The J-integral is used to develop an alternative double cantilever beam (DCB) test method
for the Mode I fracture toughness suitable for both small and large displacements. The current focus
is the experimental determination of the Mode I interlaminar fracture toughness of composite materials,
but the method is generally applicable to other similar tests and material systems, such as to the Mode
I fracture toughness of adhesives. A series of five identical specimens are tested to compare the
linear-elastic fracture mechanics method recommended by ASTM, which makes use of linear beam theory
with root rotation, large displacement, and end block corrections, with the new nonlinear-elastic
and elastic-plastic fracture mechanics method, which does not require these corrections. Experimental
results show excellent agreement between the two methods over a series of five tests of primarily
linear-elastic DCB specimens subjected to moderate to large displacements as defined in the ASTM
standard. Furthermore, an agreement is found between the results of the derivations for the two
methods being compared, whereby the large displacement equation for JIc presented
in this work is identical to the equation given by J. G. Williams (1987) and which he found to be
the true value of GIc. It is the true value of GIc that the
large displacement and root rotation correction factors were intended to approximate, and the
test method presented here allows for direct measurement of its parameters and evaluation. This
method has the added benefit that specimens can be primarily linear-elastic or nonlinear-elastic
at the crack tip and may extend to those that are elastic-plastic at the crack tip.
Enhanced simple beam theory for characterising mode-I fracture resistance via a double cantilever beam test