Blistering of Moisture Saturated Graphite/Polyimide Composites Due to Rapid Heating

Polyimide matrices extend the role of composite materials to applications in extreme temperature environments. However, composites can be susceptible to damage under extreme hygrothermal environments such as rapid heating of moisture saturated materials. Here, rapid is defined as reaching high temperature in less than the drying time at that temperature. A new method to predict initiation of steam-pressure induced damage for rapidly heated neat resin and graphite/polyimide composites is proposed. This method entails comparing the calculated, available steam pressure within the laminate to an experimentally determined critical pressure-temperature envelope. Through experiments performed in a thermal mechanical analyzer it is shown that the onset of steam-induced damage can be detected by measuring the expansion of moisture-saturated specimens subjected to a rapid temperature ramp. Optical microscopy of damaged samples shows that the process of initiation and evolution of damage in neat resin and laminates begins with void growth and coalescence in the polyimide resin matrix. Data from tests performed over a range of heating rates and initial moisture saturations are used to develop a critical pressure-temperature envelope. With this envelope we show the dependence of damage on initial moisture content and heating rate and propose an application of this envelope to failure prediction and design of laminated structures subjected to rapid heating.