Ultrasonic C-scan standardisation for fibre-reinforced polymer composites - minimising the uncertainties in attenuation measurements.

The need for an internationally-accepted standard addressing C-scanning of composite materials has resulted in a DTI-funded collaborative programme involving the National Physical Laboratory, the Defence Evaluation and Research Agency, and many manufacturing and industrial partners. As a result a draft standard has been produced and is being evaluated in a round-robin exercise. One of the main difficulties encountered in drafting the standard was in isolating the various sources of systematic uncertainty in C-scan attenuation measurements. Three main sources were found: diffraction and refraction effects, peak-frequency downshift, and nonlinear propagation in water. Although it may be possible to produce corrections for these effects, the principal purpose of the draft standard is to provide a measurement method which minimises them and to describe how to estimate the remaining uncertainties. This paper represents an experimental validation of the conclusions embodied in the draft standard, including the methods for reducing measurement uncertainties.