MILLIMETER-WAVE IMAGING FOR NONDESTRUCTIVE EVALUATION OF MATERIALS

A millimeter-wave imaging system has been developed in the W band (75- 110 GHz) for nondestructive evaluation of low-loss materials. The syst em employs a focused beam to provide spatial resolution of about one w avelength. A plane-wave model is used to calculate the effective refle ction (or transmission) coefficient of a multilayer geometry. Theoreti cal analysis is used to optimize the measurement frequency for higher image contrast and to interpret the experimental results. Both reflect ion and transmission images, based on backscattered and forward-scatte red powers, were made with Kevlar/epoxy samples containing artificiall y introduced defects such as subsurface voids and disbonds. The result s indicate that millimeter wave imaging has high potential for noncont act detection of defects in low-loss materials.