School of Engineering and Materials Science
Research Student Awards
PhD Thesis: In-situ stress/strain measurements in composites using an aramid fibre as a sensor
Author: ARJYAL, Bishwamber
Supervisor(s): Costas Galiotis
A new stress/ strain sensor for localised measurements in polymer based composites has been developed and tested. The technique used is based on the stress/ strain sensitivity of the Raman frequencies of the reinforcing fibres which are the load supporting elements of all the high performance composites. Measurements can be made in reinforcing fibres on the surface of laminates as well as in the bulk. For measurements in the bulk of composites, the exciting laser light has to be transported to the reinforcing fibres via an embedded fibre optic cable. The backscattered light is transmitted through the same fibre optic and is sent to the Raman spectrometer for analysis. The Raman stress/ strain sensor has been employed for unidirectional as well as multidirectional composites. From the measurements made, relationships between the local fibre stress/ strain obtained from the Raman sensor and the far field stress or strain measured conventionally, have been established. The effect of the direction of the fibre optic cable with respect to the axis of the reinforcing fibres in unidirectional composites is also examined.
Transparent glass-fibre cross-ply composites have been embedded with single aramid fibre strain sensors. The sensor was located at the 0°/90°/0° ply interfaces and was interrogated by a remote laser Raman microprobe. The strain magnification in 0° plies resulting from transverse cracking in the adjacent 90° ply, has been measured at different levels of applied composite strain. A maximum strain magnification value of approximately eight was detected at the ply interface whereas the affected area extended to a distance of approximately 37 m. At point of transverse crack saturation, extensive crack-crack interactions for cracks at close proximity were observed. The residual strains in the 0° plies developed during manufacturing were found to be compressive and on average -0.03% in magnitude. The strain profiles obtained using a laser Raman microprobe enabled the positions of the cracks to be determined when the laminate is loaded or unloaded, such that local measurements of the residual strain after cracking and the reduction in Young's modulus as a consequence of cracking can be made.