Interfacial dislocations in tough adhesive composites

Abstract

An adhesive interface in a glued joint or composite can fail by interfacial cracking. This is catastrophic if there is no mechanism to prevent fracture along the interface, which juts open and cracks with little energy to break at a low load. However, under certain conditions, such dangerous interfacial cracks may be stopped by conversion into interfacial dislocations. The original interfacial crack therefore does not cause failure but stops until further loads and displacements are applied to the structure, which becomes both stronger and tougher through this process. This paper explains how interfacial dislocations can be encouraged to form by applying a lateral constraint to the adhesive structure, thereby closing and healing the interfacial cracks. A theory is proposed to explain the curious jutting deformation of a constrained cracked body. Then the theory is verified by experiments on a rubber model, the cracking and healing processes leading to dislocation formation being treated in detail. Finally, it is demonstrated that this interfacial dislocation mechanism operates both in tied lap joints where the constraint is an externally applied force, and in composites where the constraint is provided by surrounding layers of material. This interfacial dislocation model consequently leads to an explanation of the structure of laminar adhesive composites such as nacre.