Size Effect in Paper Fiber-Reinforced Gypsum Panels Under In-Plane Bending
Keywords:Paper fiber-reinforced gypsum panels, in-plane bending strength, size effect, strain softening
This paper discusses the influence of the size effect on the in-plane bending strength of paper fiberreinforced gypsum panels. The incombustible composite material consists of randomly dispersed, recycled paper fibers embedded in a gypsum matrix. The brittleness of the material is considerably lower as compared to classical gypsum board. The panels are primarily employed in building industry for sheathing and bracing of timber and steel frame wall elements and for flooring applications.
The size effect study was performed on 3-point bending specimens that were identical in thickness, being 12.5 mm; depth however, with seven different dimensions was ranging from 10 to 320 mm. The experiments, for which the sample size was nominally eleven specimens per depth, were performed with a constant cross-head rate until complete separation of the specimens. The load-deflection curve was completely stable throughout the loading, even on the descending load branch beyond peak load. Thus, paper fiber-reinforced gypsum panels represent a strain-softening material. The nonlinearity of the stressdisplacement curves becomes more accentuated near peak load for smaller depths. This can be explained by the increasing ratio of the fracture process zone length to the specimen depth.
The tests revealed a nonlinear strength reduction with increasing specimen size, which was fitted by a one-parameter power law equation based on a Gauss-Newton approach. The statistical analysis of the fitted size effect curve verified the basic assumptions of normally distributed residuals and of a constant variance of the different samples. Furthermore, analysis of the variance revealed the adequacy of the chosen model function. For statistical inference on the population, confidence intervals are given for the model parameter and for the mean and individual strength values.
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