Development of matrix cracks in cross-ply laminates under bending and their detection using FBG sensors.

The development of matrix cracking damage in a double cross-ply laminate under four-point bending has been investigated for both quasi-static and fatigue loading. Under quasi-static loading, crack onset was found to develop at the interface between the outer 0° ply and the adjacent 90° ply on the tension side of the laminate. The cracks initiated at coupon edges and then extended across both the thickness and width of the 90° ply, crack growth occurring In a "stick-slip" manner. Under fatigue loading, additional damage in the form of delaminations developed, especially under "high" fatigue loads and splits developed in the outer 0° ply. The delaminations initiated at the point where the matrix cracks met the outer 0/90 interface. For both quasi-static and fatigue loading, the modulus reduction due to matrix cracking damage was in good agreement with the predictions of a model based on a onedimensional shear-lag analysis. A system for monitoring the output from a fibre Bragg grating sensor located within a 0° ply and near a 0/90 interface was developed, using a broad-band laser source and an optical spectrum analyser. The strain output from the sensors was correlated with measurements of strain in both tension and bending tests, and there was found to be good agreement between the measurements employing the sensor and those using the extensometer. The change in the spectrum recorded using the sensor when matrix cracks developed has been investigated for both tensile tests and bending tests. The reflected signal showed additional peaks on the long wavelength side when a matrix crack developed. These changes have been interpreted qualitatively in terms of the strain magnification experienced locally in the 0° ply due to the presence of the matrix crack. It has thus been shown that fibre Bragg grating sensors can both monitor strain and detect matrix crack development when the matrix cracks develop in the ply adjacent to the position of the sensor