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.

I. Introduction

The structural elements in many microelectromechanical systems (MEMS)-based sensors and actuators, e.g., polysilicon beams in accelerometers and metal torsion bars in digital mirrors, are subjected to billions of load cycles without fatigue failures. However, some transducers require lifetimes of less than 1000 cycles—an example being safing-and-arming systems [1]. Fatigue testing is normally conducted under stress control for long lifetimes and strain control for short lifetimes, but to-date fatigue testing of MEMS structures has predominantly been conducted under stress control. This letter demonstrates an optical system for measuring and controlling cyclic strain in specimens of the same size scale as MEMS products.