Frank Lam

A stochastic plasticity approach to strength modeling of strand-based wood composites

A 3 dimensional stochastic finite element technique 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 to study the effects of varying strand characteristics (such as species or geometry) on the performance of the member. The constitutive properties of the strands are found empirically and are subsequently used in a 3 dimensional finite element program. The program is formulated in a probabilistic manner using random variable material properties as input. The constitutive model incorporates classic plasticity theory whereby anisotropic hardening and eventual failure of the material is established by the Tsai– Wu criterion with an associated flow rule. Failure is marked by an upper bound surface whereupon either perfect plasticity (i.e. ductile behavior) or an abrupt loss of strength and stiffness (i.e. brittle behavior) ensues. The ability of this technique to reproduce experimental findings for the stress–strain curves of angle-ply laminates in tension, compression as well as 3 point bending is validated. # 2002 Elsevier Science Ltd. All rights reserved.

Macro-void distribution analysis in strand-based wood composites using an X-ray computer tomography technique

A database from a series of cross-sectional density distributions in a 0.16 × 0.34 × 1.28m strand-based wood composite specimen has been successfully developed using X-ray computer tomography (CT) techniques. Using conventional image processing techniques, the CT images of the specimen were analyzed with respect to the size and position of the macro-voids. Finally, CT images and the measurement results were converted and exported into MS Excel spreadsheets to provide information on the three-dimensional distribution of macro-voids so those who are not familiar with image processing and formats can handle the data easily. In future, this type of database can be used to develop a model for the prediction of macro-void presence and distributions in strand-based wood composites.

Incorporating size effects in the Tsai-Wu strength theory for Douglas-fir laminated veneer

SummaryThis paper elucidates the need to consider the effect of volume on brittle material strengths when these strengths are used in a strength theory. Specifically, Weibull weakest-link theory has been implemented with the Tsai-Wu strength theory to predict the ultimate load carrying capacity of a center point off-axis bending member made from Douglas-fir laminated veneer. Weibull theory has been used in two distinct ways to account for size effects needed to evaluate brittle material strengths (ie. tension perpendicular and parallel to grain, and shear) for the strength criterion. The analytical methods assume linear elastic, plane stress states and have been described and evaluated using probability theory as a framework. Analytical results are in reasonable agreement with experimental findings substantiating the techniques proposed herein.

Modeling Hysteretic Behavior of Wood Shear Walls with a Protocol-Independent Nail Connection Algorithm

This paper presents an extension to an algorithm called HYST to develop the hysteresis characteristics of a nail connection. The paper also discusses the implementation of the algorithm in a finite-element model of a wood shear wall, called WALL2D, to study the hysteretic wall response. The HYST algorithm is a protocol-independent and mechanics-based procedure that considers the nail shank as steel beam elements and the wood embedment medium as compression-only spring elements smeared along the nail shank. By accounting for the stiffness degradation of the wood embedment medium under cyclic loading, HYST can fully address strength/stiffness degradation and the pinching effect in the hysteresis of typical nail connections. HYST was verified by the load-slip hystereses from nail connections tested with two different loading protocols. The WALL2D application model consists of linear elastic beam elements for framing members, orthotropic plate elements for sheathing panels, linear springs for framing connecti...

Nondestructive evaluation of veneer quality using acoustic wave measurements

Abstract Two nondestructive evaluation methods, impact-induced stress wave techniques and ultrasonics, were investigated to detect lathe checks and knots in veneer, which were identified as key veneer quality properties for some engineered applications. Measurements included wave velocity and attenuation in the directions parallel and perpendicular to the grain. The results showed that both techniques were sensitive to lathe checks when using wave propagation perpendicular to grain. For wave transmission parallel to grain, signals showed some sensitivity to knots. There was no significant difference in wave velocity measurements between stress wave and ultrasonic techniques. Regression models based on stress wave velocities in these two orthogonal directions were developed to estimate the veneer quality index giving a coefficient of determination ranging between 0.39 and 0.50.

Computational modeling of material failure for parallel-aligned strand based wood composites

Abstract A three-dimensional nonlinear stochastic finite element model with size effect adjustment procedures has been developed to simulate the probabilistic distribution of tension strength of parallel-aligned strand based wood assemblies. An experimental database on the tensile modulus of elasticity (MOE) and tension strength of single and multiple ply parallel-aligned Douglas-fir strands has been developed. Based on the test data of single 2.5 × 17 mm veneer strands of varying lengths, input parameters for the model have been established. These parameters include the statistic geometric dimensions, the correlated single strand tensile MOE and strength distributions following a bivariate standard normal model. Based on nonlinear least square regression with constraints minimization, the tension strength distribution of each one of the multiple ply veneer strands (0.46 m long) has been used to establish a calibration factor, α, to account for the effects of resin, pressing and other factors on the strength of veneer assemblies and size effect. A failure criterion has also been set up within the nonlinear finite element frame with a stress update algorithm. A formula of strength modification factor α f ( n ) is proposed to predict the probabilistic distribution of the tension strength of multiple ply veneer assemblies with various sizes of finite element meshes and different number of layers.

Experimental Investigation on Lateral Performance of Timber-Steel Hybrid Shear Wall Systems

AbstractThis paper presents the results of an experimental study on the lateral performance of timber-steel hybrid shear wall systems. Such systems are composed of steel moment-resisting frames and infill wood-frame shear walls. Monotonic and reversed cyclic tests of two full-scale timber-steel hybrid structures were conducted. There were three timber-steel hybrid shear wall systems in each structure. The lateral stiffness, lateral load capacities and hysteretic characteristics of the timber-steel hybrid shear wall systems with single- and double-sheathed infill wood-frame shear walls were investigated. The load sharing effect between the timber and the steel subsystems was studied. The test results showed that the installation of the infill wood-frame shear wall produced a significant increase in the initial lateral stiffness of the bare steel moment-resisting frame. The infill wood-frame shear walls were very effective in the initial stages of loading and absorbed a substantial part of the lateral load....

Internal temperature and pressure responses to flake alignment during hot-pressing

3 and 446 kg/m3, were pressed at 180 °C to a 10 mm thickness. Steel capillary tubes and thermocouple wires were inserted during mat forming at various thickness positions to monitor gas pressure and temperature. Panel density was found to positively affect thermal conduction, and negatively affect lateral and transverse permeability and transverse thermal convection. Flake-alignment positively affected lateral permeability, and negatively affected transverse thermal convection. These results indicate that lower density mats will heat faster and have lower internal gas pressures. Less aligned mats will initially heat faster and have higher internal gas pressures, but towards the end of pressing they will take slightly longer to heat.3 wurden zu 10 mm dicken Platten gepreßt. Stahl-Kapillaren und Meßfühler wurden während des Formens in verschiedenen Plattentiefen eingefügt, um Gasdruck und Temperatur zu messen. Die Plattendichte erhöht die thermische Leitfähigkeit und erniedrigt die Gasdurchlässigkeit und Konvektion parallel und senkrecht zur Oberfläche, d.h. Platten geringerer Dichte werden schneller erwärmt und haben einen geringeren inneren Gasdruck. Weniger gut ausgerichtete Platten werden zunächst schneller erwärmt und zeigen einen höheren inneren Gasdruck; gegen Ende der Preßzeit brauchen sie jedoch längere Zeit zum Aufheizen.

Variation of tensile strength along the length of lumber

SummaryA model has been developed for the within member variation of tensile strengths parallel to grain in nominal 38 × 89 mm No. 2 Spruce-Pine-Fir lumber. Tensile strength data from two modulus of elasticity matched groups were considered. Model parameters obtained from data group 1 were used to generate tensile strength profiles of lumber 6.10 m long. Within member tensile strength cumulative probability distributions of the simulated data were evaluated by window analyses. The spatial correlation of the simulated data were evaluated by semivariogram and regression analyses. Good agreement was obtained between model predictions and test results of both groups.

Three-dimensional finite element analysis of the Japanese traditional post-and-beam connection

A three-dimensional finite solid element model for Japanese traditional post-and-beam connections was developed using the wood foundation method, which employed the concept of a beam on a nonlinear foundation. The wood foundation in the model was a three-dimensionally prescribed zone surrounding a nail shank in order to address the intricate wood crushing behavior induced by nail slip. Material models for the wood members and the foundation were developed based on the transversely isotropic plasticity from the software package ANSYS. The Japanese post-and-beam connection modeled was a ten-nail multiple connection with a mortise and tenon joint and is called the CPT (Corner Plate, T-shaped) connection. Details of the model development are presented. As a feasibility study, blind predictions of the model were compared with available connection test data and showed good results for predicting the progress of the load-deformation relations in three dimensions. However, the limitation of the model was found in simulating fracture failures such as wood splitting or nail tear-out from the wood. Model applications and the need for model improvement are discussed.