Prediction of mechanical behaviour of adhesively bonded CFRP scarf jointed specimen under tensile loading using localised DIC and CZM

The present study focuses on the mechanical behaviour of both single and double tapered scarf adhesively bonded joint of Carbon fibre reinforced polymer (CFRP) laminate as adherend subjected to tensile loading. The layup sequence of the CFRP adherend having unidirectional (UD) [0⁰]₁₆ and quasi [+45/−45/0/90]_2S are studied. The adhesive used here is Araldite 2015 supplied by Huntsman which is a two part epoxy system of intermediate toughness grade. Here, 2D digital image correlation (DIC) technique is used for capturing the whole field longitudinal, peel and shear strain distribution over the adhesive bond line of the CFRP specimen. Further, a localised DIC measurement is also carried out using microscopic tube lens for precisely capturing strain field over concentrated zones where damage initiation occurs. The evolution of whole field strain distribution with increasing load is captured to predict the mechanical behaviour and failure mechanism of a tapered scarf joint specimen. In addition, 2-D finite element analysis (FEA) of scarf joint model is carried out for validating the DIC results. In the finite element model cohesive zone elements are used for the modelling of both adhesive layer and inter/intra laminar interface of the composite laminate. Initially, to verify the proposed numerical model, joint's initial stiffness, failure load and corresponding displacement obtained from FEA are compared against the experimental load – displacement results. Later, qualitative and quantitative comparison of longitudinal, peel and shear strain values obtained over the adhesive layer by DIC and FEA is carried out to confirm the accuracy of the DIC results. A decent correlation is found to exist between the DIC predictions and numerical results thereby confirming the accuracy of the DIC technique. Analytical solutions are also derived for the same problem based on mechanics of material and further it is compared with both FEA and DIC predictions for completeness.     

Effect of TGO thickness,pores, and creep on the developed residual stresses in thermal barrier coatings under cyclic loading using SEM image-based finite element model

Thermal barrier coatings (TBC) allow the metallic internal components of gas turbine engines to operate at elevated temperatures near its melting points. Formation of thermally grown oxide (TGO) layers at the top coat (TC) and bond coat (BC) interface induces cracks in the TC that may lead to complete TBC failure due to spallation. An SEM image-based finite element (FE) model is developed using commercial finite element package ABAQUS to investigate the development of residual stresses resulting from cyclic loading of TBCs. The model includes thermo-mechanical material properties and considers the real interface between the coating layers. The model includes real pores based on an SEM image, taking advantage of image processing techniques. Effect of TC surface roughness and pores on the developed residual stresses during thermal cycling is investigated with respect to different TGO thicknesses. The analysis shows that presence of TC roughness causes stress concentration sites during heating that may force horizontal cracks to initiate and propagate with stress values that are indifferent to the TGO thickness. The pores are found to shift stress concentration regions from the TC/TGO interface to the vicinity of the pores during cooling, and that may cause horizontal cracks to start from within the TC with stresses that increase with TGO thickness. Moreover, the effect of creep for all layers on the generated residual stresses is studied. Considering creep gives lower stresses at the end of cooling, however, stress distribution remains the same with and without creep.