Three Dimensional Failure Analysis and Damage Propagation Behavior of Adhesively Bonded Single Lap Joints in Laminated FRP Composites

Abstract

A three-dimensional finite element analysis has been developed to compute the out-of-plane normal (known as peel stress) and shear stresses in an adhesively bonded single lap joint (SLJ) with laminated FRP composite plates which, in comparison to other analytical methods for bonded joint analysis, is capable of handling more general situations related to initiation of damages and its growth. Other analytical methods, such as Hart-Smith’s (Hart-Smith, L. J. (1973). Adhesive-Bonded Single Lap Joints, NASA-CR-112236.), are 1-D and mainly focus on obtaining adhesive stresses, while generally ignoring stresses in laminated adherends, particularly interlaminar stresses which are known to be the key contributors to the failure. Unlike the Volkersen’s (Volkersen, O. (1938). Die Niektraftverteilung in Zugbeanspruchten mit Konstanten Laschenquerschritten. Luftfahrtforschung, 15: 41-47.) and Goland-Reissener’s (Goland, M. and Reissner, E. (1944). The Stresses in Cemented Joints, Journal of Applied Mechanics, 11: 17-27.) analysis, the free rotation of the overlap region and adherends are considered in the present analysis. Joining of composite structures using adhesive bonding has always been a concern to the designers because the performance of the joint is severely influenced by the characteristics of the laminated composite adherends, which usually have low inter-laminar strengths. The present method computes local 3D stress fields in the most critical region, which vary along the overlap length. Adhesive layer is a linearly elastic material whereas the adherend materials are orthotropic. Thus an accurate evaluation of 3D local stress fields will enable the failure criterion to be employed effectively to predict joint strength and initiation/propagation of damages. The analysis consists of four steps. In the first step a complete three dimensional stress analysis is carried out with a special importance for the evaluation of out-of-plane stresses. Failure indices at different surfaces are calculated in the second step. The third step identifies the location of damage initiation based on the value of the failure indices. The failure index for the adhesive layer is calculated using quadratic failure criterion (QFC), whereas the Tsai-Wu’s coupled stress quadratic failure criterion is used for the interface of adherend and adhesive. Subsequently, damage propagation is analyzed by fracture mechanics based strain energy release rate (SERR) approach using virtual crack closure technique (VCCT). It is seen that the three-dimensional effects exist in the joint. The stress distributions in the joint overlap region near the free surface are quite different from those occurring in the interior. Also, it is found that the peel stresses are extremely sensitive to this three-dimensional effect, but the shear stresses are not. The value of the failure index at the interface of the loaded adherend (top) and adhesive along the free edge is the highest which indicates the location of the possibility of damage initiation. Such type of damage is considered as an adhesive failure. The propagation of such damage is governed by SERR. It is observed that the individual mode of SERR remains constant over the damage front irrespective of the damage length except at the free edge. Moreover, it is found that contribution of SERR in mode II is prominent compared to the mode I and mode III for the damage propagation due to the adhesive failure in the SLJ.

Strength prediction of single- and double-lap joints by standard and extended finite element modelling

“…However, the proposed methodology was only acceptable due to the brittleness of the adhesive since, if a ductile adhesive had been used instead, the predictions would clearly underestimate the experiments. Another handicap of XFEM modelling using the proposed technique is the mesh size dependency of the stresses/strains [50]. P-d curves for XFEM are not presented here, but there show a similar agreement to Figs.…”

“…In the early stages of bonded structures analyses, theoretical studies were popular [3][4][5][6][7], which employed simplifying assumptions in the structures geometry, materials behaviour, loading, and boundary conditions, to formulate efficient closed-form elasticity solutions for the local fields in the adhesive region. The main advantage of analytical modelling is that the structure can be analysed quickly, although with lot of embedded simplifications [8].…”

Modelling adhesive joints with cohesive zone models: effect of the cohesive law shape of the adhesive layer

“…The meshes for all FE models were automatically created by the software considering bias effects, with smaller sized elements near the overlap edges and in the thickness direction near the adhesive. Actually, it is known that the overlap edges are theoretically singularity spots with large stress variations [36]. To provide identical modelling conditions, the FE elements size in all models was made equal at the overlap edges (approximately 0.2 x 0.2 mm 2 elements), thus allowing to accurately capture stress variations [29].…”

Delamination damage analyses of adhesively bonded lap shear joints in laminated FRP composites

“…Tsai and Morton (1994) have analysed the three-dimensional deformations in the SLJ test specimen. Panigrahi and Pradhan (2007a, b) carried out a three-dimensional failure analysis and predicted the location of the damage initiation using Tsai-Hill coupled failure criterion and emphasized for the threedimensional modelling of the damaged region of the composite SLJ. FRP composite Lap Shear Joints (LSJ) are widely used in many structural applications.…”