Abstract

i) ultrasonic scanning : the main objective of the research, which in the previous years had concentrated on the problem of the localisation and the depth measurement of defects, has now shifted towards the analysis of the types of defects using advanced interpretation of the reflected ultrasonic signal, both in time and frequency domain (through FFT). Techniques of image processing are applied in order to classify the signals and correlate them to specific types of defects. To assist the user in making important decisions (e.g. which transducer to use), an expert system is under development.

ii) vibrothermography : also called SPATE (Stress Patterns Analysis by the measurement of Thermal Emission). The coupling between elastic deformation and temperature change of a body, known as the thermoelastic effect was first analysed by Lord Kelvin back in 1853. However it took more than a century before its potential as a mean for stress analysis was exploited. Though relatively young, SPATE established itself very quickly as a powerful tool to obtain full-field non-contact stress data particularly for metallic structures. Unfortunately, the use of SPATE has thus far been limited to simple comparative studies or to qualitative NDT-type applications. To fill up this lack of information, an extensive study of the heat transferring characteristics of fibre reinforced composites under cycling loading was carried out. This research has led to a doctoral dissertation (D. Van Hemelrijck): the technique, which was used for isotropic materials, has been expanded towards composite materials. Original contributions are in the description and explanation of the so called motion effect and the coupling of SPATE with other techniques for NDI. As damage processes in composite materials (e.g. delamination, matrix cracking,...) change the stress distribution in the laminate and induce localized heating (preferentially around the damaged regions), SPATE becomes the ideal solution for the detection of damage and damage evolution and so making it potentially a more powerful device for analysing composites than it is for isotropic materials.

Another subject of study was the torsional fatigue of composite materials structural elements, through a specially developed fully automated torsional test bench. A method to obtain the in-plane as well as the out-of-plane shear modulus of thin rectangular composite specimens was investigated. It was found that for some composite systems the out-of-plane shear modulus is much different for the in-plane shear modulus.