Tensile creeps have been conducted upon a woven, glass-fibre laminated epoxy composite and a 0/90° cross ply, carbon fibre reinforced epoxy composite. For the laminate loading was aligned with a fibre direction. For the ply the loading was inclined to the fibres (off-axis).
Testing to stress levels up to 200 MPa and temperatures in the range 20°- 200°C has revealed a form of creep in each material. The creep observed is essentially primary in nature but with extended time •1000 h, it may exhaust or resemble a pseudo-secondary regime with a low rate. Where the load carrying capacity is lost, through fibre breakage or tab slip, the creep rate accelerates suddenly to infinity in a few hours. Smooth creep curves apply to successful tests but many irregular curves resulted from grip failure. A phenomenological approach was used to model smooth curves using a summation of instantaneous, primary and secondary strain terms. For the mat reinforcement a consistent trend was not found between the secondary creep rate and a stress that was raised incrementally upon the same testpiece. However the cumulative instantaneous strain provided the correct elastic modulus. Creep in the solid laminate was believed to be due to a fibre straightening that yielded a limiting strain in a time beyond which the process exhausts.
Creep in cfrc was only evident when the fibres were inclined to the stress axis, indicating a viscous flow in the matrix. Moreover, it is believed that a viscous shear sliding between laminates or plies is more likely to contribute to an off-axis deformation mode which is not strain limited.http://www.brunel.ac.uk/about/acad/sed/sedstaff/design/DavidRee
