Inspection of Dielectric Materials with Microwaves

Abstract

Anisotropies in materials may result from production: During the injection moulding process of short fibre reinforced polymers, the fibres align with the mould. The resulting fibre orientation determines the mechanical behaviour of the product.

As the fibre orientation causes dielectric anisotropy whose axis is along the fibre, one can use linearly polarized microwaves for nondestructive and fast probing of fibre orientation: The interaction between the sample and the microwave field shining on it depends on the angle between the fibre and the field. Therefore the rotation of the linearly polarized microwave field with respect to the sample results in a double periodical detector signal from which both the direction and the degree of fibre orientation (or generally: anisotropy) can be calculated by Fourier transformation. The obtained result can be visualized by a line whose direction indicates the fibre orientation while the line length represents the degree of orientation. This way a two dimensional raster scan provides information on fibre orientation fields.

Further applications of microwave measurements are presented: A sharp increase of microwave anisotropy is found in damaged fibre reinforced plastics. Therefore the nondestructive microwave method allows for continuous monitoring of increasingly damaged samples.

Another microwave technique based on the reflection behaviour of dielectric materials can be used for the detection of inhomogeneities. Annual rings and knotholes in wood are distinguishable with high lateral resolution that way.