The effect of radiation exposure on the mechanical behavior of Rohacell foams used in composite structures

Rohacell foam is being utilized on a number of composite structures used for space applications. The effect of radiation exposure on these materials has been evaluated to determine the degree of structural degradation that may occur in a space environment. A ⁶⁰Co source was used on Rohacell 31HFHT, Rohacell 110WF, and Rohacell HERO 110. The exposure levels varied from 0 to 15 Mrad, in both an inert and an air environment. Tensile tests were then performed to determine the failure strength and strain. Mechanical testing of the 31HFHT specimens showed a significant decrease in strength of 50% after only a 4‐Mrad exposure. The failure strain also exhibited a 75% decrease over the same range. The 15‐Mrad specimens continued to decrease in strength, losing 95% of the initial strength. The rate of tensile strength degradation with radiation increased as the foam cell pore diameter increased. Dynamic mechanical analysis indicated an increase in the peak intensity of the tan delta curve as a function of radiation exposure, which has been attributed to radiation‐induced polymer chain fragmentation. The fracture behavior of the radiated specimens also exhibited embrittlement of the polymer cells consistent with the mechanical properties. Thermal gravimetric analysis showed a decrease in the temperature for the onset of mass loss with radiation exposure, which is consistent with chain scission of the polymer network. Even though the chemical structure of all polymethacrylimide foams is susceptible to radiation degradation, a smaller foam cell pore diameter may provide additional margin with respect to the rate of mechanical property degradation. POLYM. COMPOS., 39:2181–2189, 2018. © 2016 Society of Plastics Engineers