Investigations on fracture toughness and fracture surface energy of 3D random fibrous materials at elevated temperatures

In this study, model Ⅰ fracture toughness of three-dimensional random fibrous (3D RF) materials with a porosity of 83% is investigated from room temperature to 1273 K by performing experiments with compact tension (CT) specimens in the through-the-thickness (TTT) and in-plane (IP) directions. The experiments show that the fracture toughness in the TTT and IP directions increases from 0.0842 to 0.1162 MPa·m^1/2 and 0.4292–0.6767 MPa·m^1/2, respectively, with increasing temperature until a critical temperature (1073 K and 1223 K, respectively) is reached, following which the fracture toughness decreases from 0.1162 to 0.0819 MPa·m^1/2 and from 0.6767 to 0.6170 MPa·m^1/2, respectively. The significant changes at elevated temperatures are directly attributed to the viscous flow of the crack tip. In addition, a viscous flow contribution to the fracture surface energy of the 3D RF material is identified at high temperatures. The curves of the fracture surface energy versus temperature data for the TTT and IP directions can be partitioned into sections by fitting with experimental observations. Based on the two fitted curves, we calculate the activation energy of the viscous flow in the TTT and IP directions as 220.3 kJ/mol and 892.2 kJ/mol, respectively.