Constant Poisson’s Ratio of Thermosetting Polymers: Is It Reasonable for Accurate Thermal Stress Analysis?

For an elastic stress analysis of packages subjected to a thermal cycle, two temperature-dependent material properties are required: (1) Young's modulus and (2) Poisson's ratio. Commercial instruments such as dynamic mechanical analyzer (DMA), universal tensile testing machine (UTTM), etc., are routinely practiced to measure the temperature-dependent Young's modulus. The Poisson's ratio is, however, often assumed to be a temperature-independent constant (typically a value estimated at room temperature) because it is more difficult to obtain experimentally. In this paper, the effect of Poisson's ratio on the elastic stress analysis of packages containing thermosetting polymers is investigated first. The results from two case studies, where the polymers are subjected to multiaxial constraints, are reported to illustrate the effect. Then, an advanced method to accurately measure temperature-dependent Poisson's ratio is proposed and implemented. The method uses a robust optical strain sensor, called Fiber Bragg Grating (FBG). The sensor is embedded in a cylindrically-shaped polymer specimen, which is subjected to hydrostatic pressure. The FBG signals are documented at various temperatures, from which the temperature-dependent Poisson's ratios are determined. The results confirm the strong temperature-dependency of Poisson's ratio with an extreme gradient around the glass transition temperature.