M. Todo et al., Initiation of a mode-II interlaminar crack from an insert film in the end-notched flexure composite specimen, COMP SCI T, 60(2), 2000, pp. 263-272
Two- and three-dimensional finite-element analyses have been carried out in
order to investigate the initiation of an interlaminar crack in the end-no
tched flexure (ENF) composite specimen. It is believed that the current pra
ctice of using an insert film as a starting defect generates a blunt defect
which creates a stress pattern that attracts the crack growth towards the
fibre/matrix interface. Results from finite-element (FE) modelling support
this concept. The FE results also indicate that with a sufficiently low bon
d strength at the fibre/matrix interface, the crack can be initiated from t
he interface instead of from the starting defect. The critical interfacial
bond strength for transition of the location for crack initiation from the
starting defect to the fibre/matrix interface is 14% of the matrix strength
for the two-dimensional model, and 28% for the three-dimensional model. Al
though the two crack-initiation mechanisms are very different, the fracture
surfaces generated are similar. The FE models indicate that after the crac
k initiation its further growth is always along the fibre/matrix interface.
The above conclusions from the FE work were verified experimentally by usi
ng a glass-fibre/vinylester composite with a fibre volume fraction similar
to that used in the models. The overall conclusions from the study are that
the crack-tip bluntness plays an important role in the initiation of the i
nterlaminar fracture in the ENF specimens, and that the interlaminar fractu
re toughness measured from the ENF specimen can depend strongly on the inte
rfacial bond strength. (C) 2000 Elsevier Science Ltd. All rights reserved.