Viscoplasticity and geometric nonlinear finite element analysis of adhesively bonded LAP joints

Abstract

Adhesivebonded joints are usually preferred in composite structures due to their intrinsic advantages compared to fastener joints. Adhesivebonded joints have major advantages such as minimal sources of stress concentrations, efficient load transfer over a large bonding area, superior fatigue resistance, and a high strengthtoweight ratio compared to conventional joints. They also have the additional advantage of reducing maximum stresses in the adhesive by suitably shaping the adherends in the joint region. Recent advances in composites and adhesivebonding techniques based on strong epoxytype adhesives have led to significant improvements in adhesive joining and stiffening of structural elements. An accurate analysis of adhesivebonded joints is needed to determine stress distribution for predicting strength and failure. Some of the important factors involved in joint analysis are the nonlinear stress-strain response and ratesensitive behaviour of adhesive materials. Proper numerical modelling of the adhesive is necessary to obtain realistic stress distributions in the joint in order to ensure a good design. In the present work, adhesivebonded joints are analysed using the finite element method, considering the adhesive as an elastoviscoplastic material. Here, the timedependent behaviour of the adhesive becomes effective only after plastic yielding. Attention is focused on the behaviour of a singlelap joint, where the load acts eccentrically and the joint region is subjected to large rotations. This requires consideration of geometric nonlinearity in the analysis. A finite element computer program has been developed incorporating material nonlinearity for the adhesive and geometric nonlinearity for both the adhesive and the adherends. Assumptions of large displacement, large rotation, but small strain are included in the formulation using the total Lagrangian method. However, the adherends are assumed to be linearly elastic, modeled as either isotropic or orthotropic composites. The program has been thoroughly tested with standard benchmark problems. Investigations have been made into timeintegration schemes. The viscoplastic approach has been used to study timedependent phenomena. The same algorithm has been adopted to investigate the elastoplastic stress distribution in the joint using steadystate solutions. Extensive parametric studies have been carried out on parameters such as lap length, adhesive thickness, adherend tapering, and fiber orientation in composite adherends.