Time-dependent fracture of adhesive joints

A design methodology accounting for the time-dependent fracture of structural adhesive joints is presented. It is demonstrated how a wedge test may be used to evaluate suitable energy release rates for designing against creep crack growth in structural adhesive joints. Energy release rates corresponding to no observable crack growth were measured with mode I wedge tests and mixed mode I and mode II fixed displacement tests. The interpretation of the resulting apparent equilibrium energy release rates is explored in terms of viscoelastic fracture mechanics, and is discussed with experimental results from bulk creep tests and mixed-mode constant energy release rate tests. Using both a mineral-filled epoxy and a rubber-toughened epoxy, it is demonstrated that a wedge test may be used to determine an effective threshold energy release rate for no crack growth in planar structure adhesive joints.