Dynamic stress analysis by high-resolution reflection moire photography

Abstract

A high-resolution moiré photography system is described in which a reflection phase grating attached to a specimen is imaged onto a reference grating to form moiré fringes. A tuned camera lens has been used to resolve grating frequencies of 150 lines mm^-1, giving a sensitivity of 6.7 μm fringe^-1 and to improve fringe contrast. The fringes are formed in real time and have been recorded with a high-speed image converter camera at framing rates of up to 10⁶ frames s^-1 with a field of view of up to 35 mm diameter. Sequences of fringe patterns have been analyzed by a 2-D Fourier transform method to obtain the dynamic displacement field with submicrometer accuracy. A field lens has been used to collimate the light incident on the specimen, and it is demonstrated that this makes the system insensitive to out-of-plane specimen motion, avoiding fictitious strains. An example is presented of the application of the technique to the measurement of the mode I stress intensity factor in a cracked polymethyl methacrylate (PMMA) plate under stress pulse loading.