A novel evaluation of fracture toughness for random fibrous material

In this study, we perform three-point-bending tests for 3-dimensional random fibrous (3D RF) material specimens of two porosities (87% and 83%) with a crack. The crack with two different crack-length-to-specimen-width ratios (0.2 and 0.5) is considered. Using the tensile strength obtained from experiments and a characteristic average fiber spacing parameter from an equivalent orthotropic fiber network model, a simple theoretical model is developed to evaluate the fracture toughness of 3D RF material. This model gives the fracture toughness of 3D RF material, which is in good agreement with that from compact tension test, and better than the results calculated via the American Society for Testing Materials (ASTM) standard for three-point-bending test. This fact not only rationalizes the use of average fiber spacing size in the developed model, but also demonstrates the model efficiency in evaluating the fracture toughness of 3D RF material. Further, considering the temperature effect on the fracture strength of material, this theoretical model can be successfully used to predict fracture toughness of 3D RF materials at elevated temperatures (299 K~1073 K).