Failure investigation of composite materials using multi-instrument experimental techniques and numerical method

Official URL: https://risc01.sabanciuniv.edu/record=b2579523_(Table of contents)

Carbon fiber reinforced polymer matrix laminates are widely used in aerospace, automotive, turbine blades and marine structures due to their light weight, and high specific stiffness and strength. Another important feature of laminated composites is anisotropy, which enables one to tailor their strength depending on the stacking sequences and the constituent material combinations. However, failure analysis of fiber laminates is very complicated process due to the presence of multiple constituents and spatially heterogenous damage accumulation inside the material. Therefore, an extensive investigation is carried out in the current study to comprehensively analyze various failure modes and damage growth in carbon fiber reinforced polymer composites, under various loading conditions, by simultaneously utilizing numerous structural health monitoring techniques. The outcomes of these investigations have resulted in four distinct papers and are presented individually in this study. In the first paper, acoustic emission (AE) analysis coupled with digital image correlation (DIC) is used to study the effect of end-tabs’ material on the mechanical properties and failure accumulation in angled-ply unidirectional carbon fiber reinforced plastics (CFRPs), under tensile loading condition. The strain fields, obtained from DIC, are found to be relatively more uniform, indicating an effective load transfer to gauge section, when the difference between the elastic modulus of the tab material and test material becomes smaller. The damage accumulation rate from the evolution of AE energy results determined three distinct stages in AE activity. The duration of the second stage is observed to be longer, slower damage progression, in the specimens with smaller difference in elastic modulus between the tab material and the test material, supplementing the DIC analysis. The second paper from this thesis, utilizes a comparison between three different strain monitoring techniques, namely DIC (contact-less), Fiber Brag’s Grating (FBG) sensors (embedded) and strain gauge (surface mounted), simultaneously during tensile testing of the multidirectional (angled-ply) CFRP laminates. This assisted not only to effectively monitor the strain evolution but also directed to analyze through the thickness failure progression by correlating the inconsistencies in strain measurements from different methods with the tendency of interlaminar delamination due to distinct layup sequence in each laminate. To complement further, AE, and Infra-red thermography (IRT) are also employed simultaneously to identify and classify the characteristic failure types as matrix crack, interface failure or fiber failure. The third paper investigates the size-effect i.e., the effect of thickness variation on the mechanical properties and failure dynamics of woven CFRPs under tensile and in-plane shear loading, by simultaneously employing AE, DIC, and IRT. Wherein, it is revealed that micro‐damage initiation point is observed earliest in the thinnest laminate, showing lowest maximum tensile and shear strengths, as compared to the thicker ones. The failure accumulation in thicker laminates is found to be delayed due to activation of delamination failure in thicker laminates unlike the thinner ones, in which, the damage is predominantly controlled by comparatively lower energy release phenomenon i.e., matrix cracking. The fourth paper in this thesis presents a comparison and scrutinization of failure initiation and progression in laminates containing two interacting notches with that of single notched laminate under tensile loading, by utilizing DIC and IRT simultaneously. The physics, behind the increase in strength and the decrease in stress concentration around the notches in laminates with two holes drilled parallel to the loading direction as compared to that of single hole laminate, was successfully unearthed. It was revealed that a relatively high energy release rate mechanism i.e., interface failure was more dominant in the laminates with two holes, parallel to loading axis, as compared to a low energy release mechanism i.e., transverse matrix cracking, being dominant in single hole laminates.