Mixed-Mode Cohesive Law Estimation of Composite Joints Made of Toughened Epoxy Adhesive

Abstract

Joining composites using adhesive bonding is attractive because they reduce the weight of structure and allow to join complex shapes. These benefits encourage use of composite adhesive joints in aerospace and automotive industries. However, composite adhesive joints are seldom used for primary structures because of our limited understanding of their failure, especially under mixed-mode loads. Predicting the failure of composite joints is challenging because the failure can occur cohesive in adhesive, interfacial between adhesive/adherend, or within the composite adherend. Moreover, these failures depend on the specimen geometry, loading conditions, surface treatments, and environmental conditions. Recent studies showed that cohesive zone approach can be used to reliably predict failure, but most of these studies are limited to failure under mode I loads and further for brittle epoxy adhesives. In this study, traction-separation laws (TSL) were extracted for composite joints made of toughened epoxy adhesive through fracture tests and by applying the digital image correlation (DIC) technique. These TSLs were used for strength prediction of composite joints subjected to mixed-mode loading. Composite adhesive joints were made of carbon fiber/epoxy composite adherend and Araldite 2015 epoxy adhesive. Mode I and mode II fracture testing were conducted using the double cantilever beam and end notch flexural specimens, respectively. From these fracture tests, TSLs were extracted by using a direct method based on the DIC technique. These TSLs were used in a finite element (FE) model of a lap shear joint model in ANSYS to predict the failure strength. This FE predicted failure strength reasonably agreed with the experimentally determined failure strength of the toughened adhesive joint.