The effects of simple load histories and specimen sizes on crack closure measurements are presented. Tests are conducted on compact type C(T) specimens having various thicknesses and in-plane dimensions. The simple load histories are composed of constant amplitude load and stress intensity ranges. Closure load is determined from data collected from three measurement techniques. A clip gage is used to determine the load-displacement behavior at the crack mouth. A strain gage placed on the backface of the C(T) specimen is used to determine the load-strain behavior. The third technique uses a laser interferometric displacement system where load-displacement data are collected at various locations on the side of the specimen in the vicinity of the crack tip.
The results indicate that the closure load determined by the interferometric system at various locations along the length of the crack is not uniquely determined. Even though the interferometric technique provides a resolution of displacement less than 0.01 μm, the sensitivity of the technique impacts the evaluation of the closure load. Under constant amplitude stress intensity loading conditions, the values of the closure load varied with crack length and with in-plane and thickness dimensions of the test specimens.