PSI - Issue 14

Mohd Tauheed et al. / Procedia Structural Integrity 14 (2019) 354–361 Mohd Tauheed/ Structural Integrity Procedia 00 (2018) 000–000

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1. Introduction Manufacturing industries like aircraft, marine and automotive industries are replacing the classical joining methods with some modern techniques such as adhesive bonding this is due to their high structural integrity and reliability. However, the formation of the reliable adhesive joint in between the composite components is not an easy task and requires greater insight. To study the failure of adhesive joint and modelling of failure, cohesive zone models (CZM) has been extensively used as described in Constante and Moura (2015). Some other researcher studied to predict the fracture toughness of degraded adhesive joints by cohesive zone modeling with fracture data of accelerated aging tests. Degraded fracture toughness predictions were done by calculating the exposure index values and thereby the degraded cohesive parameters across the width of the closed joints as described in Patil et al. (2017). Some researcher successfully studies the critical energy release rate was the major parameter characterizing the fracture of adhesive. They investigated numerically and experimentally of mixed-mode fracture testing of CFRP composite joints with a film adhesive Balzani et al. (2012). Several studies have been done to evaluate mode I and mode II traction-separation laws using different methods to find the cohesive properties near the crack tip of the specimens. Few works focus on the shape of the traction-separation laws to model the adhesive layers as described detail in Campilho et al. (2013) for the best results in strength prediction. Several studies have shown that shape of the traction-separation laws are triangular in brittle adhesives, trapezoidal in ductile adhesives and exponential shapes are also fitted with different properties of adhesives as presented by Campilho et al. (2015). Some other researcher investigated the effect of adhesive layer thickness on the fracture behaviour of the adhesive joint. A recent study Carlberger et al. (2010) successfully investigates the effect of adhesive thickness on the traction separation law that indicates the fracture energy is more sensitive to the adhesive layer thickness than cohesive strength. When the failure is cohesive, the CZM parameters arises due to the plastic dissipation in the adhesive layers. Several studies also explore the effect of strain rate on the CZM parameters as described in Desai et al. (2015). The determination of the cohesive parameters and selection of CZM law also depends on several other parameters such as the adhesive material, moisture and temperature as deailed described in Budhe et al. (2017). Though practical joints are loaded under mixed-mode conditions, there is seldom work and this represents a research gap in the literature. In this study, traction-separation laws (TSL) were extracted for composite joints made of toughened adhesive 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. Initially, a suitable surface pre-treatment was established to be lateral sanding with 220 grit sandpaper followed by acetone cleaning. Mode I and mode II fracture testing were conducted using the double cantilever beam and end notch flexural specimens, respectively. In this study, fracture energies were taken only at the crack initiation. From these fracture tests, TSLs were extracted by using a direct method based on the digital image correlation technique. These TSLs were used in a finite element (FE) model of a single lap shear (SLS) 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. Nomenclature m compliance calibration coefficient B specimen width P or ��� maximum load �� , ��� critical energy release rate for mode I and mode II G I , G II energy release rate depends on the current state of normal and shear stress at the crack tip , � initial crack length effective crack length to correct for rotation of DCB arms at crack front displacement T traction separation, normal separation and shear separation � � curve fit polynomial equation � maximum separation in the adhesive layer.