Mode I fracture along adhesively bonded sinusoidal interfaces

• We examine sinusoidally-patterned adhesive joints using DCB geometries.
• All the experiments and simulations consider remote mode-I conditions.
• The sinusoidal DCBs exhibit “stick–slip” conditions during crack propagation.
• Stick–slip is caused by a snapthrough mechanism under displacement control.
• We show that sinusoidal DCBs can be substantially tougher than the flat DCBs.

Fracture along sinusoidally-patterned and flat interfaces in AA7075-T6 double cantilever beam (DCB) adhesive joints was investigated with an experimental/theoretical approach. Sinusoidal profiles of A/λ = 1/4, 1/3 and 1/2 (A = amplitude, λ = wavelength) were prepared with wire EDM followed by application of a 0.3 mm adhesive layer. Crack propagation from remote mode I loading occurred as the DCBs were separated under displacement control. All tests exhibited crack propagation within the adhesive. Experimental analysis, analytical and finite element models of crack propagation along a cohesive patterned interface provided fundamental insights into the differences between the sinusoidally-patterned and flat DCBs observed in the experiments. For the sinusoidal DCBs, crack propagation is delayed relative to the flat DCBs and the peak load increases with A/λ. The sinusoidal DCBs induce intermittent crack extension that resembles “stick–slip” conditions with slow (stable) crack propagation and fast (unstable) crack propagation. The intermittent crack propagation is facilitated by the release of strain energy though the viscous response of the adhesive in a non-equilibrium “snapback” mechanism which enhances energy dissipation through the crack propagation process. Such release is also associated with a drop in the applied load leading to a serrated load–displacement behavior. The size of the fracture process zone also plays an important when it is comparable with the sinusoidal characteristic length scale. These results demonstrate that patterned adhesive joints can be substantially tougher than joints with no pattern.

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