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Fracture toughness testing of ductile materials can be difficult in situations where
it is not possible to measure crack extension during the test, such as under high
rate loading or in aggressive environments. In these situations, an alternative
method of inferring crack extension must be used to generate the tearing
resistance curve, and thereby determine ductile crack initiation. ASTM test
method E1820 uses the Normalization method to generate the plasticity function
for the specimen, which can then be used to calculate crack extension. This
method relies heavily on choosing a particular functional form for the plasticity
function. If there is a large amount of crack extension in a test, the uncertainty in
the derived plasticity function increases, which can lead to non-conservative J-R
curves. This is demonstrated using test records for two steels and a titanium
alloy.