Thermoelastic stress analysis of laminated composite materials

摘要

In this work thermoelastic stress analysis (TSA) is used to obtain quantitative stress/strain data from a variety of multi-directional laminated composites. In order to interpret the thermoelastic signal correctly the source of the thermoelastic response has been investigated in detail. In this thesis four possible routines to extract quantitative stress/strain information from thermoelastic data have been explored. A set of carefully selected glass/epoxy composite specimens with designated stacking sequences provided a scheme to identify the source and nature of the thermoelastic response. All of the material properties of the composite laminate were obtained experimentally, to aid an accurate assessment of each routine. The variation in the stress experienced by the laminate in the surface resin layer and ply by ply thereafter leads to large variations in the temperature change through the thickness. The thermoelastic measurements from different laminates revealed a local non-adiabatic condition within the layered medium.Therefore, the implication of applied loading frequency on the heat conduction properties of the laminates was studied. Based on the experimental observation from a representative specimen, numerical models have been developed to understand the nature of theheat transfer in the glass/epoxy material considered in this work. An analysis of the effect of holes in a variety of laminated components is presented to provide stress concentration factors (SCF's) based on TSA data. The conventional, orthotropic surface ply model most often used for thermoelastic stress analysis of composite material is revisited in order to elucidate the invariant nature of the equation. This is an important base for the analysis of structures which are better notated in coordinate system other thanCartesian, or as ratio of thermoelastic measurements in two different coordinate systems. The nature of the thermoelastic response in the presence of the in-plane stress gradient is investigated with the aid of numerical and analytical models. An introductory work for quantifying the SCF's around pin-loaded holes in laminated composite based on TSA measurements is also presented. The work presented in this thesis provides a step forward in the application of TSA to the composite materials in a quantitative manner.