Cryogenic fracture toughness of 3D random fibrous materials

This study aimed to evaluate the fracture toughness of three-dimensional random fibrous (3D RF) materials with 87% porosity and shallow surface cracks at room temperature (20 ℃) and cryogenic temperature (− 100 ℃). The ratios of crack length to specimen width of three-point-bending (3-p-b) specimens were 0.2 and 0.5, respectively. The experimental results revealed that the average fracture toughness K_IC at − 100 ℃ was higher than that at 20 ℃. In addition, the experimental data were analysed using the boundary effect model (BEM), with almost all experimental data included in the upper and lower boundaries with 96% reliability, as specified by a normal distribution. Furthermore, based on the fracture toughness K_Q computed using the formula of American Society for Testing Materials (ASTM), the expression of the fibre gap size G is derived. When the confidence level was 95%, no significant difference was observed in the fracture toughness model when the fibre gap size G was replaced with the average fibre spacing size D₂ in the fracture toughness model. Thus, this study is expected to provide a reference for the safety assessment of the insulation layer of space shuttles at cryogenic temperatures.