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Computational modeling of strand-based wood composites in bending Clouston, Peggi Lynn

Abstract

A stochastic finite element approach is presented herein for simulating the nonlinear behaviour of strand-based wood composites with strands of varying grain angle. The approach is based on the constitutive properties of the individual strands making it practical and versatile in application. It can be used to gauge the effects of varying strand characteristics in product manufacturing or it may be useful in the design of wood composite structures. Both 2 and 3 dimensional, stochastic, materially nonlinear finite element codes are developed. The nonlinear constitutive behaviour of the wood strands is characterized within the framework of rate-independent theory of orthotropic elasto-plasticity. The constitutive model employs the Tsai-Wu yield criterion with the associated flow rule of plasticity. Failure is marked by an upper bound surface whereupon either perfect plasticity or an abrupt loss of strength and stiffness ensues. This constitutive model is implemented into the finite element codes. The programs are based on the conventional displacement formulation using linear isoparametric elements. The nonlinearities are resolved by a modified Newton-Raphson procedure. The programs are further formulated in a probabilistic manner using random variables as input for principal material strengths and stiffnesses. To generate entire data samples for comparison with experimental samples, the programs were written with extended capacity to perform Monte Carlo simulations. The mechanical properties of the strands are derived through both experiment and analysis. An experimental database of principal material strengths and stiffness parameters is acquired for Douglas fir heartwood strands. Statistical parameters for shear strength and stiffness as well as the interaction parameter of the Tsai-Wu criterion are estimated, however, through a least square minirriization of error between simulated and experimental compression strength of [+15]s angle-ply laminates. Weibull weakest-link theory is employed to adjust experimental tensile strength values for size effect. The general performance of the programs is verified through comparison of results for several analyses solved using analytical techniques or alternate programs. Following this, the ability of the models to reproduce experimental findings for angle-ply laminates in tension, compression and 3 point bending is validated. A preliminary investigation is conducted to compare numerical simulations with experimental data for Parallam* in tension and 3 point bending. The favourable comparisons of the model to experimental results attest to the effectiveness of the proposed technique.

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