Modeling matrix failure in composites due to cryogenic temperatures

A drastic reduction in structural weight is an indispensable prerequisite to realize future high altitude area defense systems or single stage to orbit space vehicles. Boost defense programs such as the Airborne Laser (ABL) and Space-Based Laser (SBL) as well as numerous optical-based systems require the storage, transport and dispensing of large amounts of cryogenic fluids. All elements contributing to the mass of the cryogenic systems must be as light as possible, particularly the tanks which are one of the most challenging parts. Today, the majority of cryogenic tanks are made of insulated aluminum, stainless steel, or a metal liner with composite overwrap. While these tanks have high damage tolerance and chemical resistance, they do so with a relatively high mass penalty. Advanced composite materials can provide significant mass reduction due to their high specific strength and stiffness ratios, compared to metals, but are susceptible to cracking under thermal cycling. Thermally induced cracking is the result of large internal stresses generated at cryogenic temperatures due to a mismatch in coefficients of thermal expansion between the fiber and the matrix. Cracking of composite cryogenic tanks is classified as a component failure because the increased permeability of the tank results in significant loss of stored cryogens. Current state-of-the-practice analysis technologies, such as linear elastic fracture mechanics, micromechanics, and classical lamination theory have been found to be lacking in their capability to be applied in the general design of composite cryogenic tanks. Primarily in their ability to accurately and easily model thermally induced cracking in advanced composite materials. Faster and more accurate analysis techniques were needed as enabling technologies for improving composite cryogenic tank design. Thus, a research effort was initiated by the Missile Defense Agency to improve prediction and analysis methods. An alternative analysis method, called Multi-Continuum Theory (MCT), uses a classic Hill strain decomposition technique to solve for the phase averaged stress and strains in each constituent of the composite with a minimum of computational complexity and virtually no time penalty. Constituent information is valuable because thermal damage in a composite begins at the constituent level and may in fact be limited to only one constituent. Accurate prediction of constituent failure at a single point enables the analysis of progressive damage growth throughout the composite. In this paper, we describe how MCT was used to predict thermally induced composite damage, primary matrix cracking, of carbon fiber reinforced plastic composite specimens. The MCT results were benchmarked against existing analytical methods to assess any improvements in accuracy and ease of use.