Fig. 9 shows the shear stress–horizontal displacement relationships
from the ISTs for OTW-A/FRP and VBS-A/FRP
interfaces. The curves for OTW-A/FRP interfaces have a relatively
obvious peak, whereas the curves for VBS-A/FRP interfaces
do not. In other words, OTW-A/FRP interfaces exhibit
postpeak displacement-softening behavior, whereas VBS-A/
FRP interfaces have approximately postpeak plastic behavior.
This phenomenon is attributed to the amount of particle rearrangement
that can occur. Since VBS is an angular material
and has a higher internal friction, the particles on the interface
surface are less likely to be rearranged during shearing if the
shear stress is not large enough to overcome the internal friction.
The restriction of particle rearrangement results in the
interface reaching the steady-state strength at low deformation.
However, rearrangement of particles near the interface surface
is possible for round materials like OTW-A. The curves also
show that VBS-A has stress-hardening behavior, especially at
higher normal stress, which may result from the plowing of
angular particles into FRP material surfaces. The images taken
from the FRP specimens before and after interface tests clearly
showed a number of scratches left on the specimen surface
under high normal stress tests (Han 1997). For OTW-A, the
transition of the curves from the prepeak to the postpeak behavior
is ‘‘brittle’’ because of the quick development of slippage
for the rounded material. Fig. 9(a) also shows that the
OTW-A/FRP interface exhibits a more obvious peak in the
shear stress–horizontal displacement curves at high normal