An Optical Method for Strain-Controlled Fatigue Testing of Microspecimens

This letter presents an initial version of an optical method for measuring and controlling strain over very short gauge lengths. Reflective indentations which are 200 μm apart are put in a steel microspecimen which is 3 mm long. When illuminated with a laser, interference fringes are produced; their motion is monitored with photomultiplier tubes. A computer system converts fringe motion to strain and controls the test. The specimen preparation and measurement system is described, and the control of peak cyclic strain at 1140 ± 10 μstrain is demonstrated.