Understanding Failure Mechanisms in Hybrid Fiber Reinforced Laminates through the Combined usage of DIC, AE, Thermography and Optic Based Systems

Official URL: https://risc01.sabanciuniv.edu/record=b2553610_(Table of contents)

Carbon fiber reinforced polymer matrix laminates are widely used in automotive, aerospace and transportation structures due to their light weight, high specific stiffness and strength, and chemical durability. However catastrophic failure of these laminates limits their capability for critical engineering applications. Fiber hybridization is a low cost and effective way to overcome this issue and improve the reliability of Carbon fiber reinforced polymer matrix laminates by introducing Hybrid Effect. Nevertheless, failure analysis of hybrid fiber laminates is very difficult task due to presence of multiple constituents inside the laminated structure and spatially heterogenous damage accumulation inside the material. Therefore, this study aims to comprehensively analyze the failure mechanisms and damage development inside glass/carbon fiber hybrid laminates through simultaneous usage of structural health monitoring techniques under various loading conditions in distinct investigation given as three papers. In the first paper, acoustic emission analysis is used to monitor the damage growth during tensile and flexural tests for hybrid and nonhybrid specimens. The acoustic emission data is clustered by using Kmeans method based on weighted peak frequency and partial power parameters. Four different clusters are associated with four different failure types namely, Matrix cracking, fiber/matrix interface failure, fiber pull out and fiber breakage for each laminate. An finite element model based on Refined Zigzag Theory (RZT) is utilized to predict the linear behavior of composites and it is shown that onset of major damages recorded by acoustic emission sensors is corresponding to deviation of experimental stress-strain curve from model predictions. The second paper of this thesis uses full field strain measurements to analyze damage development under in-plane shear condition where glass/carbon fiber hybrid and nonhybrid laminates sequentially show linear and nonlinear response to the applied shear stress. Comparison of strain maps obtained from three-dimensional digital image correlation (DIC) system show the effect of stacking sequence on damage development behavior. Moreover, by selecting appropriate regions of interest for full field strain measurement technique i.e. DIC it is shown that accurate monitoring of shear behavior and failure at V-notch region is possible. The third part of this investigation shows that differences in tensile stress-strain curves obtained for hybrid and nonhybrid laminates by different strain measurement systems namely, surface mounted strain gauges, digital image correlation and two embedded FBG sensors inside the laminated composite material. The fluctuations in stress-strain curves are well-described using strain and thermal maps obtained by digital image correlation and thermal camera. It is shown that due to the nature of strain measurement techniques, global or local, some failures such as edge splitting might not affect all of strain measurement techniques and therefore cause miscalculation of initial failure point in hybrid laminates, i.e. overestimation of hybrid effect. Furthermore, DIC displacements are smoothed by Smoothing Element Analysis (SEA) and it is demonstrated that using such a mathematical modification can help to remove inherent noise of obtained data from full field measurements at low stress levels. Besides, smoothing analysis has successfully enabled early prediction of failure region in the composite material at stress levels 30% below the strength of laminate. Poisson’s ratio evolution monitored through digital image correlation is used for the first time as a damage index and compared with that of biaxial strain gauges for each sample. It is shown that strain gauges indicate faster damage accumulation inside the laminates due to their direct contact with the material under loading condition