applied stress intensity factor along the crack face free surface. The crack opening displacement patterns and measured closure loads are found to be strongly influenced by the formation of a void internal to the crack surfaces. Three distinct closure loads are identified and measured for surface flaws of various crack lengths. Upon loading the specimen, the first closure load is defined as the load that initially opens the crack free surface. A second closure load corresponds to the opening of the crack tip in the depth direction along the axis of symmetry. The third closure load corresponds to opening of the crack tip along the entire boundary. The crack closure loads, applied and effective stress intensity factors, and crack growth rates are presented as a function of crack length.

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