13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- A coupled stress and energy model for mixed-mode cracking of adhesive joints Philipp Weißgraeber1,*, Wilfried Becker1 1 FG Strukturmechanik, TU Darmstadt, Germany * Corresponding author: weissgraeber@fsm.tu-darmstadt.de Abstract This work is concerned with the analysis of failure of adhesive joints. Typically adhesive joints fail due to cracking of the adhesive with cracks originating from the reentrant corner of the adherend and the adhesive. A failure model based on a coupled stress and energy criterion settled in the framework of Finite Fracture Mechanics (FFM) is proposed in this work. The main idea of coupled criteria in FFM is that cracks of finite size are predicted when a stress criterion is fulfilled on all points of the considered crack and simultaneously an energy criterion is fulfilled. A failure model for adhesively bonded single lap joints is worked out that makes use of an extended weak interface model. Its closed-form analytical nature allows for an efficient formulation of the non-linear failure criterion. The effects of the involved geometric parameters are examined in detail. A comparison of the failure load predictions to experimental results is given and shows a good agreement. It is shown that the effect of the adhesive layer thickness is incorporated correctly. The failure model and its implications on the understanding of failure of adhesive joints are discussed in detail. Keywords Brittle fracture, Finite Fracture Mechanics, adhesive joints 1. Introduction Adhesive joints can be an interesting alternative joining method. They have several distinctive advantages over other joining techniques as e.g. welding or bolting. Adhesive bonding allows for large surface joining of thin-walled structures that can have dissimilar materials. The resulting joint has a smooth surface and a sealing function is given. But uncertain failure load predictions and lacking knowledge of failure mechanisms still hinder the widespread use of adhesive joints. To make use of their advantageous features a better understanding of this joining method must be achieved. In the last decades many researchers have worked on an improved understanding of adhesive joints. An overview on the performed research can be found in the comprehensive textbooks of this field, e.g. [1,5]. Failure models for adhesive joints mainly make use of three different approaches, namely strength of materials, fracture mechanics and damage mechanics. Most of the works given in literature focus on strength of material approaches as it has the longest history and it is well known. Unfortunately these approaches lack in the correct description of some important features. Various effects of the involved geometric parameters of a simple joint configuration cannot be fully described by these approaches. A typical outcome is for example that the strength of the adhesive seems to depend on certain geometric parameters. No solid physical explanation can be given for such a relation. The aim of the present work is to present a new failure model for adhesive joints that correctly describes the effect of the geometrical parameters and makes accurate predictions for the failure load. 2. Theoretical background 2.1. Coupled stress and energy criterion In this work a coupled stress and energy criterion [11] is used to predict crack initiation. The criterion requires two basic material parameters: the strength of the material and the fracture toughness. The criterion is settled in the framework of Finite Fracture Mechanics (FFM) [9] that
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