Thermal and mechanical analysis of polymer matrix composite materials exposed to a concentrated heat source for a short duration

Properties and performance of composite materials are well known at ambient temperatures but need to be characterized when exposed to high temperature for short durations. The material may either show physical damage or show no physical changes in terms of discoloration of the resin, delamination or significant weight loss. In the latter case, it is necessary to understand the impact of the exposure on its mechanical performance. The objective of this work is to predict and validate the temperature history and to assess the extent of damage of a composite plate exposed for short duration to a concentrated heat source in terms of mechanical property changes.; A laminate of a thermoset polymer matrix composite material fabricated using carbon fibers and Bismaleimide (BMI) resin is exposed to two different types of heat sources for a short duration: a gas flame and an infrared heater. Gas flame models the fire effect; the infrared heater exposes a large area of the sample to a uniform temperature. The influence of this fire/heat on composites materials is predicted using a thermal model. A parametric study is conducted with a finite element based software, Wintherm, in which appropriate boundary conditions are implemented. An approach to predict the temperature distribution in a composite plate exposed to a gas flame was developed numerically. The convection parameters were calculated from modified empirical expressions used for plumes which account for the change in air temperature from the centerline to the outward edges in the region between the source and the laminate. Validation experiments were conducted; the temperature distribution was measured with an infrared camera. The results compare well with the numerical predictions.; A methodology has been developed to assess the degradation of structural composites which don't have any visual flaws after being exposed to a concentrated heat source for a short time period. The degradation of the structural composite is defined as a reduction of mechanical properties from the designed value. Flexural tests were conducted on composites materials exposed to an infrared heater. As the methodology is based on the thermal history of the material, this work focuses on high temperature exposures (up to 454°C (850°F)) for short duration (1 to 20 minutes). Results show that there are temperature and time combinations for which significant mechanical performance degradation occurs, though there is no detectable change in the material. For example, a drop of 30% in flexural strength is observed for an exposure of 360°C (680°F) for 20 minutes even though there is no visible change in the material or significant weight loss. The results are compared with the resin changes which are observed for long time exposures at high temperatures, which show similar trends. Indeed, previous works have reported a correlation between drop in mechanical properties and the resin chemistry.; Thus, for given boundary conditions of a composite material exposed to a concentrated heat source for a short duration, the degradation of the material can be estimated in terms of mechanical property changes, from the predicted thermal history of the exposed sample.