Optical fibres for crack detection in composites

Materials World magazine
2 Aug 2007

Optical fibres may one day aid crack detection in composite structures, says Jasson Gryzagoridis, Professor of Mechanical Engineering at the University of Cape Town, South Africa.

Mechanical deformation caused by static and/or dynamic loading can cause crack propagation in materials. The subsequent mechanical failure of these components can be catastrophic in sectors such as aerospace, aviation, construction and mining. But early fault detection in composites is problematic as cracks often develop from the inside of the material and are difficult to visualise.

Gryzagoridis says, ‘The analysis of crack creation and propagation for most materials is normally handled through considerations of fatigue loading. Composites don’t behave like other materials that are of homogenous structure. It is difficult to predict the propagation of a crack in a composite because it can initiate at any part of the [material].’
Existing non-destructive testing techniques, such as magnetic particle detection, acoustic emission and ultrasonic tools, are not entirely effective, he explains, while optical techniques, such as electronic speckle pattern interformetry and shearography, appear to work better.

Gryzagoridis and his team, which includes researchers at the Cape Peninsula University of Technology in Cape Town, are proposing to exploit the mechanical flexibility of silica glass single mode optical fibres for testing future composite structures. Embedded into the part, the fibres’ high elastic yield to tensile forces allows them to elongate in response to crack widening and lengthening. This enables more accurate and early detection of potential mechanical failure.
The ends of the fibres, with diameters of about 125µm to allow light wavelengths of about 610-770nm to pass through, are embedded into ferules. One end is connected to the light source (a red helium neon laser) and the other a hotodetector. ‘The ends are smoothed and polished to allow efficient transmission of light [and] the fibre is stripped of its clad (plastic cover) so that a strong bond can be effected between the fibre and the host material,’ explains Gryzagoridis.
‘The fibre necks or its diameter changes between the bonded positions as the crack opens perpendicular to it and underneath it. This affects the [transmission] of light, and losses occur at the neck. The photodetector picks up the attenuation.’
For example, elongation and crack opening in each fibre of about 1.7mm (just prior to fracture) reflects a total crack length in the composite of about 26mm (see graph).

The optical fibres could be applied in composite applications where techniques that are reliant on the electrical conductivity of the material are not as efficient.

While the initial experimental work has been completed, further research is required to monitor crack propagation and increase the accuracy of the results. Self-healing mechanisms could becombined with the system to repair damage on detection.