Anisotropic fracture analysis of the BX-265 foam insulation material under mixed-mode loading

The National Aeronautics and Space Administration (NASA) uses a closed-cell polyurethane foam to insulate the external tank (ET) which contains the liquid oxygen and hydrogen for the Space Shuttle main engines. This is a type of spray-on foam insulation (SOFI), similar to the material used to insulate attics in residential construction. In February of 2003, the Shuttle Columbia suffered a catastrophic accident during re-entry. Debris from the ET impacting the Shuttle's thermal protection tiles during liftoff is believed to have caused the Space Shuttle Columbia failure during reentry.;NASA engineers are very interested in understanding the processes that govern the breakup/fracture of this complex material from the shuttle ET. The foam is anisotropic in nature and the required stress and fracture mechanics analysis must include the effects of the direction dependence on material properties. Over smooth, flat areas of the ET the foam can be sprayed down in a very uniform fashion. However, near bolts and fitting points it is possible for voids and other defects to be present after the foam is applied. Also, the orientation of the foam, as it rises from non-uniform surfaces, can be significantly different from the orientation over typical acreage sprays. NASA believes that air present in these small voids is liquefied and then turned into a gas during liftoff. While the Shuttle is ascending to space, the pressure in these cavities can become large enough to force a subsurface crack toward the exterior of the tank, thus freeing portions of foam insulation.;As a first step toward understanding the fracture mechanics of this complex material, a general theoretical and numerical framework is presented for computing stress intensity factors (SIFs), under mixed-mode loading conditions, taking into account the material anisotropy. The effects of material orientation and mode mixity on the anisotropic SIF solution are analyzed. Crack turning predictions under mixed mode loading are presented. Furthermore, the influence of temperature gradients on the SIF solution is studied, in view of the thermal gradient present through the foam thickness on the ET. The results presented represent a quantitative basis for evaluating the strength and fracture properties of anisotropic BX-265 foam insulation material.