  An assessment is made of the J-integral test procedure for initial crack
growth in an open-cell aluminium alloy foam by combining finite element (FE)
simulations with experiment. It is found experimentally that a zone of randomly
failed struts develops ahead of the primary crack tip, and is comparable in
size to that of the plastic zone. Hence, a crack tip J-field is absent at the
initiation of crack growth from the primary crack tip. This implies that the
measured J_IC value and the J versus crack extension Da curve cannot be treated
as material properties despite the fact that the specimen size meets the usual
criteria for J validity. The toughness tests were performed on a single-edge
notched bend specimen, and crack extension was measured by the direct current
potential drop method, by digital image correlation and by X-ray computed
tomography. The crack growth resistance of the foam is associated with two
distinct zones of plastic dissipation: (i) a bulk plastic zone emanating from
the crack tip (containing a cluster of randomly failed struts), and (ii) a
crack bridging zone behind the advancing crack tip. The applicability of a
cohesive zone model to predict the fracture response is explored for the
observed case of large scale bridging. To do so, FE simulations are performed
by replacing the discrete lattice of the open-cell metallic foam by a
compressible, elastic-plastic hardening solid while the fracture process zone
in the foam is represented by a cohesive zone, as characterised by a tensile
traction versus separation law. A detailed comparison of the cohesive zone
model with experimental observations reveals that it is possible to capture the
load versus displacement response but not the details of the fracture process
zone using a single set of process zone parameters.
