Abstract :
A thin skin of low tensile failure strain, if
bonded to the tensile surface of an un-notched impact
bend specimen of much tougher material, can change
the global failure mode from ductile to brittle. A novel
model of this well-known effect is developed and
applied to results from impact tests on a tough core
of polyamide-polyethylene blend, with a single skin
of brittle EVOH. At a fixed crosshead speed, notched
specimens of the blend become brittle at a relatively
low temperature Tbt. Un-notched bilayer specimens
continue to show skin fracture up to a considerably
higher temperature Tfs; above this temperature they do
not fail at all but below Tbt they too fail in a brittle
manner. Within the temperature range from Tfs down
to Tbt there is a transition from crack arrest, either at
the skin/core interface or further into the core where
a crack would not normally propagate, to brittle fracture.
This brittle fracture temperature is predicted by
modelling the process as a three-phase impact event.
In the first phase, the striker bends the bilayer quasistatically.
The second phase begins with instantaneous
fracture of the skin at its failure strain. The skin ends
retract at finite speed, and a craze grows in the adjacent
core material to accommodate the local strain singularity.
The last phase is a striker-driven impact event
similar to that in a notched bend specimen of the core
material, except that the crack-tip craze already bearsthe adiabatic temperature distribution generated while
it was driven open by skin retraction. The criterion for
craze decohesion, and hence for a crack jump, is the
same adiabatic decohesion criterion which accounts for
the speed-dependence of impact fracture in notched
monolayer specimens. Applied computationally, this
model predicts whether a bilayer structure fails in a
brittle way or whether cracks initiated in the skin are
arrested, either temporarily or permanently, at the
skin/core interface.
