Monitoring micro-damage accumulation based on reduction in elastic properties using fiber bragg grating sensors, and acoustic emission and thermography methods in fiber reinforced polymeric composites
Yılmaz, Çağatay (2018) Monitoring micro-damage accumulation based on reduction in elastic properties using fiber bragg grating sensors, and acoustic emission and thermography methods in fiber reinforced polymeric composites. [Thesis]
Laminated Composite Structures (LCS) have been used increasingly in recent years in engineering applications such as aircraft and shipbuilding. However, it is important to note that despite the constant increase in the usage of LCSs, a number of certain aspects need to be investigated in detail. These aspects can be summarized in two different topics: the effect of different micro-damage types on LCSs and novel test methods for the measurement of certain mechanical properties. Here, to touch on these two issues, we investigate micro-damage accumulation and its effects on elastic constants of the laminated composite as well as we introduce a Finite Element Analysis (FEA) of a possible test method to calculate certain mechanical properties of thin-laminated composites. Unlike metals, LCSs can develop different types of micro-damage under load. These different types of micro-damage play a critical role in the failure of LCSs. Once initiation and accumulation of different types of micro-damage are understood, an adequate understanding of LCSs under operational loads can be established. This understanding results in the prediction of possible failure time of LCSs and usage of a number of elastic constants as a micro-damage monitoring index. For that purpose, we present an experimental study on the monitoring of the reduction in Poisson's ratio coupled with micro-damage initiation and accumulation. Poisson's ratio is monitored with a novel an embedded-biaxial Fiber Bragg Grating (FBG) sensor during the static tensile test of composite coupons. Accumulation of different types of micro-damage are monitored with Acoustic Emission (AE) set-up. It is seen that as Poisson's ratio reduces under tensile loading, composite coupons emit a higher number of acoustic waves due to the micro-damage formation. It is also demonstrated that reduction in in-plane shear modulus under in-plane shear loading is due to micro-damage accumulation. For this purpose, two different fiber reinforcement, S-glass and E-glass, are chosen to produce laminated composites. The total amount of micro-damage incurred as well as the average temperature change measured by thermography are higher for the E-glass reinforced LCS. Under the applied in-plane shear load, a significant reduction in inplane shear modulus is observed both for the E-glass and S-glass-reinforced LCS, where the E-glass reinforced LCS shows a greater reduction. In addition, a FEA of Lamb wave propagation in thin-laminated composites is also performed. The performed FEA is a numerical simulation of a novel, nontime-consuming, non-destructive evaluation method for thin-laminated composite structures to calculate the components of stiffness matrix and laminate elastic properties. For that purpose, the group velocities of A0 and S0 modes of Lamb wave are measured on a transversely isotropic thin-laminate. Then, these group velocities are used in Christoffel's equation to calculate the components of stiffness matrix. In turn, the components of stiffness matrix are used in an inverse formulation to calculate thin-laminate elastic constants.
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