Thermomechanical interaction effects in polymer foam cored sandwich structure

Abstract

Polymer foam cored sandwich structures are frequently exposed to elevated temperatures in the range of 50-100°C. Recent theoretical studies showed that elevated temperatures may shift the behaviour of sandwich structures from linear and stable to nonlinear and unstable. Although this prediction has not been experimentally validated, it has aroused concerns on the performance of sandwich structures at elevated temperatures. Thereby, a focused experimental study is required to confirm the thermomechanical interaction effects. The work described in this thesis provides experimental assessments on the thermomechanical interaction effects in polymer foam cored sandwich structures.

As a starting point, a novel methodology is proposed to obtain the elastic properties of polymer foam materials at elevated temperatures based on digital image correlation (DIC). The tensile, compressive and shear properties were characterised at temperatures from room temperature to 90°C. It is identified that the thermal degradation of the elastic modulus of polymer foams only depends on the base polymer, regardless of the deformation type or foam density. A master curve is derived which shows the temperature dependence of the Young's and shear moduli of Divinycell H100, H130 and H200 foam. An experimental apparatus is constructed to allow sandwich beam specimens to be subjected to a bending load as well as a well-defined temperature gradient through the beam thickness. The bending deflection, inhomogeneous core shear deformation and the wavy deflection of face sheet at the onset of localised buckling/wrinkling are accurately characterised using DIC. High-speed imaging was also adopted to view the rapid evolution of the wrinkles. It was found that, at elevated temperatures, the core shear strain can be nonuniform through the core thickness and the failure mode can shift to wrinkling instability. Simple analytical models are also developed to predict the behaviour of sandwich beams with transverse temperature gradients and consequently core stiffness gradients.

A modification of the classical sandwich beam theory is proposed to predict the stress/strain state, load-defection response and failure mechanism with respect to the core shear yielding and face sheet yielding/fracture. Another analytical model is developed to predict the critical wrinkling stress. Both of the two models agree well with the experiments. The work described in the thesis demonstrates that the stiffness and load carrying capability of polymer foam cored sandwich structures are deteriorated by elevated temperatures. The deterioration can be confidently predicted by the experimentally validated analytical models introduced in the thesis. The study thereby provides a significant step towards an improved understanding of thermomechanical interaction effects in polymer foam cored sandwich structures.

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