Markus Lukacevic

Numerical simulation tool for wooden boards with a physically based approach to identify structural failure

Knots and the resulting fiber deviations are the main influencing parameters of the effective stiffness and strength behavior of wooden boards on a structural scale. Depending on the size, shape and arrangement of knots/knot groups, certain effective mechanical properties can remain unaffected or change significantly. For this reason, a reliable prediction of the mechanical behavior of wooden boards is a basic requirement for efficiently designed wood products and timber structures. Within this work, a Finite-Element simulation tool was developed, which is able to consider a realistic three-dimensional fiber course in the vicinity of knots and also accounts for density and moisture dependent strength and stiffness properties. The estimation of effective strength values within this tool is done by examining the qualitative stress changes in predefined volumes, and is based on the formation of failure zones predominantly caused by perpendicular-to-grain tension in the vicinity of knots. Comprehensive test series, comprising tensile and four-point bending tests were carried out and used for validation. In general, a very good correlation between numerical and experimental results was obtained.

Discussion of common and new indicating properties for the strength grading of wooden boards

The naturally grown material wood exhibits, in addition to its orthotropic material structure, several types of inhomogeneities, where most of them can be allocated to knots and the resulting local fiber deviations. Since they generally lead to a reduction in strength properties, wooden boards must be subjected to a grading process before they can be used as load-bearing elements. Within this process so-called indicating properties are recorded and used to assess the wooden board strength. Common indicating properties are almost exclusively based on surface information of wooden boards while the 3D position and orientation of knots within a board is hardly considered. Thus, algorithms for the 3D reconstruction of wooden boards based on already available surface scans, laser scanning, X-ray or computer tomography data are assessed first within this work. This new knot information allows then the development of novel indicating properties, which consider the knots, the resulting fiber deviation regions and, for bending conditions, the knot location information using height-dependent weighting functions. The statistical evaluation of combinations of the new indicating properties, separately for tensile and bending load conditions, shows that the correlations to experimentally obtained strength properties could be improved significantly with such an approach.

Application of a multisurface discrete crack model for clear wood taking into account the inherent microstructural characteristics of wood cells

Abstract A more accurate prediction of the mechanical behavior of wood is needed to increase its ability to compete with other building materials. Especially, when it comes to estimate failure loads, the lack of appropriate prediction tools becomes obvious. The present work contributes to this goal in two different ways: First, a damage concept for wood is revisited, which allows for transferring information about failure processes through different scales of observation. In this concept, the failure behavior of clear wood is linked to the different characteristic of earlywood and latewood layers in softwoods. This reduces the number of empirically determined strength parameters, while the definition of multisurface failure criteria is still possible. Secondly, it will be demonstrated that the combination of these models with discrete crack modeling based on the extended finite element method provides a numerical simulation tool capable to describe failure mechanisms more realistically than existing approaches. The results obtained by numerical calculations and experiments by means of a micro wedge splitting test show very good agreement, especially, if the load capacity and failure mechanisms are in focus. The presented approach shows a much better performance compared to linear elastic or elastoplastic simulations.

A numerical simulation tool for wood grading model development

Wood is a naturally grown material that has growth irregularities, especially knots and site-related defects. The former result in a pronounced reduction in stiffness and strength of boards. The lack of knowledge regarding the effects of growth irregularities on the mechanical behavior of boards motivated the investigation of such defects by means of physically-based numerical simulations. A model which is based on the combination of the finite element method with sophisticated descriptions of the fiber course and of the material behavior has been developed. Comparisons of model predictions with corresponding experimental results in terms of relations between tensile strength and knot area ratio-values show good agreement and underline the predictive quality of the simulation model. The enhanced insight into the mechanical functionality of a stem–branch junction gained by the newly developed numerical multiscale model might contribute to improve the grading criteria and, thus, to achieve an economic benefit for the wood-processing industry.

A numerical simulation tool for wood grading: model validation and parameter studies

Growth irregularities such as knots always result in a pronounced reduction in strength in wooden boards. The presented paper deals with the experimental validation of a newly developed numerical simulation tool (Hackspiel et al., Wood Sci Techol, submitted, 2013) which allows the investigation of such defects by means of physically based numerical simulations. Thereby, advanced models for the description of the three-dimensional fiber course and for the mechanical material behavior are used. The stepwise validation covers the validation of the model for the elastic behavior covering the model for the grain course in the first place. For the validation of the model’s strength prediction capabilities, a total number of 52 boards were tested in tension and bending. The corresponding strength predicted by the simulation tool showed good agreement with the test results. The validated tool was then used to perform parameter studies in which the influence of various knot-related parameters on strength was investigated. The observed trends help to identify decisive knot parameters for board strength, which should receive particular attention in the grading process.

Experimental study on glued laminated timber beams with well-known knot morphology

Nowadays, the impact of knots on the failure behaviour of glued laminated timber (GLT) beams is considered by subjecting the single lamellas to a strength grading process, where, i.a., tracheid effect-based laser scanning is used to obtain information about knot properties. This approach single-handedly defines the beam’s final strength properties according to current standards. At the same time, advanced production processes of such beams would allow an easy tracking of a scanned board’s location, but, at this point, previously obtained detailed information is already disregarded. Therefore, the scanning data is used to virtually reconstruct knot geometries and group them into sections within GLT beams. For this study, a sample of 50 GLT beams of five different configuration types was produced and tested under static four-point-bending until failure. As for each assembled lamella the orientation and position within the corresponding GLT beam is known, several parameters derived from the reconstructed knots can be correlated to effective GLT properties. Furthermore, the crack patterns of the tested beams are manually recorded and used to obtain measures of cracks. A detailed analysis of the generated data and their statistical evaluation show that, in the future, dedicated mechanical models for such timber elements must be developed to realistically predict their strength properties. A potential approach, using fluctuating section-wise effective material properties, is proposed.