Characterization and modeling of oriented strand composites

A strong relationship exists between the spatial structure of a wood-strand composite and its compaction characteristics, density, mechanical properties, dimensional stability, and fastener performance. The following research proposes a physical model to represent an oriented strand composite panel and applies a mechanics based approach to estimate its elastic properties. The objectives of this research were to (1) review literature on recent progress in characterizing the structure and modeling of wood-strand composites, (2) understand the effects of undulating strands on the elastic behavior of a wood-strand composite and develop a general constitutive model to predict the elastic properties as a function of strand waviness, (3) characterize the elastic properties of strands and develop a response model to account for the effects of hot pressing, (4) quantify key random variables for characterizing the three dimensional structure of an oriented strand composite, and (5) validate the fiber undulation model to predict the elastic properties of an oriented strand composite.; The fiber undulation model (FUM) was shown to predict the elastic properties of wood-strand laminates with predetermined strand undulations reasonably well. Strand undulation degrades Young's modulus in both tension and compression as the undulation angle increases. Model estimates of E_x agreed very well with compression E_x (2 to 4 percent error), but were on the average 12% lower than tensile E_x. Difference between tension and compression Young's moduli is attributed to phenomenological differences in behavior of undulating strands when subjected to tension versus compression and deformation of wood at microstructure level. Response models based on mixture design, considering the hot pressing effects, were developed to predict E₁ and ν₁₂ of aspen strands. This study indicates a direct relationship between densification during hot pressing and E ₁. The results of the simplex analysis suggest the addition of resin tends to increase stiffness by restraining springback of strands.; The structure of a wood-strand panel was characterized with the out-of-plane undulation of strands in both the longitudinal and transverse directions, and the void volume between strands. Strand undulations were represented quite accurately with discrete Fourier expansions. A series rule of mixtures with probability density functions of undulation angle distributions was utilized to account for the effects of out-of-plane strand waviness and in-plane strand deviations on the elastic constants of a wood-strand composite. The FUM was validated for unidirectional wood-strand panels. Model predictions consistently over-predicted elastic properties from compression tests and under-predicted elastic properties from tensile tests. Average reduction in E_x due to strand undulations was approximately 6.9%. The results of this study show that random and incidental strand undulations in the longitudinal and transverse directions, which are inherent attributes of oriented strand composites, are significant and could potentially influence the physical and mechanical behavior of composite panels.