Temperature effects on material properties and structural response of polymer matrix composites

Structural components used in civil engineering applications are often subjected to compressive loads. Unlike the tensile strength of fiber-reinforced polymer (FRP) materials, their compressive strength is resin-dominated, exhibiting lower values and more scatter due to initial imperfections in the resin and fibers caused during fabrication. The failure of FRP materials under compression resulting from the synergistic effect of failure mechanisms such as kinking and buckling is more complicated than that under tension. Furthermore, the material properties of composite structural components under compression are sensitive to temperature elevations occurring in engineering applications. This thesis includes experimental and analytical investigations in order to deepen the knowledge and enhance the understanding of the effect of temperature on the polymer and FRP material properties and the structural response of FRP composites in thermomechanical compression loading conditions. An introduction including a literature review on the topic is presented in chapter 1, while in the second chapter of the thesis a new physically-based model for the simulation of the imaginary part of the shear modulus-temperature (G"-T) curve of thermosets is introduced. The model is based on the modification of the G"-T equation used for thermoplastic polymers through the introduction of two new formulations regarding the calculation of the configuration probability and the velocity matrix of the examined thermosets. In addition, an arctangent function is proposed to consider the effect of temperature on the mean square separation of the ends of the sub-molecules. The introduced model is based on a sound physical background, and can be used for the investigation of the effect of the molecular structure of thermosets on their G" modulus and their glass transition temperature (Tg). In the third chapter the influence of thermal lag on the Tg of polymers as measured under different heating rates during dynamic mechanical analysis (DMA) is investigated. The observed thermal lag results were experimentally quantified and numerically simulated by a developed finite volume model. A numerical procedure was adopted to calculate the influence of the thermal lags on the storage modulus-temperature curve. It was found that when the thermal lag effects are excluded, the storage modulus of a specimen and Tg decreases with increasing heating rates. The fourth chapter focuses on the investigation of the kinking failure mechanisms of non-slender glass fiber-reinforced epoxy prismatic specimens subjected to axial compression loadings. Digital image correlation is used to map the surface strain fields in order to identify the failure modes. The kink initiation mechanism and the kink band formation can be clearly observed. In the fifth chapter the kink behavior of non-slender glass fiber-reinforced epoxy prismatic specimens of variable length and fiber volume fraction at elevated temperatures is investigated. Three different failure modes are observed and attributed to different temperature ranges and the corresponding material states. Different kink initiation and the kink band formation mechanisms are observed at the glass transition and the rubbery states of the examined material, however, the different specimen slenderness and fiber volume fractions does not influence these mechanisms.