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The effect of out of plane constraint on the fracture toughness of metals has long
been appreciated. The extreme cases of out-of-plane constraint are typically held
to be plane strain and plane stress: the stress intensity factor at fracture is lowest
for thick (plane strain) specimens and greatest for thin (plane stress) specimens.
However, situations that give higher levels of out-of-plane loading have rarely
been considered, yet such conditions may occur in practical loading of cracked
components. This combined numerical and experimental study explored the
effects of applied out-of-plane loading on a cracked cruciform specimen tested in
a biaxial test machine. It is observed that a stress intensity factor significantly
below the plane strain fracture toughness can be measured at fracture when the
applied out-of-plane load is sufficiently tensile.