Experimental and Computational Micro-Characterization Techniques in Wood Mechanics

Lectures


Wood anatomy

John BARNETT (The University of Reading, UK)

Professor Emeritus of Structural Botany at University of Reading, UK; PhD of University of Leeds, UK; DSc of University of Reading, UK; 2002-2005 President of the International Academy of Wood Science
Research interests: wood fibre cell walls; anatomy of wood

Michael GRABNER (Vienna University of Natural Resources and Applied Life Sciences, Austria)

Diploma and PhD at the University of Natural Resources and Applied Life Sciences, Vienna - BOKU.  
Research interests: Dendrochronology, Wood anatomy, Historical wood utilization, Wood quality

Lectures on the vascular cambium and formation of wood, anatomy of soft- and hardwoods), and variations in anatomy (reaction wood, juvenile and mature wood, sapwood and heartwood)

A series of four lectures which will introduce students to the basics of wood anatomy will be presented. The first lecture will look at the development of wood cells from the vascular cambium, and the structures of the main cell types found in wood: fibres, vessels elements, tracheids, fibre tracheids and parenchyma. The evolutionary relationships between these cell types will also be briefly mentioned. The second lecture will describe the anatomy of the wood of coniferous trees (softwoods) while the third lecture will cover the anatomy of angiosperm dicotyledonous woods (hardwoods). The way in which the structure of the cell wall in fibres and tracheids affects timber properties will be described, and the ecological advantages and disadvantages of the various anatomical types will be discussed. The final lecture will look at variations from "normal wood anatomy", dealing with juvenile wood and mature wood, compression wood and tension wood. Emphasis throughout will be on teaching students to recognize the different cell types in sections and the different basic anatomical arrangements, rather than on wood identification.

Solid mechanics

Herbert Mang (Vienna University of Technology, Austria)

Professor of Strength of Materials at Vienna University of Technology; PhD from Vienna University of Technology and from Texas Tech University; 2003-2006 President of the Austrian Academy of Sciences. 
Research interests: computational mechanics of solids and structures; computational geomechanics.

Lectures on basic concepts (deformation, stress, strain), fundamentals of elasticity, elasto-plasticity, and visco-elasticity, and beams.

In a series of lectures, the basic principles of mechanics of materials and structures will be reviewed. The focus is placed on providing the fundamental knowledge required to understand and suitably apply the experimental and numerical characterization techniques treated in later lectures. Moreover, reference will be made to how mechanical principles can help to understand the anatomy and morphology of wood tissues, since plants and trees evolved in reaction to mechanical influences of their local environment.

Lectures will start with a brief outline of the fundamental concepts of deformation, strain and stress, which serve as the basis for defining the mechanical behavior of a material or a structure. Besides mathematical definitions, the physical meaning of these quantities will be discussed. Thereon, constitutive equations relating stresses and strains will be derived. Besides the simplest case of linear elasticity (described by Hooke's Law), also nonlinear elastic and elasto-plastic material behavior will be treated and the basics of the corresponding mathematical formulations explained. Finally, phenomena of time-dependent material behavior, such as creep and relaxation, will be touched on.

Due to the importance of rod-shaped structures in nature, beams will receive special attention. Basic relations between loads and the resulting deformations and stresses will be outlined, and simple example calculations will be performed. In this context, the influence of shape and size on the mechanical behavior will be investigated.

 

Experimental techniques

Olivier ARNOULD (University of Montpellier 2, France)

Ph.D. in Mechanics of Materials (LMT Cachan/UPMC Paris, France); Lecturer in Mechanics engineering and mechanics of material (LMGC/University of Montpellier, France).

Research interests: (Micro)mechanical characterization (DMA, AFM, etc.), experiment design, cell wall rheology

Johannes KONNERTH (Vienna University of Natural Resources and Applied Life Sciences, Austria)

Ph.D. in Wood Science at University of Natural Resources and Applied Life Sciences, Vienna, Austria; University Assistant at the Institute of Wood Science and Technology at University of Natural Resources and Applied Life Sciences, Vienna, Austria.

Research interests: Basic study of bond line formation - Chemistry, Structure and Mechanics

Andreas J√ĄGER (Vienna University of Technology, Austria)

Ph.D. at Vienna University of Technology, Austria; Research Assistant at the Institute for Mechanics of Materials and Structures, Vienna University of Technology, Austria.
Research interests: nanoindentation, wood cell wall, transversely isotropic material

Lectures on Atomic Force Microscopy (AFM) and nanoindendation (NI).

The lectures focus on the measurement of viscoelastic properties at the lowest scale level by using nanoindentation and AFM devices. It begins with an introductory lecture on the basic concept of contact mechanics based on the Hertzian theory for linear elastic isotropic solids and its extension to the case of depth-sensing indentation. The extension to the case of elastic anisotropic solids will be briefly tackled. Then, nanoindenter and Atomic Force Microscope's (commercial) implementation, calibration(s) and classical operating principles for mechanical measurements (single point or mapping mode) are described together with a brief review on data interpretations. The lecture will end with a short discussion on measurement and scale of observation possibilities, limitation of each technique for measuring mechanical properties of wood components at the submicrometre scale and nowadays developments.

 

 

Stephan PUCHEGGER (University of Vienna, Austria)

Ph.D. at University of Vienna; University assistant in the group Dynamics of Condensed Systems of the Factulty of Physics/University of Vienna. 
Research interests: ultrasound based measurements of elastic constants both on the macroscopic and microscopic level and structural investigations via X-ray diffraction.

Lectures on Scanning Acoustic Microscopy (SAM). 

The lecture will provide an overview over the theoretical concepts behind Scanning Acoustic Microscopy (SAM), the experimental setup and the possibility to substitute nano-indentation with SAM. This technique is all about sending short ultrasound pulses onto a surface and measuring the amplitude of the reflected signal as a function of the distance between the lens and the surface and possibly the orientation of the lens. The beginning of the lecture will therefore feature a short excursion into the mathematics necessary to use the physics behind SAM. Thereafter the theories of reflectivity and Rayleigh surface waves will be examined, which are crucial to understand the reflected signal amplitude as a function of the lens surface distance. This theories also enable us, based on certain assumptions, to calculate the elastic constants from the measured reflectivity for isotropic materials and anisotropic systems. We will then take a close look on the experimental setup, the do's and don'ts of measuring, how the samples need to be prepared and what possible countermeasures against imperfect surface preparation exist. The measurement and evaluation of a bone sample will be shown as an example and discussed in detail.

Hubert MAIGRE (INSA de Lyon, France)

Engineer from Ecole Polytechnique (France); PhD in Mechanics from Ecole Polytechnique (France); Researcher of CNRS (National Scientific Research Center) from 1990.
Research interests : Dynamic propagation of cracks, Identification of cracks using inverse problems technics, Mechanical behaviour of wood using multiscale approach.

Lectures on application of image correlation to measure deformations in wood. 

Wood is a very heterogeneous material whatever the scale considered (cells, growth rings, timber). The consequence is that conventional testing machines give only mean values for strain and stress which are very different from local values. On the contrary image correlation technique gives the full field of strain with almost no restriction on the shape and the size of the sample. This technique consists in comparing two images of the sample one before loading and the other after loading. The precision depends on the texture of the specimen and for many cases an artificial texture is used. With one camera we have only access to in plane deformation but with two cameras we get the whole 3D deformation. The efficiency relies on the texture of the specimen, the quality of the optic, the numerical algorithm of correlation. Different applications on wood at different scales will be presented.

Michaela EDER (Max-Planck-Institute of Colloids and Interfaces, Germany)

PhD at BOKU - University of Natural Resources and Applied Life Sciences, Department of Material Sciences and Process Engineering, Vienna, Austria; Currently Post Doc in the research group "Plant Biomechanics and Biomimetics" at the Max-Planck-Institute of Colloids and Interfaces, Department of Biomaterials, Potsdam, Germany.
Research interests: cell wall structure and function (primary and secondary cell wall), with focus on mechanical properties.

Lectures on microtensile tests on wood fibers. 

To gather information about tensile properties of wood at the micro- and nanoscale, sufficient small samples are required in order to exclude effects from other levels of hierarchy (length scales). For this reason the lecture starts with a brief introduction of appropriate sample preparation (thin tissue slices and single fibres), with regard to particular questions.

One of the first microtensile experiments on single pulp fibres was performed by Jayne in 1959. He used an Instron testing machine and clamped the fibres by using abrasive papers. The cross sections of the fibres were determined by observation under a compound microscope. Since then a number of further experimental setups for testing single fibres and tissue sheets were developed and described. The lecture should overview selected tensile testing equipment; their possibilities and limitations should be discussed.

A very promising tool in studying the properties of biological materials, such as wood, is the investigation of structural changes during deformation by using a combination of various techniques, e. g. light microcopy, scanning electron microscopy, X-rays or spectroscopic methods. Examples thereto are given very briefly.

 

Numerical and analytical techniques

Kristofer GAMSTEDT (KTH - Royal Institute of Technology, Sweden)

Associate professor at the Department of Fibre and Polymer Technology at KTH in Stockholm, Sweden. 
Research interests: structure-property relations of wood materials and the use of solid mechanics as a tool in development of cellulose-based composite materials;  fatigue of carbon-fibre composite laminates; micromechanics of biobased materials.

Lectures on composites micromechanics applied to wood materials.

Over the last few decades, a wide variety micromechanical models for fibre composites used in high-end applications, such as aerospace structures, have been developed to relate the microstructure to the mechanical properties of the composite material, such as stiffness, dimensional stability, strength and fracture toughness. The ultra- and microstructure of wood ressemble those of conventional manmade composites; load-carrying fibrils in a polymer matrix, a cellular structure such as in sandwich cores, fibre bridging as a toughening mechanisms in crack propagation, etc. These lectures will highlight some robust micromechanical composite models that are applicable to wood materials. Starting from a small dimensions the hygroelastic properties due to the ultrastucture can be modelled by rules of mixtures, Halpin-Tsai and concentric cylinder assemblage, i.e. Hashin-type of models. Next, the cell wall-level may be modelled by classical laminate theory used for composite laminates. The cellular microstructure of wood can be descibed by beam theory starting by the use of basic approaches of Gibson and Ashby. Furthermore, cohesive fracture mechanics will be touched upon to show how the toughness of cross-over fibre-bridged cracking wood can be quantified."

 

 

Karin HOFSTETTER (Vienna University of Technology, Austria)

PhD at Vienna University of Technology; senior researcher (University Assistant) at Institute for Mechanics for Materials and Structures, Vienna University of Technology, Austria.
Research interests: micromechanical concepts and homogenization techniques, mechanical characterization of wood.

Lectures on homogenization techniques.

Wood shows a hierarchical architecture. Characteristic structural features vary at different scales of observation and result in an extremely heterogeneous and anisotropic material behavior at the macroscale. Much effort has already been put into the exploration of structure-function relationships in wood mechanics, aiming at a better understanding of the material behavior and at establishing a firm basis for the design and development of innovative wood-based materials.

Homogenization techniques are powerful tools to relate quantitatively macroscopic material properties to corresponding properties of microscale constituents and to their geometrical arrangement. The basic idea of homogenization techniques is to replace a micro-heterogeneous material by an equivalent homogeneous one. The lectures will sketch the fundamental principles of homogenization and point out possible fields and also limitations of applications. After a short survey of available homogenization techniques, referring also to the lecture on composite micromechanics, two techniques will be discussed in more detail: the mean field approach, which is suitable for estimation of effective properties of materials with a random (statistically homogeneous) microstructure, and the unit cell method, which characterizes (approximately) periodic heterogeneous materials. Simple analytical example calculations will demonstrate the practical use of the techniques and will deliver insight into basic features of the homogenization schemes.

 

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