Detection of subsurface cracks and delaminations in polymeric materials by electron thermoelastic acoustic microscopy

Abstract

Electron Thermoelastic Acoustic Microscopy (ETEAM), also termed Electron Acoustic Microscopy and Thermal Wave Microscopy, detects subsurface cracks, delaminations, and voids at depths up to at least 100 µm, as first reported by Cargill, The technique has previously been applied only to beam-insensitive materials, since high beam currents of about 1 µamp (1000 times greater than for routine SEM) are required to obtain adequate signal intensity. We present here a procedure for extending the technique to beam-sensitive polymeric materials.In the ETEAM technique, a periodic electron beam (frequency V = 10 kHz-10 MHz) generates an acoustic wave, leading to a periodic force on a transducer attached to the specimen. The ETEAM image is derived from the transducer voltage output as the electron beam is scanned over the sample in an SEM. The inability of an acoustic wave to travel through a delamination results in it being transmitted around the delamination as a transverse shear wave (Fig. 1).