Real-Time Measurement of Small-Crack Opening Behavior Using an Interferometric Strain/Displacement Gage

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Abstract
A procedure for real-time measurement of the opening displacement of small fatigue cracks is reviewed. The method employs a computerized, laser-based, interferometric strain/displacement gage (ISDG) to monitor the relative displacement between two tiny indentations placed across small surface cracks. Complete load versus crack opening displacement curves are obtained for surface cracks of length ranging from 50 μm to several millimetres. The method is applicable to naturally or artificially initiated cracks in smooth or notched specimens. The load-displacement measurements provide a detailed record of crack closure behavior, and estimates of crack size are obtained from measurements of elastic compliance. The resolution of the opening displacement measurements is on the order of 5 nm. Computerization of the system allows measurements to be obtained in real time, facilitating feedback control of testing. Four versions of the ISDG that are used in different laboratories are described. The instrument's characteristics are presented along with some practical considerations associated with its use. Examples from studies of a number of materials are included to demonstrate applications of the ISDG to the measurement of closure, crack shape, and to automation of small-crack testing.