A Laser Interferometric Strain/Displacement Gauge(ISDG) was used to measure the crack opening load with Al 7075-T6 CCT specimen. The guage consits of laser and two photodiode. Indentations are marked with a diamond pyramid indenter using microhardness tester and are 200㎛ apart. Coherent, monochromatic light from a 10mW He-Ne laser incident on these indentations will produce fringe pattems. The position of these pattern in space is related to the distance between the two indentations. As this distance changes, the fringes shift. Measurement of these fringe shifts enables one to determine the local strain of specimen.
Laser Interferometric Strain/Displacement Gauge has the following features : a very short guage length, no electrical or mechanical contact, no temperature sensitivity, measurement of displacement near the crack tip, relatively easy application.
In this paper, procedure for measure the crack opening load using Laser Interferometric Strain/Displacement Gauge is described. It was found that Laser Interferometric Strain/Displacement Gauge can be used to measure the crack closure and that crack closure concept must be considered when crack growth rate is expected.