Fracture Toughness of Wood Fiber Gypsum Panels from Size Effect Law

This paper addresses the size effect of inplane bending strength as well as Mode I fracture toughness and process zone length of wood fiber-reinforced gypsum panels. Wood fiber gypsum panels represent an incombustible short fiber composite material composed of recycled paper fibers embedded in a gypsum matrix. The material, which is used for sheathing and bracing of timber frame constructions, exhibits marked fracture softening supposedly resulting in a considerable size effect. In the paper presented, in a first step Bazʹantʹs size effect law for quasi-brittle materials is derived. The parameters of this size effect law are then determined by means of nonlinear regression analysis applied to a test series with scaled single edge notched beam specimens. Detailed consideration is given to the adequacy of linear confidence intervals of the model parameters in comparison to nonlinear inferential results. Finally, the probability densities of fracture toughness and fracture process zone length are determined from the distributions of the size effect parameters by means of theory of random variables.