Non-destructive testing methods and apparatus

A non-destructive testing apparatus for analyzing a sample comprising a composite material, the apparatus comprising:

• an emitter configured to emit an electromagnetic signal onto the sample, the electromagnetic signal having a range of frequencies;
• a detector configured to detect a response signal transmitted and/or reflected by the sample in response to the electromagnetic signal;
• a network analyzer configured to process the response signal to determine variation with frequency of a dielectric permittivity of the sample over the range of frequencies;
• and
• a computing device configured to determine an indication of a structural characteristic of the sample from a measure of the variation with frequency of the dielectric permittivity of the sample.