A study of stress-induced whitening in glass fibre reinforced epoxy laminates.

The aim of this study has been to investigate stress-induced whitening in glass fibre/epoxy (0,90)s laminates and to develop a technique to monitor the low strain damage associated with it. The effect of resin cure, laminate geometry, heat treatment and surface finish of glass fibres on the extent and development of damage has also been studied. It has been found that the whitening is caused by the development of micro-cracks in the resin. The micro-cracks developed in the 90° ply and occurred predominantly at or near points of contact between fibres where the strain magnification in the resin is highest. The cracks occurred near the fibre/matrix interface and extended into the resin with the crack faces lying at 90° to the loading axis. Careful observation under oblique illumination conditions has shown that the whitening effect is preceded by other colour effects so that the laminate is observed to gradually change colour from an original blue through to red or white depending on the level of cure of the matrix resin. The whitening is a result of ordinary diffuse reflections from cracks that are larger than the wavelength of light and occurs when the level of cure of the matrix system is low while the reddening is the result of Rayleigh or Mie scattering from cracks that are smaller than the wavelength of light and occurs when the level of cure of the matrix is high. An off-axis LASER diffraction technique has been developed to monitor the amount of damage in the transverse ply of laminates by measurement of the intensity of light diffracted by the micro-cracks. In addition to confirming the results obtained from photomicroscopy, it revealed that decreasing the inner 90° ply thickness in (0,90)s laminates resulted in a substantial increase in the amount of micro damage in the laminate. Heat treatment of the laminate after testing resulted in the disappearance of whitening and healing of micro-cracks in the resin which continues to cure during heat treatment. Successive heat treatments reduced the size of micro-cracks which developed on reloading. In addition to decreasing the rate of re-development of the original cracks on reloading, the heat treatment reduced the rate of development of "new" cracks formed at higher applied strains