Fracture mechanisms and failure criteria of adhesive joints and toughened epoxy adhesives

Abstract

Adhesive bonded applications are used widely in industry because of significant advantages such as uniform stress distribution, and the ability to join different materials. However most epoxy structural adhesives are brittle at room temperature and it is required to improve their toughness. The objective of this work was to understand the fracture of adhesive joints, failure criteria and rubber toughening mechanisms via a series of experiments and FEA modelling. Double lap joints (DLJ) bonded by commercial AV119 adhesive were studied. It was found that local strain and failure path were controlled by adhesive thickness. In order to model adhesive joints accurately and efficiently, systematic fracture tests were implemented to determine the fracture criteria. Mode-I, mode-II and mixed mode fracture energy release rates were obtained by Fixed Arm Peel, 4-point End Notched Flexure (ENF) and Mixed Mode Bending (MMB) tests. Numerical analysis was applied to determine the parameters of the Drucker-Prager material model and Cohesive Zone Model (CZM). The 3D FEA results showed good agreement with experimental results of DLJ and MMB. FEA results successfully demonstrated bonding strength, stress and strain distribution and plastic deformation; and further details were found using sub models. The rubber toughening mechanism was studied by modelling different face-centred micromodels. The stress distributions ahead of the crack tip in global DLJ models were extracted and used as the loading condition for the micromodels, so that a relationship between macromodel and micromodel has been established. It is found that Von Mises and hydrostatic stress play very important roles in the toughening mechanisms and also predicted that rubber particles with multi-layer structure have more potential to toughen epoxy resin than simple rubber particles.