Configuration of the ELS test for the determination of the mode II fracture toughness in adhesive joints

Abstract

In order to obtain Mode II fracture toughness of unidirectional fiber composite materials, the End-Loaded Split (ELS) has enjoyed great success because of its higher stability. As a result, crack growth initiation and propagation values are obtained, the latter being especially used in interlaminar progressive damage models. The propagation values must be obtained when the Failure Process Zone (FPZ) has completely developed. In delamination tests of unidirectional composites, such as those contemplated in ISO 15114, the dimension of the FPZ can be neglected against the rest of the dimensions of the specimen, whereby the self-similar crack growth is reached just after the initiation of the crack propagation. However, in adhesive joints, the dimension of the FPZ increases significantly due to the plasticity of the adhesives. And even it can be larger than the entire length reserved for crack propagation. When this occurs, only initiation values are obtained, always inferior to the values of propagation, being these values too conservative to reproduce the behaviour of the joint. In this paper we present a methodology to define the configuration of the ELS test and the specimen dimensions required to obtain propagation results in adhesive joints. The methodology defined is based on two criteria that take into account the length of the FPZ and the stability of the test. As a result, a domain of validity of the test is obtained, which, for a certain type of adhesive and specimen configuration, it indicated is there will be propagation or not.