Overview of microwave NDE applied to thick composites

Application of composite materials in a wide variety of areas continues to grow at a high rate. Advanced engineering and manufacturing approaches have promoted composite uses when thick section components are required. These strong, light-weight materials offer many benefits over traditional monolithic materials. However, they also present a significant challenge when nondestructive evaluation (NDE) methods are applied. NDE difficulties arise from inherent composite material properties, for example, anisotropy, inhomogeniety, and acceptable flaws and defects resulting from manufacturing or induced in service. In addition, most thick composite materials are highly absorbing and/or scattering to traditional NDE energy probes such as heat, sound, x-rays, etc. Microwave NDE techniques offer some novel solutions for the inspection and evaluation of thick dielectric composites. This paper will present an overview of microwave NDE applications for these thick materials, describing theoretical and experimental results from materials ranging in thickness from one centimeter to over 10 centimeters. The experiments were performed on well characterized standard materials containing intentionally introduced flaws and defects including: holes/voids, delamination/disbonds, contaminating materials, and impact damage. Comparison of microwave NDE results will be made with other NDE methods such as ultrasonics, radiography, thermal imaging, and optical methods using the same standards.