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A glass fibre epoxy resin laminate was prepared from a flexible thermosetting resin and a 'plain weave' glass cloth. Experiments in
simple tensile creep were carried out on strips cut with their long
dimensions at various angles to the warp threads in the glass cloth. It
was found that each of these strips showed, over the limited range of
loads and times covered, essentially linear creep behaviour. The creep
compliance varied systematically with direction being as much as twenty
times smaller in the warp and weft directions as at 45° to these directions.
It was found that the shape of the creep compliance versus orientation
curves was similar for all times and the behaviour can therefore conveniently
be described by two curves, a master curve of reduced creep compliance as
a function of direction and a curve of reduction factor versus time.
The significance of both these curves is discussed in terms of an
extension of linear viscoelasticity theory to the case of antisotropic
materials. It is shown that the variation of creep compliance with
direction is similar in form to the variation of elastic compliance with
direction in orthorhombic anisotropic elastic materials and also that the
results are consistent with a similar variation of relaxation time spectrum
with direction. |
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