Modelling for Ultrasonic Inspection of Aerospace Composites

Modern engineering materials require carefully controlled microstructures in order to ensure that the desired material properties are achieved. Leading manufacturers in the aerospace industry are working towards the introduction of new composite materials such as metal matrix composites (MMCs) for engine components and non-crimped fabric composites (NCFCs) in the next generation of military and civil aircraft. MMCs have improved stiffness and strength-to-weight ratio for high temperature applications and NCFCs have better damage tolerance and lower cost compared with traditional unidirectional prepreg composites. Deviations from the desired microstructure may lead to a catastrophic degradation in the properties of these materials. These can arise due to manufacturing errors and also in-service degradation. Ultrasonic non-destructive evaluation (NDE) is potentially the most powerful and appropriate method for evaluation of the microstructure and defects in these materials. It is however recognised that new NDE techniques are needed, based on a better understanding of wave propagation in these complex materials, before engineers can be confident in introducing MMCs and NCFCs into service. Overcoming the problems associated with the inspection of these novel composites will provide a great impetus towards even more widespread adoption of some of these advanced materials. Complex microstructures present a formidable problem for ultrasonic inspection, since by their very nature they strongly scatter the incident ultrasound. This can have many different effects depending on the nature of the microstructure and the frequency of the ultrasound. In some aerospace applications it is highly desirable to have a very regular layout for the reinforcing fibres in MMCs. Modelling elastic wave propagation through these materials demonstrates that they can behave as phononic crystals and exhibit elastic band-gaps. The purpose of this investigation is to see if in the range of frequencies commonly used for the non-destructive evaluation of these structures we can observe interesting band structures and whether the appearance and characteristics of these bands can be used for the characterisation of defects within the material. We have investigated the degree of regularity needed in the distribution of the fibres for the appearance of pass bands. Other effects investigated are changes of pass-band characteristics due to the presence of defects, the interaction length and the size of the receiving aperture. The study has been performed using a Finite Element model which has also been compared with a plane wave expansion method. The results demonstrate very interesting features in the transmission and reflection spectra with potential important implications for the quantitative NDE of these composite materials.

کلیدواژه ها:

Ultrasonic NDE ، aerospace composites ، elastic band gaps