CONTENTS Young Stress and Vibration Analyst Competition Finalists A validated point-wise approach to the analysis of stresses and strains around complex composite geometries using Digital Image Correlation and Thermoelastic Stress Analysis

Introduction Composite materials are weak normal to the plane of the laminate due to a lack of through-thickness reinforcement, with strength dictated by the brittle epoxy matrix. Therefore it is important to evaluate the through-thickness load transfer in composite bonded joints because of their discontinuous nature to improve confidence in the join and inform more efficient joint designs. A double butt strap joint (DBSJ), Figure 1, was constructed with 800g/m 2 unidirectional and 450g/m 2 chopped strand matt glass fibre in a [CSM 8 90 4 ] s sequence using Gurit Prime 20lv epoxy resin using the resin infusion process. Araldite 2015 epoxy adhesive was used to bond the adherends. Figure 1: Schematic of Double butt strap joint Meso scale analysis Component strains within the DBSJ were evaluated using 2D DIC. A Canon mp-e65 macro lens connected to a 5Mp LaVsion 5Mp ELite camera was used to image an area of 3.1 mm x 2.6 mm around the discontinuity between adherends. The specimen was mounted in an Instron 5569 test Inner adherend Outer adherend (strap) Adhesive Root of discontinuity Area of analysis Strap length Overlap length 1 machine and loaded at 1mm/min up to failure. Complex localised strain distributions were revealed, shown in Figures 2 and 3. Detailed analysis of the strain fields identifies small, yet critical, through-thickness and shear strains evolving within the joint. To validate the DIC strain values, Thermoelastic Stress Analysis (TSA) was conducted. TSA provides a fast and accurate experimental technique, to capture the complex behaviour around the join at similar length scales as the DIC. The two independent experimental data sets provide sufficient detail to fully inform numerical models. When a material experiences a stress change it is accompanied by a small temperature change [1], ΔT. The relationship between ΔT and the change in the sum of principal stresses, σ 1 , σ 2 is as follows for an orthotropic material () (1) where α 1 and α 2 are the coefficients of thermal expansion in the principal stress directions, ρ is the material density, C p is the specific heat at constant pressure, and T is the specimen temperature. These constants form the 'thermoelastic constants' in the principal stress directions K 1 , K 2 , providing the relationship between the sum of the principal stresses,