Constitutive model calibration for the thermal viscoelastic properties of laminated glass interlayer materials
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The use of blast-resistant glazing such as laminated glass in buildings can greatly reduce, if not eliminate, the hazard of flying glass shards. In a failure event, fractured glass shards adhere to the polymer interlayer and do not fly or fall. Under dynamic loading scenarios such as blast, the interlayer deforms largely providing post-cracking energy absorption to the laminated glass system. When properly designed, laminated glass polymer interlayers can maintain the integrity of the building envelope in extreme events such as blasts or hurricanes, protecting the interior from damage. The polymeric interlayers are often viscoelastic materials, and therefore are highly strain-rate-dependent and temperature-dependent. As part of the building envelope, the laminated glass system is subjected to environmental effects such as temperature-humidity cycles and water infiltration which can significantly alter the mechanical behavior of the polymer interlayer and its bonding with glass. Interlayer materials often exhibit high heterogeneity in the deformation field and may undergo necking phenomenon, necessitating advanced methodologies such as digital image correlation (DIC) for understanding and predicting their behavior under mechanical stress. Experimental evaluation of laminated glass under blast loading is constrained by its exorbitant cost and the formidable difficulty of execution, compounded by the fact that most researchers lack the requisite capabilities. Finite element analysis software, such as LS-DYNA, can be used in place of experimental testing to research the behavior of laminated glass under blast loading. However, material models that adequately capture the viscoelastic behavior of the polymeric interlayer materials used in laminated glass are not available in the widely used finite element analysis software. Developing material models that capture the strain-rate and temperature dependent behavior of laminated glass interlayer materials is vital to further the understanding of laminated glass window systems subjected to dynamic loading such as blast. Therefore, an extensive research plan to experimentally evaluate the high strain rate, temperature, and environmental effects on the mechanical responses of six different laminated glass interlayer polymers was performed. Quasi-static tensile tests were conducted using the two most widely used polymeric interlayer materials in LG panels for structural applications, PVB and SG6000, and digital image correlation (DIC) was utilized for observation of possible necking phenomenon, precise strain distribution measurement along the gauge length, stress-strain analysis, and ultimately determination of the localization length which is a key parameter for predicting the response of materials that undergo necking in their mechanical response. The dynamic resistance functions derived from the experimental testing at various strain rates and temperatures were used to calibrate new viscoelastic material models for the dynamic responses PVB and SG interlayer. The material models were then implemented into finite element simulations of the high strain rate tensile tests to validate their accuracies for predicting the highly temperature and strain rate dependent dynamic responses of PVB and SG laminated glass interlayer materials. The results of this dissertation will improve the existing blast design and analysis methods for predicting the dynamic response and dynamic reactions of laminated panels, which will enhance building safety under extreme loading caused by explosion threats.
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Ph. D.