Mixed mode intra-laminar fracture in glass/epoxy laminates
The development of matrix cracking in (0/8/0) glass fibre reinforced epoxy
laminates, where 8 = 45°,54 0, 75° and 90°, has been investigated under quasistatic
and cyclic loading. All laminates were fabricated using a wet lay-up
process to impregnate the fibres before curing.
Uniaxial quasi-static tension tests were carried out on samples of each laminate
type. Crack initiation and crack propagation in the 8° ply were studied
separately using unnotched samples (tested with both as-cut and polished
edges) and notched samples, in which the notch introduced is a drilled hole
parallel to the fibres in the 8° ply. The ply stresses at which cracking events
occurred were calculated from laminate theory and these stresses are discussed
for all four laminate types along with investigations of notch depth effects and
crack growth patterns. Material non-linearity was also considered and a method
presented to account for material non-linearity in shear. Ply stresses were
recalculated to include the effects of non-linearity and the presence of thermal
stresses. It was shown that crack propagation is controlled principally by the
stress transverse to the fibres and that the stress was independent of notch
length. The stress state at crack initiation did not satisfy any of the
conventional failure criteria.
Uniaxial tension-tension fatigue tests were carried out on samples of each
laminate type. Samples were cycled to various maximum load levels and crack
propagation from a single notch in each sample was monitored as it grew full
width to provide crack growth rate data. Crack growth data were presented on
Paris-type plots. The crack growth data were compared to experimental data
obtained by other authors for 90° cracking in cross-ply samples and 90° and
+45° cracking in quasi-isotropic samples. There was good agreement between
the sets of data after accounting for differences in transverse ply thickness.
A series of compliance experiments were carried out to obtain the change in
compliance with crack length for samples of each laminate type. The
experimental compliance changes in (0/90/0) samples were compared to
theoretical, calculated compliance changes.