Josef Füssl

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.

Towards a microstructural model of bitumen ageing behaviour

When it comes to describe the mechanical behaviour of hot mix asphalt (HMA), the change of the mechanical properties over time due to environmental impacts known as ageing has to be considered. Hardening and embrittlement of bitumen lead to a reduced resistance against cryogenic cracks and premature formation of fatigue cracks in bituminous layers. Within this work, the microstructure of bitumen is investigated by conducting static shear creep tests on artificially composed bitumen with asphaltene contents varying between 0 vol-% and 26.71 vol-% as well as on a paving grade bitumen 70/100. Both are considered in unaged and laboratory-aged (rolling thin-film oven test + pressure ageing vessel) conditions to be able to identify and describe ageing effects. While the properties of the considered material phases of bitumen (identical to saturates, aromatics, resins and asphaltenes (SARA) fractions) seem to remain unaffected, an increase of the asphaltene content due to ageing can be observed. A micromechanical model is proposed that allows a prediction of the consequences of these microstructural changes on the mechanical behaviour of bitumen. Atomic force microscopy and environmental scanning electron microscopy images confirm a composition consisting of a micelle-like structure in a contiguous matrix, a structural representative volume element concept based on SARA fractions is suggested, extending an existing multiscale model for HMA. A very good accordance between model predictions and experimental results indicates the models ability to reproduce as well as to describe the effects related to ageing.

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.

Effective stiffness prediction of GLT beams based on stiffness distributions of individual lamellas

Abstract The mechanical properties of structural timber—particularly in terms of stiffness and strength—are subject to high variability, which also affects the properties of timber products made from structural timber, e.g., glued laminated timber (GLT). In this paper, the influence of the longitudinal stiffness variability of wooden lamellas on the effective stiffness of GLT is investigated. In a first step, the local fiber orientation on the surfaces of 350 lamellas of Norway spruce was determined by an optical scanning device. This fiber angle information in combination with a micromechanical model for wood was used for the generation of a longitudinal stiffness profile of each lamella. Recording the position and orientation of each lamella, a total number of 50 GLT beams were assembled (with 4, 7, and 10 laminations) and tested under four-point bending. Knowing the stiffness profile of each board and its location within the GLT beam allowed for an accurate numerical finite element model, which is able to predict the effective GLT stiffness with high accuracy. Interesting insights into the relation between the stiffness of lamellas and the resulting GLT beams could be gained, and finally, a numerical simulation tool which is able to reproduce the experimental results appropriately was obtained.

Towards an optimised lab procedure for long-term oxidative ageing of asphalt mix specimen

Ageing of bitumen leads to increased stiffness and brittleness. Thus, bituminous bound pavements become more prone to failure by low-temperature and fatigue cracking. Therefore, the ageing behaviour of bitumen has a crucial impact on durability, as well as recyclability of pavements. To assess ageing of bitumen, the rolling thin film oven test and pressure ageing vessel are standardised methods for short-term and long-term ageing in the lab. For lab-ageing of hot mix asphalt (HMA), various methods have been developed in the last decades. This paper presents a study on the potential of employing a highly oxidant gas for simulating the long-term oxidative ageing of asphalt mix specimens in the lab. Based on the results, an optimised lab-ageing procedure (Viennese Ageing Procedure – VAPro) for compacted HMA specimens to assess mix performance of long-term lab-aged specimens is developed. Thus, it is possible to optimise mix design not only for short-term performance but to take into account effects of oxidative ageing during its in-service life. VAPro is based on a triaxial cell with forced flow of a gaseous oxidant agent through the specimen. The oxidant agent is enriched in ozone and nitric oxides to increase the rate of oxidation. It is shown by stiffness tests of unaged and lab-aged specimens, as well as by Dynamic Shear Rheometer tests of recovered binder from aged specimens that asphalt mixes can be long-term aged at moderate temperatures (+60°C) and within 4 days and a flow rate of 1 l/min by applying VAPro. Thus, an ageing procedure is at hand that can simulate long-term ageing at conditions that are representative of conditions that occur in the field within an efficient amount of time.

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.

Stochastic finite element approaches for wood-based products: theoretical framework and review of methods

As a result of the natural growing process of wood, each timber lamella shows a high variability in its mechanical properties, particularly strength and stiffness. It is assumed that those properties are governed by a random process and, subsequently, that the effective stiffness of glued laminated timber also varies randomly. Therefore, a probabilistic approach is necessary. Hence, the latest achievements in probabilistic timber engineering are reviewed and compared. Numerous works rely on random process models for the representation of stiffness and/or strength distributions in single timber lamellas. The statistical evaluation of those random process models, however, is limited almost exclusively to Monte Carlo simulation (MCS) so far. Therefore, this work aims at giving an overview of alternative ways to compute the effective stiffness, by reviewing the framework of stochastic finite element methods. Random process models for the representation of the stiffness distribution in single lamellas are discussed, and the two most promising alternatives to the MCS for computation of effective stiffness parameters, the perturbation and the spectral stochastic finite element method, are evaluated in terms of accuracy and efficiency. Finally, this paper shows alternative and more efficient ways of exploring the stochastic nature of wood, delivering a new basis for more reliable design concepts for timber products.

Alternative Approach Toward the Aging of Asphalt Binder

Awareness that natural, financial, and energy resources are scarce goods has increased. Thus demand is growing for infrastructure that is not only of high quality but also efficient. Efficiency, in this case, aims to optimize cost and energy consumption over the complete life cycle of a structure. The objective is to build long-lasting infrastructure with low maintenance demands and with high recycling potential after it has reached the end of its service life. For bituminous bound materials, the aging of asphalt binder has a crucial impact on durability and recyclability. Because asphalt binder is organic by nature, the thermal and oxidative aging processes affected by chemical and structural changes occur when asphalt mixes first are produced and applied and continue over the course of their service life. Increasing stiffness and brittleness of the binder make pavement more prone to thermal and fatigue cracking. The interdisciplinary research project reported here worked toward a better understanding of the physicochemical fundamentals of asphalt binder aging, as well as of the impact of binder aging on the mechanical properties of asphalt binder and asphalt mixes. Through extensive chemical and mechanical analyses, a new model was developed to explain the aging process comprehensively (i.e., on the physicochemical and mechanical levels). Aging can be determined mathematically by micromechanical modeling. With the model presented in this paper, changes in asphalt binder as a result of aging (i.e., increasing brittleness and stiffness) can be explained.

Experimental identification and mechanical interpretation of the interaction behaviour between concrete paving blocks

In recent years, concrete block (CB) pavements have become a favourite alternative to asphalt pavements, mainly in intra-urban regions due to their architectural design possibilities. Unfortunately, this trend is restrained by a lack of adequate design methods to assess the load capacity and durability of such pavements. Especially, the mechanical performance of the vertical joints between CBs is often not depicted realistically enough. For this reason, in this work three new experiments are proposed to determine the mechanical behaviour of the joints between the CBs, and thus the load transmission capability of different joint formations. Mechanical models and the corresponding material parameters to describe the joint behaviour are identified from the experimental results. Finally, performance optimisation of block pavements with respect to their jointing behaviour should become possible.

The performance of paving block structures with mortar filled joints under temperature loading, accessed by means of numerical simulations

Paving block structures with mortar filled vertical joints can exhibit a significantly higher load capacity as comparable structures with sand filled joints. For this reason, this pavement construction method is increasingly being used or would be used, respectively, if its performance could be estimated reliably. Specifically, the exact cause of cracks due to cooling in winter has not been fully understood yet and appropriate prediction tools do not exist. This motivated the development of a numerical simulation tool for such paving block structures under temperature loading, which is able to take the brittle failure mechanisms in-between paving blocks and paving blocks and the underlying mortar bed into account. By means of the proposed simulation tool in this paper, basic structural failure mechanisms of such paving block structures, due to different temperature events, could be identified and relationships between crack widths and different bonding strengths as well as installation temperatures were obtained. Finally, estimates for necessary bonding strengths between paving blocks and mortar bed to prevent large (visible) cracks due to temperature loads are given.