Pattern and rigidity effects of adhesive on the stress distribution of the adhesively bonded composite joints

Adhesively bonded composite joints have attracted considerable attentions in industrial applications due to notable advantages such as, easy integrated manufacturing, low mounting weight, minimal stress concentration, smooth load transfer, and excellent fatigue and damage tolerance performances. However, the joints are potential weakness under load conditions due to discontinuities of structures. In adhesively bonded joints structures, stress concentrations are occurred both ends of the adhesive regarding to both literature and performed finite element model analysis stress results. The stress concentrations in all mechanical systems should be prevented and provided uniform stress distribution due to durability of the structures. In this study the single lap joints and double lap joints are investigated at first. Stress distributions and deformations of both joints are discussed and presented in order to reveal difference behavior of the single lap joint and double lap joint. Unlike the double lap joints, the single lap joints have unbalanced structures. As a results of this, the higher stress levels are observed at critical regions in the single lap joints. Therefore, the research concentrates on the single lap joints and reductions of the stress concentrations. The study suggests several ways to minimize the peak stress levels in the critical regions and provides more uniform stress distributions on the adhesives. The pattern and rigidity effects of adhesive on the stress distribution are examined in details. The different adhesives which are ductile (low elastic modulus according to the general adhesives), general (normal elastic modulus for adhesives) and brittle adhesives (high elastic modulus according to general adhesives) and different shape of adhesives patterns are generated and performed analyses via finite element method in ABAQUS. The general adhesive is used for only in master model. On the other hand, both ductile and brittle adhesives are used together in the other all configurations. Except master adhesive model, five different configurations are generated in order to realize the pattern and rigidity effects of the adhesive on the stress distribution. Due to revealing importance of the ratio of the ductile and brittle adhesive usage, different ratios of the ductile-brittle adhesive are used in the configurations. Three models of five have same adhesive pattern shape but different ratio of ductile-brittle adhesive usages. The other two models of five have also same adhesive pattern shape but different ratios of ductile-brittle adhesive usages, too. The purpose of different ratios of ductile-brittle adhesive usage is to define the most suitable configuration for stress reduction of the stress concentration regions. The ductile adhesive is used for overlap specific ends region due to minimizing stress concentration by the help of more flexible structure. On the other hand, the brittle adhesive is used for overlap specific center region for shifting stresses in center region of the overlap and enhancing durability of the structure. There is a balanced ratio of the ductile-brittle adhesive usage for strength of the adhesive bonding joints. In order to examine the stress concentrations changes for different adhesive and shapes, the finite element models for each configuration are generated and analyzed. All models' peel and shear stresses are compared with the general master adhesive model and each other. As it is expected the maximum stress for both peel and shear stress are observed both ends of the adhesive for master adhesive model. On the other hand for both peel and shear stress levels of center region of overlap are almost zero, unlike the stress concentration region where both peel and shear stress levels are around 50 MPa and 40 MPa, respectively. The stress reduction of the critical regions for different adhesives and pattern shapes models are varied 25% to 50% according to master adhesive model. Both peel and shear stress levels are around 30 MPa and 20 MPa, respectively for adhesive shape models. On the other hand, center region of the overlap, especially the shear stress levels are around 25 MPa. The shear stress levels are around 5 MPa for the same region of master model. The stress concentration regions shift towards the center region of the overlap. Therefore, the more uniform stress distributions can be obtained. The other way to decreasing stress levels is taper adherends model and spew fillet model. Both models are examined in details and compared each other and master single lap joint model. Especially, the low stress levels in critical regions on the spew fillet model are observed according to master single lap joint and taper adherends model. Finally, the fiber angles effects on the stress distributions of the adhesives are investigated and compared each other and master single lap joint model. The stress analysis results and stress distributions on the adhesives are effected significantly by fibers angles changing. According to the orthotropic elastic mechanics theory and plane strain assumption, composite plies in all models are generated via the equivalent engineering constants which are obtained only by the material properties of unidirectional composites.