NDTnet 1998 Aug, Vol.3 No.8

Abstract

Composite materials are being increasingly used as construction materials because of their excellent strength to weight ratio. Taking this into account it is not surprising that most of the applications of these materials are found in areas where weight is an important factor (e.g. ground or air transportation). A safe use of composite materials, however, requires procedures or techniques that can reliably monitor the damage state during the life-time of the components. Up to now traditional NDT techniques like radiography or ultrasonic testing are used to this extent, the main disadvantage of these techniques being that they can not be used continuously during service.

A solution to this problem that has received considerable research attention during the last years is the incorporation of optical fibres in the composite components during manufacturing. During service gradual damage evolution could then be detected by monitoring the evolution of the properties of laserlight transmitted through the optical fibres.
In this paper the first steps of the development of a damage detection sensor for composite materials based on optical fibre technology will be presented. The material under investigation was a quasi-isotropic [0,45,-45,90]_s CFRP with optical fibres embedded in the different interfaces, transversely to the loading direction. In a first step an investigation was made of any influence the optical fibres may have on the mechanical properties of the resulting structure, since literature results have shown that embedding optical fibres can result in considerable stress concentrations. Both static (tensile and three point bending) and dynamic (fatigue) testing were carried out. The results of the tensile tests showed a 7 % degradation of strength when optical fibres were embedded in the 0/45-interface. During three point bending considerable strength degradation was observed when the optical fibres were embedded in the 0/45 and the 45/-45-interfaces with all specimens failing in the zone of maximum compression stress. The fatigue tests revealed a degradation in properties (number of cycles to fracture and stiffness degradation) when optical fibres were embedded in the 0/45 and the 45/90-interfaces.

The main result of the investigation into the mechanical properties was an optimum optical fibre configuration that could in a next step be used for the development of an actual optical fibre sensor. The configuration that was chosen was the one where the optical fibres were embedded in the 90/90-interface. Interrupted tensile tests were carried out during which damage development was monitored making use of the embedded optical fibres and using the well established acoustic emission technique as a calibration tool. An attempt was made to correlate the results of both techniques with the actual damage development.
The main conclusion of this work is that it has shown the feasibility of using optical fibre technology as a tool for monitoring gradual damage development in composite materials. This research should ultimately lead to a damage detection sensor that can monitor composite components in situ and continuously during service, thus eliminating the main disadvantages of the classic NDT techniques.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

Publication Contact:
7th ECNDT, Park Alle 345, DK-2605 Broendby, Denmark, Fax: +45 46 26 70 11, Email: 101373.3414@compuserve.com