A laser-based interferometric technique(which is named ISDG : laser Interferometric Strain/Displacement Gage) was used to measure displacements on a center cracked tension AL 7039-T61 specimen. The displacement was measured between two indentations 300㎛ apart(one on each side of an existing initial notch tip )under fatigue loading.
The theory and process of experimental setup is described, and a crack opening point is detected.