Master's Thesis in Structural Engineering

Friction of wood on steel

Authors: Radek Koubek, Karolina Dedicova

Surpervisor LNU: Michael Dorn, Erik Serrano Examiner, LNU: Johan Vessby

Course Code: 4BY05E

Semester: Spring 2014, 15 credits

Linnaeus University, Faculty of Technology

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Abstract

This thesis deals with the experimental description of friction between steel and wood materials, specifically laminated veneer lumber (LVL) and pine wood with two types of annual rings. It studies the influence of a number of different parameters on the coefficient of friction such as contact pressure, moisture content, fiber orientation in relation to the load direction, steel surface roughness, and horizontal load rate. First, the theoretical mechanical and physical properties as well as the coefficient of friction itself are described. This is followed by the description of the test setup including the test method and how the obtained data is exported, handled and processed and how the coefficient of friction is determined.

The results study the influence of different parameters and show that the coefficients of friction for the smooth sliding plate tests vary in between 0.1 and 0.3, whereas tests with the rough sliding plate vary around 0.7.

Factors influencing the coefficient of friction were found to be the different moisture content under all tested pressures, the different fiber direction under low contact pressure, the contact pressure itself, though under higher pressures the influence was found to be low, and the horizontal load rate under low pressures. The outcomes are further discussed in the discussion chapter.

Key words: Friction, wood, steel, moisture, coefficient of friction, contact pressure, LVL, laminated veneer lumber, pine, fiber direction

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Acknowledgement

We would like to thank the supervisors of our thesis Michael Dorn and Erik Serrano for providing us with time, guidance and assistance during the conduction of the experiments and for contributing with valuable information, comments and advice during analysis of the data and the actual writing process. Furthermore we would like to thank to the personnel of the laboratory of Linnaeus University for providing us with the necessary equipment and machinery.

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Table of contents

1. INTRODUCTION ............................................................................................ 1

1.1 BACKGROUND ...................................................................................................................... 1 1.2 PURPOSE AND AIM ................................................................................................................ 2 1.3 HYPOTHESIS AND LIMITATIONS ............................................................................................ 2 1.4 RELIABILITY , VALIDITY AND OBJECTIVITY ........................................................................... 3 1.5 LITERATURE REVIEW ............................................................................................................ 4

1.5.1 History of friction determination .................................................................................. 4 1.5.2 Research on friction between wood and steel .............................................................. 5 1.5.3 Wood friction characteristics during exposure to high pressure ................................. 5

2. THEORY .......................................................................................................... 8

2.1 MECHANICAL PROPERTIES OF WOOD..................................................................................... 8 2.1.1 Strength and stiffness of wood ...................................................................................... 8 2.1.2 Compression parallel to the fiber direction ................................................................. 8 2.1.3 Compression perpendicular to the fiber direction ....................................................... 9 2.1.4 Compression stresses at an angle to the grain ............................................................. 9 2.1.5 Orthotropic elasticity ................................................................................................. 10 2.1.6 Stress at an angle to the grain - Hankinson's formula ............................................... 11

2.2 PHYSICAL PROPERTIES OF WOOD ........................................................................................ 12 2.2.1 Moisture and wood ..................................................................................................... 12 2.2.2 Density ....................................................................................................................... 13 2.2.3 Shrinkage and swelling .............................................................................................. 14

2.3 DESCRIPTION OF FRICTION AND THE COEFFICIENT OF FRICTION .......................................... 14

3. METHOD ....................................................................................................... 17

3.1 MATERIALS ..................................................................................................................... 17 3.2 SPECIMEN PREPARATION AND PHYSICAL PROPERTIES ...................................... 19 3.3 EXPERIMENTAL PART .................................................................................................. 24

3.3.1 Test procedure............................................................................................................ 20 3.3.2 Methodology of experiments ...................................................................................... 22

3.4 ANALYTICAL PART....................................................................................................... 24

4. RESULTS ....................................................................................................... 25

4.1 OBTAINING THE COEFFICIENT OF FRICTION OF A SINGLE SPECIMEN .................................... 25 4.2 STATISTICAL EVALUATION OF A SINGLE TEST SERIES .......................................................... 28 4.3 STATISTICAL EVALUATION FOR VARYING CONTACT PRESSURE ........................................... 28

5. ANALYSIS ..................................................................................................... 29

5.1 VARIATION OF CONTACT PRESSURE .................................................................................... 29 5.2 VARIATION OF MOISTURE CONTENT .................................................................................... 31 5.3 VARIATION OF FIBER DIRECTION ......................................................................................... 33 5.4 VARIATION OF ROUGHNESS OF THE STEEL SLIDING SURFACE .............................................. 35 5.5 VARIATION OF LOAD RATE .................................................................................................. 36

6. DISCUSSION ................................................................................................. 38

7. CONCLUSIONS ............................................................................................ 39

REFERENCES ................................................................................................... 40

APPENDICES .................................................................................................... 42

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1. Introduction

Friction is everywhere where there is contact between two surfaces of materials. Depending on the amount of friction, it plays an important role in determining the behavior of the materials in contact. In timber applications, contact between wood and steel appears nearly in every construction, mostly in connections.

A commonly used connection is the dowel-type steel-to-timber joint in structural timber engineering. Joints are often considered as the weaker part of the structure. Therefore estimating the load-bearing capacity and stiffness of the connection should be done in an accurate and reliable manner (Dorn, 2012). Previous researches on the influence of friction between wood and steel in connections has proved that using different values of the coefficient of friction can give a significant difference in the final results (Sjödin et al. 2008; Dorn, 2012).

Nowadays, Eurocode 5 (EC5) is used for designing timber constructions. EC5 does not include the coefficient of friction as a parameter in the designing. Friction, of course, exists and should be applied during the designing process, but the coefficient of friction changes depending on various parameters. But also the properties of wood widely vary from one tree to another and from sawn wood to engineered wood products, which again affect frictional properties.

Whether and how the coefficient of friction changes in dependence on different parameters can bring very important knowledge for timber design.

1.1 Background

Broadly speaking, the current calculations and literature for timber constructions provide relatively poor information about the coefficient of friction and its values are seldom precisely determined. For contact between wood and a steel surface, coefficients of friction between 0 and 1 (and above) are commonly found in literature (American Forest & Paper Association, 1997; Residential structural design guide, 2010), but the values given are seldom rationalized. In general, frictional resistance to slipping of connection members is conservatively ignored in design equations, although in some cases coefficient of friction is taken into account (American Forest & Paper Association, 1997).

Friction is usually not accounted for in wood connection design because the amount of frictional force is difficult to predict and in many instances may not exist, if a wood member shrinks or a connection relaxes. (American Forest & Paper Association, 1997). The effect of friction between wood and steel is supposed to be dependent on a variety of parameters such as surface

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texture, roughness, wood density, moisture content, applied pressure, orientation of annual rings in respect to the sliding plane and the direction of sliding, etc. (Sjödin et al., 2008) show, in their experimental and numerical study of effect of friction in single dowel joints, how the coefficient of friction is changing by using different surfaces of dowels. From the work of (Sjödin et al., 2008) it is obvious, that there, in some cases, can be a substantial effect of friction that could be taken into account in calculations. Nevertheless, the current version of Eurocode 5, does not explicitly involve friction in design as a parameter.

1.2 Purpose and Aim

The purpose of this thesis is laying down a range of coefficients of friction for different situations involving pine wood and laminated veneer lumber (LVL) coming into contact with steel. For obtaining results it is also necessary to design the experiments including the MTS machine used in order to obtain reliable data and being able to evaluate the coefficients of friction.

The aim of this work can be split into three sub-goals:

1) Carry out experiments on two wooden materials with different combinations of selected parameters (moisture content, surface of the steel sliding plane, applied pressure, orientation of annual rings and load rate).

2) Calculate the coefficient of friction for different types of wood species and combinations of parameters.

3) Generate data from the measurements (forces) and evaluate the data.

1.3 Hypothesis and Limitations

Hypotheses

1) The machine setup and the way the specimens are mounted will provide data that makes it possible to obtain the necessary information regarding differences between e.g. wood species and different steel surfaces.

2) There is a correlation between all the studied parameters – contact

pressures, fiber directions, moisture contents of pine wood and LVL materials - and the coefficient of friction measured.

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Limitations and assumptions

In regard to the hypotheses above, there are several limitations and assumptions that have to be considered in this work. For instance:

1) There is only one machine setup.

2) A limited amount of tests for each parameter variation can be conducted

3) There is no possibility to measure roughness of the used materials.

4) Some mechanical properties are taken from literature.

1.4 Reliability, validity and objectivity

The experiments have been performed using a MTS Frame machine 322, which has high reliability in functions provided by a hydraulic system. The load cells and actuators of the machine are designed for laboratory testing of materials and every test is controlled by a computer. The setup of the MTS Frame machine 322 has been done by professional employees of Linnaeus University laboratory.

During preparation of the experiments, there was an effort to obtain as much numerical data as possible. Stress was put also on the selection of specimen and their preparation. All specimens were cut by professional personnel of Linnaeus University Växjö, measured by appropriate tools and marked and placed in a climate chamber with constant temperature and moisture content. All measured values were noted to excel files.

The specimens are made from two materials (pine wood and LVL) and they have approximately the same dimensions (30 x 30 x 10 mm3). Each specimen dimensions has been measured before the actual testing using a digital sliding gauge and all data are noted in an Excel document.

For each combination of parameters used in this work, five repetitions have usually been made. All data has been recorded by the machine computer and afterwards evaluated with help of suitable software (Matlab, Excel).

All the above described procedures aimed at minimizing variability and assuring the methodology used was objective.

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1.5 Literature review

This section addresses previous research relating to measuring coefficients of friction and its dependency on various parameters. It includes a short history of determination and establishment of friction in a general way and later research about friction between wood and steel.

1.5.1 History of friction determination

Looking back into history, (Jost, P., 1966) found that huge financial losses were occurring as a result of wear, friction and corrosion. The ancients as far back as Paleolithic times understood the need to control these forces. Also drawings from ancient Egypt, 2400 years BC, show that lowering friction by using grease in transporting heavy statues and building the pyramids facilitated the works.

However, defining friction as an actual science started 500 years ago. (Beek, A., 1995) describes that the first deep study of friction was conducted in the 15th century by Leonardo Da Vinci (1452-1518) who realized that it is important to take friction into account, while he was designing his machines (Figure 1.1). He stated two basic laws of friction - “1st the size of the areas in contact has no effect on friction and 2nd if the load of an object is doubled, the frictional force will also be doubled”. Also other scientists who were concerned with the definition of friction, observing its values and its dependency on various parameters; the most famous are Guillaume Amontos (1663-1705) whose work was based on the theory of friction as a result of roughness of two surfaces and he rediscovered the basic laws of friction of Da Vinci. Charles August Colomb (1736-1806) who expanded Amontons’ work by stating “strength due to friction is proportional to a compressive force” so the “Amontons-Coulomb Law” was established (Beek, A., 1995).

Figure 1.1: Sketches from da Vinci's notebook, ca. 1480 demonstrating some of his coefficient of friction experiments (Hart, 2011).

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In the late 1930s, F. Philip Bowden and David Tabor found that the true area of contact is formed by asperities and with the increasing normal force the area of contact increases. They also gave the researched discipline about friction the name tribophysics, but that term was never generally accepted, leaving the way clear for H. Peter Jost, author of the eponymous report, to give the science its name: tribology (Hart, 2011).

Tribology is defined as the science and engineering of surface phenomena such as friction, wear, lubrication, adhesion, surface fatigue and erosion (Sinha S.K., 2010). Today, friction is mainly studied in scientific tribology.

1.5.2 Research on friction between wood and steel

In regard to friction between wood and steel, experiments mainly addressing issues related to the mechanics of cutting wood have been done.

(Murase et al., 1980) conducted extensive evaluation of the influence of change in various factors such as surface roughness, sliding speed, moisture content on the friction characteristics during processing (cutting/shaving) wood with tools during exposure to relatively low pressure (up to 0.1 MPa).

(McKenzie and Karpovich, 1968) worked on determining the more important variables affecting friction between wood and steel by using a milling machine for cutting wood; they found that sliding speed as well as moisture content and roughness of steel have a significant influence on the value of the coefficient of friction.

(Ning, et al., 1982) studied the friction between Swedish wood and steel by using the same set-up as (McKenzie and Karpovich, 1968) and they obtained the same results, in addition they obtained evidence for two basic relations: a) the softer the wood surface, the higher the coefficient of friction and b) the higher the surface roughness of steel, the higher the coefficient.

1.5.3 Wood friction characteristics during exposure to high pressure

The friction between wood and steel is becoming a widely discussed phenomenon, especially due to the increased use of wooden structures with e.g. dowel type joints, where high pressure occurs. Proof about the fact, that friction is an important factor that should be taken into account has been verified for example in (Seki et al., 2012). (Sjödin et al., 2008) showed by experimental and numerical methods that the value of the coefficient of friction has a significant effect on the load-bearing capacity of single dowel connections.

(Seki et al., 2012) published a study on friction of wood and steel during exposure to high pressure, where contact pressures 1, 5 and 10 MPa were used. The effect of metal surface (polished and grinded) and effect of wood

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surface (planed, rim sawn, band sawn) was tested on two moisture states of wood specimens; oven dried and water saturated. In the case of different surfaces of the steel, the value of the coefficient of friction was approximately twice as high for a grinded steel surface as compared to a polished surface on oven dried wood during exposure to different pressure. For water saturated wood the value of the coefficient of friction was the same or higher in comparison to the oven dried wood at all load levels. Studying the effect of the wood surface on the friction characteristics it was shown, that wood surface finishing has less impact on the coefficient of friction. The irregularities on a coarse wood surface were deformed and became smooth since the wood surface is much softer than the metal tool surface. In every case, the value of the coefficient of friction tended to be higher with water saturated wood than with oven dried wood.

In case of dowel connections used in structural engineering, a lot of previous research has been done, e.g. by (Sjödin et al., 2008) and (Dorn, 2012). (Sjödin et al, 2008) estimated the coefficient of friction between the dowel and the surrounding timber for two groups of dowels – with a smooth surface of the dowel and with a rough one. For joints with smooth dowel surfaces, the value of the coefficient of friction µ was estimated to lie between 0 and 0.3. For joints with a rough surface dowel µ, was estimated to be between 0.3 and 0.5. Experimental investigations also show, that the load-bearing capacity increases for single dowel joints when rough surface dowels are used compared to when smooth dowels are used.

(Dorn, 2012) performed structural experiments on single dowel-type timber connections using different roughness of the dowels and density of the wood and studied the influences of the coefficient of friction and the effects on the behavior and failure of the connection. The outcomes verified the expected influence of increased dowel roughness on connection behavior: increase of both maximum load and maximum displacement at failure. In finite elements simulations of dowel connections, (Dorn, 2012) used variations of the coefficient of friction from 0.0 up to 0.8 and observed the results being significantly influenced; increased friction positively affected load bearing capacity, while stiffness was less significantly influenced (see Figure 1.2).

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Figure 1.2: Load-displacement curves and the stiffness course for variation of frictional properties (Dorn, 2012).

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2. Theory

The first two parts of the theory chapter is oriented towards mechanical and physical properties of wood, such as behavior in compression with different fiber directions as well as moisture influence on wood properties. The third part describes friction itself and the coefficient of friction including formulas necessary to obtain its value.

2.1 Mechanical properties of wood

The mechanical properties such as strength and stiffness of wood, in compression parallel to the fiber direction and in compression perpendicular to the fiber direction, are often given for "clear" wood. The term “clear” refers to the fact that effects of growth features, such as spiral grain, knots, splits or checks are not included. (Kretschmann, D., 1999) describes in wood mechanics that clear wood specimens are considered as homogeneous.

2.1.1 Strength and stiffness of wood

(Kretschmann, D., 1999) says that wood may be described as an orthotropic material which means that it has unique and independent mechanical properties in different directions of the three mutually perpendicular axes (Figure 2.1). The longitudinal axis L is parallel to the fiber (grain); the radial axis R is perpendicular to the grain direction (normal to the growth rings); tangential axis T is perpendicular to the grain but tangential to the growth rings.

Figure 2.1: Three principal axes of wood with respect to grain direction and growth rings.

2.1.2 Compression parallel to the fiber direction

Loading wood parts parallel to the fiber direction is (together with bending) the most common method of straining wooden constructions. It is necessary

Radial R

Tangential T

Longitudinal L

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to take the difference between buckling pressure and simple pressure into account.

There are some aspects that have to be taken into account in designing and evaluating wood in compression parallel to the grain:

1. load capacity is calculated based on a linear relation between stress and deformation,

2. strength and stiffness are dependent on moisture content, the rate of loading and the duration of loading,

3. particular damages vary and depend on many factors, for example on the layout of the volume of loaded particular cells among other things (i.e. that kind of damage is hard to predict for untested specimens)

Characteristic strength of wood in compression parallel to the fiber direction is given by standards and it is in turn referred to Eurocode 5.

2.1.3 Compression perpendicular to the fiber direction

Compression perpendicular to the fiber direction is a very common kind of loading. Current knowledge about this type of loading is based on long-time experience and rules are empirically determined. These are used in contemporary standards and design rules for wooden constructions.

For deriving and assessing wood loaded in compression perpendicular to the fiber direction it is necessary to take the following aspects into account:

1. strength and stiffness is dependent on the moisture content and its changes, and on the duration of loading as well,

2. loading capacity is given by a relation between stress and deformation, which is non-linear.

The maximum stress level in compression loading perpendicular to the grain is between 3 and 5 MPa and failure stress is defined as the stress level that gives 10% remaining deformations (Johansson et al. 2011). There is a difference in the amount of deformation depending on the orientation of the annual rings – the modulus of elasticity perpendicular to the grain is higher in the thick-walled latewood than in the thin-walled earlywood.

2.1.4 Compression stresses at an angle to the grain

Being a strongly orthotropic material angle of loading in relation to the fiber direction must be taken into account in design.

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According to (Johansson et al., 2011), the strength of the wood material changes considerably depending on the angle between the applied load and the grain direction. A correlation for calculating compression strength depending on angle (α) was proposed by Hankinson. The relationship between the failure strength and the angle (α) according to Hankinson is shown in Figure 2.2.

Figure 2.2 - The relationship between the failure strength f and the angle α between the fiber direction and the force direction according to Hankinson (Johannson et al, 2011).

2.1.5 Orthotropic elasticity

Applying the theory of orthotropic elastic materials involves several assumptions. The tree log is for instance idealized to have the shape of a perfect cylinder, of which the longitudinal axis is identical to the fiber direction. Another idealization is the assumed concentric orientation of the annual rings.

Orthotropic solid occurs when the three mutually perpendicular symmetric planes of elastic properties go through each point of its body. If there is such a case in a solid, it holds true that there are three mutual perpendicular axis directions, e.g. longitudinal (L), radial (R) and tangential (T). Hooke's law establishes a linear relation between each stress component and all the strain components. The relation between stresses and strains can then be written as:

0° 90°

α

f0

f90

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(1)

In simple linear elasticity symmetry gives

(2)

From where it is concluded that: (3)

Due to the symmetry, nine independent constants define the compliance matrix:

EL, ER, ET, GLR, GRT, GLT, υLR, υRT, υLT.

The moduli of elasticity in tension and compression, EL, ER, ET, the shear moduli GLR, GRT, GLT, and Poisson’s coefficients υLR, υRT, υLT

2.1.6 Stress at an angle to the grain - Hankinson's formula

There is no general formula describing cracking of wood. Empirical relations for the determination strength boundaries have been used until now. These can give relatively accurate results. One of these methods is Hankinson's formula for plane stress giving the strength at angle α to the grain:

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�� = ��,�.��,���,��� ����,��� � (4)

Where:

��,� ... Compression strength parallel to the grain.

��,�� ... Compression strength perpendicular to the grain.

� ... Deviation of grain from the axis system given by two mutual perpendicular planes.

n … Exponent which value can be determined experimentally, for the time being the most common value used is n = 2.

2.2 Physical properties of wood

There are a number of physical properties of wood that influence the behavior of wooden materials and they have to be taken into account. The well-known are moisture content, density, shrinkage and swelling.

2.2.1 Moisture and wood

Wood is a hygroscopic material; therefore the moisture content plays a very important role. The amount of water absorbed in wood is determined primarily by the relative moisture of the surrounding environment.

The mechanical properties of wood are affected by the moisture content therefore its influence is usually taken into account in design code by reducing strength values for timber used in environments where high moisture content can occur (Johannson et al. 2001).

There are two kinds of water in wood, bounded water and free water. Free water does not influence strength or elasticity or other aspects, while bounded water has a significant effect on entire characteristics including all kinds of strength. Bound water is the moisture absorbed within the cell wall; this water is molecularly bound to the wood molecules of the cell (Figure 2.2) (Department of Natural Resources, 2013).

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Figure 2.2 – Anatomy of longitudinal cells, in relation to moisture loss (Department of Natural Resources, 2013).

There is no effect on strength of wood in the loss of free water, however when wood loses its bounded water, most strength properties increase (Department of Natural Resources, 2013). Therefore it is important to relate e.g. all mechanical tests to the moisture content (MC). This is usually expressed as a percentage and can be calculated from

�� = �����������

. (100%) (6)

Where mwater is the mass of water in wood and mwood is the mass of the ovendry wood.

The moisture content of a given piece of wood can be calculated by

�� = ����$���%���%

. (100%) (7)

Where mwet is the mass of the specimen at given moisture content and mdry is the mass of the ovendry specimen.

2.2.2 Density

Wood is a porous material made of cells of various kinds; therefore depending on the nature of these cells, some wood have more or less solid wood substance for a given sized piece. Density is determined by the amount of wood substance for a given volume. Density is dependent on volume and weight, which are in turn dependent on the moisture content; it is also an indicator of wood strength – the higher the density the stronger the wood (Department of Natural Resources, 2013).

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Values of density for wood are usually determined for moisture content 12% which is referred to as a standard condition. The density of wood varies significantly between species; between about 320 and 721 kg/m3 (Glass et Zelinka, 1999).

2.2.3 Shrinkage and swelling

With respect to dimensional stability, wood is an anisotropic material. It shrinks (swells) most in the direction of the annual growth rings (tangentially), about half as much across the rings (radially), and only slightly along the grain (longitudinally) (Glass et Zelinka, 1999).

Shrinkage occurs when the moisture content is reduced; the micro fibrils (surface of cells where water is bonded) come closer to each other. The shrinkage is usually very small but for large lengths this can be necessary to take into account (Johannson et al. 2011).

2.3 Description of friction and the coefficient of friction

There are several types of friction, but only dry static friction is relevant for this thesis since it is a study of solid surfaces in contact (wood and steel).

According to (Wikipedia, 2014), dry friction is defined as the force resisting a relative motion between two solid surfaces and it is subdivided into static friction (between non-moving surfaces) and kinetic friction (between moving surfaces).

Friction converts kinetic energy into heat whenever relative movement between two surfaces in contact occurs. Another inevitable consequence whenever friction occurs is wear, which may lead to damage of the surfaces exposed to friction and/or performance degradation.

According to (Persson B.N.J., 2000) friction is not itself a fundamental force but arises from fundamental electromagnetic forces between the charged particles constituting the two contacting surfaces. The interactions between these particles results in the calculation of friction from first principles being impractical, thus use of empirical methods for analysis and the development of theories are required.

The coefficient of friction, denoted μ, is defined as the ratio between the friction force F and the normal force N, both acting in the contact surface (Figure 2.3), resulting in

& = '( = )

* (8)

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Figure 2.3 - The friction force F and the normal force N

As can be seen from the equation (8), there is no area that would influence the friction, meaning that for a small block of wood, the calculated coefficient of friction will be the same as for a larger one, if the ratio of acting forces remains the same. Further observations made in (Persson B.N.J., 2000) show that the coefficient of friction is also often almost independent of velocity, except for extreme cases of low or high velocity. The coefficient of friction is also nearly independent of the surface roughness, except for extreme cases, where either of the surfaces is smooth or rough.

The friction force equals the shear stress integrated over the area of real contact. Because of surface roughness, the area of real contact is usually much smaller than the apparent area of contact.

Macroscopic bodies always have rough surfaces, at least on a microscopic level, and if one places two solid materials in contact, some regions on their surfaces will be so close together that the surface atoms of one material "touch" the surface atoms of the other material, while in other regions, the surface atoms are separated by relatively large distances. The regions of contact are referred to as junctions, and the sum of all the junctions is called the area of real contact. The rest of the apparent area of contact is usually much larger than the real area of contact, but plays essentially no part in determining the sliding friction.

The real area of contact in most practical cases can be estimated accurately by assuming that plastic deformation has occurred at each junction and that all the junctions are in a state of incipient plastic flow. This assumption gives

∆, = -/�� (9)

where N is the load and σc (the penetration hardness) the largest compressive stress that the materials can bear without plastic yielding.

F

L

N

P

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Coulomb’s friction law then states that the force F necessary to shear the junctions with the total area ΔA equals

/ = 0�∆, (10)

where τc is the yield stress in shear. Inserting this into equation (9) gives

/ = (0�/��)- (11)

i.e. the coefficient of friction

& = 0�/�� (12)

This derivation not only explains why the friction force is proportional to the load but also why it is independent of the surface area A and of the nature of the surface roughness (as long as the surfaces are not too rough or too smooth). This follows from the fact that the 0� and �� are of similar magnitude.

Coulomb’s law in (Persson B.N.J., 2000) states that

F=μ N (13)

The coefficient of friction tends to increase with increasing velocity.

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3. Method

This thesis work is divided into two parts - experimental and analytical.

The experimental part describes the testing machine MTS Frame machine 322 used for applying force in both vertical and horizontal direction. The experimental part also shows test parameters and used formulas. All variations are shown in tables for easy illustration of each experiment. The descriptions of the used materials, parameters and methodology for the generation of data of the tested combinations are also in the experimental part. Characteristics of the used combinations follow the description above.

The analytical part shows statistical formulas used for analyzing the obtained data and how it was processed in Matlab in terms of finding the coefficient of friction.

3.1 Materials

The test specimens as well as the steel sliding plates have been provided by Linnaeus University.

There were two kinds of wooden materials used in this work: pine wood and laminated veneer lumber (LVL) (Figure 3.1). Both were cut into small specimens with a cross section of approximately 30 x 30 mm2 and a thickness of approximately 10 mm. All specimens were carefully selected in order to not include any knots or cracks. The wood was dry, without any damages and its moisture was approximately 12%, which is considered as the reference conditions.

The influence of change in moisture content on the coefficient of friction was studied. In these tests, dried and wet specimens, respectively, have been used as well as specimens with the reference moisture content. Dry wood was achieved by using an oven with temperature set to 105°C, where the already cut specimens were stored for at least 24 hours before testing. Wet material was achieved by soaking the specimens in water for at least one week before cutting and testing. The surfaces were made wet again right before the test itself.

Area and thickness of each specimen were measured by a sliding gauge and the dimensions noted in an Excel file. The sliding steel plates were carefully cleaned by acetone before each test.

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Figure 3.1: Tested specimens, from left: laminated veneer lumber (LVL), pine wood with wide annual rings and pine wood with narrow annual rings.

Pine wood

Pine (Pinus sylvestris) is softwood mostly available in the Northern Hemisphere. Despite being lightweight, pinewood is a structurally strong and inexpensive material, which is used for a variety of purposes such as carpentry, general constructions, furniture and boat/ship building.

Like all woods, pine products must be sawn and machined from felled trees, which puts some limits on the range of shapes for which it is suitable. The structure of pine wood gives the material very low density and good durability. Pine wood boards frequently contain a number of knots, which can be problematic during cutting, but the wood is otherwise very easy to work with.

For this work, pine wood has been divided into two groups with wide and narrow annual rings. Only clear wood (wood without any knots or damages) has been used.

Laminated veneer lumber (LVL)

In general, LVL is known as a material with high bending, tension, compression and shear strength. Also a relatively high modulus of elasticity is observed. These characteristics are accomplished by gluing thin (3-5 mm thick) veneer sheets together using a phenolic resin-based adhesive (phenol formaldehyde). The veneers are dried and the grains of each veneer are oriented in the same direction. This makes LVL stronger, straighter and more uniform than solid timber and hence overcomes some of timber’s natural limitations such as strength-reducing knots. LVL is less prone to shrinking or wrapping. LVL is mainly used for permanent structural applications including beams, lintels, purlins, truss chords and formwork and its main advantages are that it can be manufactured to almost any length and it can also support heavier loads and span longer distances than normal timber (Wood Solutions, 2013).

30 mm

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Sliding steel plates

Two kinds of steel plates have been used as a sliding part. One plate was polished and thus had a smooth surface. The surface of the second plate was sandblasted and it is considered as a rough surface (Figure 3.2).

a) b)

Figure 3.2: a) The sandblasted rough steel sliding plate and b) the polished smooth steel sliding plate used in the tests

3.2 Specimen preparation and physical properties

For determining the physical properties of the specimen, mathematical calculations and special machines have been used. The digital sliding gauge and a digital scale used to measure the dimensions and mass of the specimens in order to determine the volumetric mass density 1

1 = �2 456

�78 (14)

The specimens were cut from a long piece of wood on a circular saw down to the thickness of 10 mm and, where necessary, grinded to an approximate size of 30 x 30 mm2. After cutting and to achieve identical conditions, all specimens were stored in a climate chamber before being tested.

The steel surface was cleaned by Acetone prior to each test.

3.3 Experimental part

The experimental part has focused on five main types of tests to investigate the dependence of friction on contact pressure, moisture content, angle of fiber direction relative to the sliding direction, different roughness of the sliding plate and different sliding speed. In order to estimate the influence on individual parameters, different contact pressures have been used (Tables 3.1 – 3.5). In the case of different fiber directions, the tests have been done

20

for a low load pressure (0.3 MPa) and for a high level of contact pressure, which was different for each angle (Table 3.3). For each variation, the testing was done for all three groups of wood: LVL, pine wood with wide rings (PW) and pine wood with narrow rings (PN). Individual experiments are described in the following chapters.

Table 3.1: Applied pressures for the experiment with dependence on contact pressure for all three materials (LVL, PW, PN).

Fibre direction Pressure (MPa) 0° 0.3, 1.0, 10, 30 90° 0.3, 0.6, 1.0, 2.5, 5.0, 7.5, 8.5, 10

Table 3.2: Applied pressures in experiments of dependence on moisture content for all three materials (LVL, PW, PN).

Fiber direction Pressure (MPa) 0° 0.3, 5 90° 0.3, 1.0, 2.5

Table 3.3: Applied high pressures in experiments with different fiber directions for all three materials (LVL, PW, PN).

Angle (°) -90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90

Pressures (MPa)

5 5.3 6.3 8.6 13.3 22.5 30 22.5 13.3 8.6 6.3 5.3 5 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

Table 3.4: Combinations of pressures and fiber directions for LVL material in experiments with different roughness of sliding plate.

Pressure (MPa) Angles (°) 0.3 -90,-45, 0, 45, 90

Table 3.5: Combination of pressures and load rates in experiments with sliding speed

Sliding speed (mm/min) Pressure (MPa) 1 0.3, 5.0 10 0.3, 5.0 100 0.3, 5.0

3.3.1 Test procedure

The experiments were performed on a MTS Frame machine, Model 322, which had special components designed and constructed for purposes of these experiments. The tests were performed in the Laboratory of the Department of Building Technology at Linnaeus University, Växjö.

The MTS Frame machine, Model 322 is able to perform a wide variety of tests such as tension, compression, fatigue and fracture mechanics tests. MTS manufactures a variety of grips, mounting fixtures and test area guards.

21

Main components of the MTS Frame machine 322 used for this experiment are shown in Figure 3.3. Further details about the machine and its particular components can be seen in Appendix A.

Figure 3.3: Set up for friction testing: 1 – Vertically moving part. 2 – Sliding steel plate fixed to part 1. 3 – Wooden specimen. 4 – Attachment for wood specimens. 5 – Horizontally moving bottom part.

All tests were performed as compression tests with horizontal loading. As was previously stated, some of the components were specifically manufactured for these experiments: the horizontally moving part which can fit a plate (see also Figure 3.4) on which the wood specimen is placed and the vertically moving part for the attachment of the sliding steel plate.

22

Figure 3.4: Horizontally moving machine components: 1 – File plate, mostly used in this work. 2 – Very rough file, used for verification reliability of this set up. 3 – Smooth plate, used for glued attachment. 4 - Bearing plate.

Experimental data was automatically recorded to a text file and subsequently processed to obtain the friction coefficient for each test. All parameters and their range is shown in Table 3.6.

Table 3.6: Range of values of parameters set for experiments on the MTS 320 machine

Range of contact pressure 0.1 – 30 MPa Sliding speed (horizontal movement) 1, 10, 100 mm/min Sliding length 10, 30, 60 mm

3.3.2 Methodology of experiments

The purpose of testing using the MTS Frame machine 322 was to do primarily a qualitative and, if possible, a quantitative investigation of the ratio between vertical load N and horizontal load P (Figure 2.3), to estimate the coefficient of friction µ.

The principle of the tests is to press the steel plate to the surface of a wood specimen. The steel plate is fixed to the upper part of the machine and the wood specimen is attached to the bottom part of the machine by a very rough steel plate (or glued to a smooth steel plate in some tests). The upper part moves only vertically while the bottom part moves only in the horizontal direction. The coefficient of friction is obtained by calculating the ratio of horizontal to vertical load.

23

The machine is controlled by a PC. The system enables the user to define the testing profile with personalized settings. The following automated steps were executed during the experiments:

1. The vertical displacement was set to the position where the loading surface of the machine is only a couple of millimeters away from the tested surface of the wood.

2. The procedure is switched to vertical load control and the specimen is loaded until the desired vertical load is achieved. During this period the horizontal load is being kept constant at 0 N.

3. After the vertical loading procedure is complete, the machine waits for a couple of seconds to balance both loads

4. A horizontal displacement is initiated with a defined rate (1, 10 or 100 mm/min) during which the vertical load is kept constant at a predefined value. For the normal speed of movement (10 mm/min) the horizontal displacement continues until the entire length of one specimen is reached (30 mm) or until the specimen can no longer withstand the load and fails. For 1 mm/min rate the displacement is only a third of the element length (10 mm) and for 100 mm/min rate the horizontal displacement is two element lengths (60 mm).

After setting all automated steps it is necessary to select output in terms of graphs to be plotted. Four graphs were used for this work – vertical and horizontal force and vertical and horizontal displacement as a function of test time.

An illustration of the methodology of these experiments can be divided into six steps:

1. Preparing wooden specimens - cutting, grinding, marking and measuring.

2. Setting up the test procedure on the computer such as defining loads and displacements to be applied during the test.

3. Cleaning the steel sliding plate with acetone.

4. Placing wooden specimen on the mounted parts of the machine.

5. Running the test procedure.

6. Export and analysis of obtained data.

24

3.4 Analytical part - statistical evaluation

A large amount of data was obtained for each group of measured specimens. This data is necessary to evaluate with statistical relevance. For each tested group the arithmetic mean (average) was used:

9: = ;� ∗ ∑ 9�

��>; (15)

where n is the number of tested specimens and Xi is the i-th value in the group of tested values. Regarding the fact that the arithmetical average hides and smoothens extremes and is also influenced by extremes, it is very convenient to expand the final results with range R, variance s2, standard deviation s and average absolute deviation d. The range R is defined as

? = @�AB − @��� (16)

where xmax is the maximum and xmin the minimum value, respectively, within the sample. Thus it measures the distance between the extreme values, but doesn’t say anything about the concentration of the extremes around the mean. For such a measure the variance s2 is used

DE = ;�$; . ∑ (9� − 9:)E�

�>; (17)

The standard deviation s is defined as the square root of the variance. It is used together with variance s2 for characterizing.

For statistical evaluation it is also possible to use the average absolute deviation d, which is less sensitive to extreme values than the standard deviation

F = ;� . ∑ |9� − 9:|�

�>; . (18)

All characteristics above have an absolute value and therefore it is necessary to consider the magnitude of the characteristics with respect to the nominal value of data in the group of the obtained data, (Hanousek, 1992).

25

4. Results

Large amounts of data were obtained in the tests and these were subsequently analyzed in Matlab and Excel. The basic data presented here is given in terms of the static coefficient of friction µ. The experiments were performed for different conditions such as varying contact pressure, moisture content, fiber direction, roughness and sliding speed and they are divided into separate sections.

For clear understanding of the evaluation of the data, the first part of this chapter describes the step by step process of obtaining the coefficients of friction by means of a Matlab script for a single specimen. The second part shows the further evaluation of the results in Excel and the derivation of the statistics. This is done using LVL specimens in otherwise standard conditions (0.3 MPa contact pressure, fiber direction 0°, normal moisture, smooth sliding plate, 10 mm/min load rate).

4.1 Obtaining the coefficient of friction of a single specimen

From each single test the raw data from the MTS Frame machine 322 is first imported from the text files into Matlab in the form of matrices. In addition, the Excel file containing all information about the actual specimen is loaded, which contains specimen number, date of the experiment, dimensions, applied pressure, moisture content, attachment, plate roughness, fiber direction, type of wood, and sliding speed.

The first plot (the examples in this chapter are for LVL, fiber direction 0°, contact pressure 1 MPa, normal moisture, smooth plate, sliding speed 10 mm/min) is then created showing both vertical and horizontal forces as can be observed in Figure 4.1. Following that, the coefficient of friction matrix is calculated by dividing the horizontal force by the vertical force. This is plotted again as a function of time (Figure 4.2).

As can be observed in Figure 4.2 a lot of noise is in the very beginning and the very end of the test. The noise in the beginning is removed by deleting the first 50 seconds of the data since at least this timespan of each experiment contains only data about the loading process. Next all values with a coefficient below 0.1 is deleted. This eliminates any discrepancies during the rest of the loading process after the first 50 seconds and also to find the point where the first slip (movement) of the tested specimen is measured (marked with a circle in Figure 4.2 and Figure 4.3). Next the first decreasing value (first occurrence of slip) after the coefficient of friction reaches 0.1 is selected as a rough estimate of the coefficient of friction. This serves also as the starting point for finding the static coefficient of friction in the script.

26

Figure 4.1: An example of the vertical and horizontal force output of the experiment.

Figure 4.2: The coefficient of friction over time plot calculated from the exported horizontal and vertical forces.

From that point only the next five seconds are considered in the search for the coefficient of friction. The Matlab function diff is then used to create a matrix with differences between the neighboring friction values. In that matrix the values that are smaller than an adjustable threshold value (0.01 was found to be working properly) are deleted, which only leaves the big jumps (if there are any more) in the coefficient of friction matrix. By this the stick-slip motion is removed that can be observed in Figure 4.3 at around 67.5 s before the coefficient is again rising.

The last step is to select the highest value, meaning the next biggest slip in the selected data (marked with a cross in Figure 4.2 and Figure 4.3). The

-50

150

350

550

750

950

0 50 100 150 200 250

Fo

rce

[N

]

Time [s]

Vertical Horizontal

27

value of the coefficient of friction and its timestamp is then recorded into a separate text file.

Figure 4.3: Detail of the significant difference between the occurrence of the first movement (red circle) and the actual selected coefficient of friction (black cross), and of the stick and slip motion.

4.2 Statistical evaluation of a single test series

Figure 4.4 and Table 4.1 show the statistical parameters discussed previously in Section 3.4. Arithmetical average is labeled as @̅, average deviation as d, range as R and the value of standard deviation as s in both positive and negative direction, which is a square root of variance s2.

Conditions: material LVL, normal moisture, smooth plate, sliding speed 10 mm/min, fiber direction 0°, contact pressure 0.3 MPa.

Table 4.1: Statistical results for a single test series LVL 0°

x R s2 d

0.3 MPa 0.251 0.0479 0.00024 0.0126

28

Figure 4.4: Graphical interpretation of the statistical results.

4.3 Statistical evaluation for varying contact pressure

The results are then presented and compared varying only a single parameter, in the following example the variation of contact pressure.

Conditions: material LVL; normal moisture; smooth plate; load rate 10 mm/min; fiber direction 0°; contact pressure 0.3, 1, 10 and 30 MPa.

In Figure 4.5, all measured values are shown as well as the derived average values, which are connected to show the course of the curve of the coefficient of friction depending on varying contact pressure.

Figure 4.5: Pressure variation of LVL, fiber direction 0°, smooth plate, normal moisture, sliding speed 10 mm/min.

0,20

0,22

0,24

0,26

0,28

0,30

0

Co

eff

icie

nt

of

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tio

n μ

[-]

s d

Rs

@̅

0,00

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0 5 10 15 20 25 30

Co

eff

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d

29

5. Analysis

This chapter shows and describes the average values obtained in accordance with the previous chapter. The following experiments are analyzed with variations of individual parameters such as contact pressure, moisture content, fiber direction, steel plate roughness and load rate.

Only the average values for each nominally equal series will be shown. More detailed figures showing the exact measured values are included in the Appendix B in form of plots and Appendix C in statistical form.

5.1 Variation of contact pressure

Conditions: smooth steel plate; load rate 10 mm/min; normal moisture content; fiber direction 0° (Figure 5.1) and 90° (Figure 5.2); attachment by rough plate; contact pressure in the range of 0.1, 0.3, 10, and 30 MPa.

Figure 5.1: Dependence of the coefficient of friction on the contact pressure for fiber direction 0° for LVL and pine wood (with wide and narrow annual rings).

According to the course of the curve of average values it is visible that the coefficient of friction is decreasing while contact pressure is increasing, but after a certain pressure is reached, the decrease is negligible.

0,00

0,10

0,20

0,30

0 5 10 15 20 25 30

Co

eff

icie

nt

of

fric

tio

n μ

[-]

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LVL PW PN

30

Figure 5.2: Dependence of the coefficient of friction on the contact pressure for fiber direction 90° for LVL and pine wood (with wide and narrow annual rings).

Behavior of the coefficient of friction in the case of fiber direction 90° evinces a similar course of the curve as in the case of fiber direction 0°. The highest coefficient of friction has shown in the lowest contact pressure, since then, only lower values are detected.

0,00

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0 1 2 3 4 5 6 7 8 9 10

Co

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LVL PW PN

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5.2 Variation of moisture content

Conditions: smooth steel plate; load rate 10 mm/min; fiber directions 0°° and 90°; attachment by rough plate; contact pressure 0.3, 1, 2.5, and 5 MPa; normal, dry and wet wood.

a) 0.3 MPa b) 5 MPa

Figure 5.3: Dependence of the coefficient of friction on the moisture content with contact pressures 0.3 MPa (a) and 5 MPa (b), fiber direction 0°.

In the experiment with varying moisture content for fiber direction 0°, two levels of contact pressure were used (0.3 MPa and 5 MPa). For fiber direction 90° the influence of moisture content was measured at three different levels of contact pressures: 0.3 MPa, 1 MPa and 2.5 MPa (Figures 5.4 a, b, c).

a) 0.3 MPa b) 1 MPa c) 2.5 MPa

Figure 5.4: Dependence of the coefficient of friction on the moisture content with fiber direction 90°.

0,00

0,10

0,20

0,30

dry normal wet

Co

eff

icie

nt

of

fric

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n μ

[-]

Moisture content

LVL PW PN

dry normal wet

0,00

0,10

0,20

0,30

dry normal wet

Co

eff

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[-]

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dry normal wet

Moisture content

PW

dry normal wet

PN

32

It is evident from Figure 5.3a that moisture content has a significant effect on the coefficient of friction: the value for PW in wet conditions is twice as high as in dry conditions. When high contact pressure is used (Figure 5.3b), the coefficient of friction has a lower value for each material, but its value still increases with the increase of the moisture content.

The significant influence of the moisture content in wood is clearly visible in both angle directions of 0º and 90º. The coefficient of friction increases with increasing moisture content, which was expected as a result of this experiment. It is also visible that, for higher contact pressures, the values of the coefficients of friction are smaller than the values of the coefficients of friction during exposure to the low contact pressure.

33

5.3 Variation of fiber direction

Conditions: smooth steel plate; load rate 10 mm/min; contact pressure 0.3 MPa; normal moisture content; fiber directions from - 90° up to +90°.

Figure 5.5: Distribution of values for each group of tested specimens with fiber direction variation for LVL.

The data in the experiment with varying fiber direction show a high variability of the coefficient of friction in each group. Bigger deviation was recorded compared to the data from other experiments therefore presenting only average values would not be valuable so that all determined coefficients are shown (Figure 5.5 and Table 5.1).

Table 5.1: Statistical values for LVL under contact pressure of 0.3 MPa. x R s2 d

-90° 0.189 0.0580 0.00052 0.0158 -75° 0.190 0.0417 0.00023 0.0105 -60° 0.162 0.0539 0.00040 0.0143 -45° 0.154 0.0396 0.00027 0.0114 -30° 0.169 0.0422 0.00035 0.0155 -15° 0.202 0.0727 0.00083 0.0212 0° 0.251 0.0479 0.00024 0.0126 15° 0.161 0.0351 0.00021 0.0119 30° 0.14 0.0556 0.00043 0.0143 45° 0.157 0.1004 0.00149 0.0299 60° 0.170 0.0641 0.00076 0.0214 75° 0.183 0.0700 0.00078 0.1835 90° 0.189 0.0580 0.00052 0.0158

Average values of the coefficient of friction for all three materials under 0.3 MPa contact pressure (Figure 5.6) and under high contact pressure (Figure 5.7) are presented. The values differed for each fiber direction. The chosen value of contact pressure was approx. 80% of the respective maximum load. The applied contact pressures are shown in Table 3.3.

0,00

0,10

0,20

0,30

-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90

Co

eff

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nt

of

fric

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n μ

[-]

Angle [°]

34

Figure 5.6: Dependence of the coefficient of friction on fiber direction under the contact pressure of 0.3 MPa, average values.

Figure 5.7: Dependence of the coefficient of friction on different fiber direction under a high level of contact pressure.

Comparing Figure 5.6 and Figure 5.7 shows that the coefficient of friction varies with fiber direction during exposure to low contact pressure. The highest value of the coefficient of friction is detected in 0° for all three materials. The coefficient of friction decreases in both fiber directions from 0°, but slight increase is observed again towards ±90°. The coefficient of friction under high contact pressure does not vary as much with fiber direction and the differences between the coefficients of friction are negligible.

0,00

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-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90

Co

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LVL PW PN

0,00

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-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90

Co

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LVL PW PN

35

5.4 Variation of roughness of the steel sliding surface

Conditions: polished and sandblasted surface of steel plate; load rate 10 mm/min; attachment by rough plate; contact pressure 0.3 MPa; normal moisture content; fiber direction 0°, ±45° and 90°.

Figure 5.8: Dependence of the coefficient of friction on the roughness of the sliding plate.

Figure 5.8 shows that roughness of the steel plate significantly influences the coefficient of friction. The sandblasted rough plate almost triples the coefficient of friction compared to the polished surface. The difference between both surfaces can be seen in Figure 3.4.

0,00

0,20

0,40

0,60

0,80

Smooth Rough

Co

eff

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[-]

Plate roughness

LVL 0.3 MPa 90° LVL 0.3 MPa, 0°

LVL 0.3 MPa, -45° LVL 0.3 MPa, 45°

36

5.5 Variation of load rate

Conditions: polished steel plate; load rate 1, 10 and 100 mm/min; attachment by rough plate; contact pressure 0.3, and 5 MPa; normal moisture content; fiber direction 90°.

a) 0.3 MPa b) 5 MPa

Figure 5.9: Dependence of the coefficient of friction on different loading rate for fiber direction of 90°.

Figure 5.10: Typical plot of the coefficient of friction over time for a load rate of 100 mm/min under low contact pressure 0.3 MPa for LVL.

The results from the experiments with different load rate show that the coefficient of friction changes significantly under low pressure as can be seen in Figure 5.9a, showing very high values at a rate of 100 mm/min. Figure 5.10 allows a more detailed look into the evolution of the coefficient

0,00

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37

of friction over the course of a single test under this high load rate. It shows that there is a pronounced peak with a coefficient of friction of approx. 0.58 but the ratio between vertically and horizontally applied forces immediately drops afterwards to levels of around 0.20, the level that has been observed for the other load rates.

When the high pressure (5 MPa) is applied, the coefficient of friction is less influenced by the different loading rate, see Figure 5.9b.

38

6. Discussion

This thesis studies influencing parameters on the coefficient of friction between wood and steel surfaces. The parameters chosen for the experiments were contact pressure, moisture content, fiber direction, roughness of the steel plate and loading rate.

The research of the issue results in the following findings:

The coefficient of friction changes due to a variation of the parameters. Big differences are observed mainly under lower contact pressure. However, under higher contact pressure the coefficient of friction does not show big changes (Chapter 5.1).

In the experiment with different moisture content it is again clear that the value of the coefficient of friction is lower under high contact pressure than it is when low contact pressure is applied. The coefficient of friction is still increasing with increasing moisture content for all tested contact pressures.

Other proof of the influence of high pressure on the coefficient of friction is the experiment with different fiber directions. The results show that under low contact pressure (0.3 MPa) the coefficient of friction ranges between 0.15 and 0.30, whereas under high contact pressure the coefficient of friction ranges between 0.12 and 0.22.

Fundamentally influencing the coefficient of friction is the use of the sandblasted rough steel sliding plate in comparison with the smooth steel sliding plate. The experiments proved significant changes of the values of the coefficient of friction for all applied combinations, where the coefficient of friction was almost tripled on the rough steel sliding plate.

The coefficient of friction for the three tested wood materials (LVL, PW, PN) only showed slight differences and the shapes of the curves were very similar for all three wood types.

The results of the experiments used in this thesis are enclosed in the appendix.

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7. Conclusions

The results obtained in the experimental part correspond to the expected outcomes, for example an increasing coefficient of friction with increasing moisture content of the specimens was found.

While using the sandblasted rough sliding plate the measured coefficients of friction were almost doubled in comparison to the smooth sliding plate. The importance of the roughness of the steel in dowel connections was tested in (Sjödin et al, 2008), where the outcomes showed that using dowels with higher surface roughness increased the load bearing capacity of the connection.

It becomes clear that the coefficient of friction is influenced by more factors and by multiple factors at the same time and that it would be useful to conduct deeper research into this topic and carry out more experiments with more combinations that are shown to be most influential (for example moisture content and steel plate roughness) on the coefficient of friction.

Considering the machine set-up and the experiment itself it could be beneficial to create and try out new ways of attaching the wood specimen to the bearing place (by for example gluing) to see how much the set-up is influenced by the attachment of the specimen.

It is important to continue with research on this topic. It is obvious that the changes of coefficient of friction are significant and might be influential for the calculation and design of timber constructions. The aim should be to integrate the coefficient of friction into Eurocode 5.

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References

American Forest & Paper Association (1997): General Dowel Equations for Calculating Lateral Connection Values. American Wood Council Available at: http://www.awc.org/pdf/tr12.pdf [Accessed 2014-05-28] Eurocode 5 (2004): Eurocode 5 – Design of timber structures. EN 1995-1-1:2004(E) Glass, S.V. and Zelinka, S.L. (2010): Moisture Relations and Physical Properties of Wood. General Technical Report FPL-GTR-190 Available at: http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_04.pdf [Accessed 2014-03-01] Hanousek, J. and Charamza, P. (1992): Moderni metody zpracovani dat (engl.: Modern methods of data processing). Praha, GRADA 1992 Hart, M. (2011): Tribology Makes the World Go ´Round. Available at: https://www.asme.org/engineering-topics/articles/tribology/tribology-makes-the-world-go-round [Accessed 2014-02-11] Jost, H.P. (1966): Lubrication: Tribology; Education and Research; Report on the Present Position and Industry's Needs (submitted to the Department of Education and Science by the Lubrication Engineering and Research) Working Group, H.M. Stationery Office, 1966 Johannson, M. et al. (2011): Design of timber structures. Swedish Forest Industries Federation, ISBN: 978-9-637-0055-2 Kretschmann, David E. (1999): Mechanical properties of wood, General Technical Report FPL-GTR-190 Available at: http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_05.pdf McKenzie, W.M. and Karpovich, H. (1968): The Frictional Behavior of Wood, Wood Science and Technology, Vol. 2, pp.138-152 MTS System Corporation (2004). Available at: http://www.mts.com/en/index.htm. [Accessed 2014-03-01] Murase, Y. (1980) Frictional properties of wood at sliding speed. Mokuzai Gakkaishi 26:61-65 Ning, G, et al. (1982): The Friction between some common Swedish wood species and steel. Royal Institute of Technology, Wood Technology and Processing, Stocholm. Trita TRT 0019

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Persson, B.N.J. (2000): Sliding friction Physical Principles and Applications. Springer-Verlag, Berlin. Heidelberg Rabinowicz, E. (1995): Friction and Wear Materials, 2nd edition, John Wiley and Sons, New York Seki, M. et al (2012): Wood friction characteristics during exposure to high pressure: influence of wood/metal tool surface finishing conditions. The Japan Wood Research Society (2013) 59:10-16, DOI 10.1007/s10086-012-1295-1 Sjödin J, Serrano E, Enquist B (2008): An experimental and numerical study of the effect of friction is single dowel joints. Holz Roh. Werkst. 66:363-372 Sujeet K. Sinha (2010): Engineering Materials in Mechanical Design – Principles of Selection with Q & A, Indian Institute of Technology Kanpur, India. ISBN: 978-981-08-2314-6 Van Beek, A. (1995): History of Science Friction. Available at: http://www.tribology-abc.com/abc/history.htm [Accessed 2014-02-11] Wikipedia (2014): Friction. Available at: http://en.wikipedia.org/wiki/Friction. [Accessed 2014-02-25] Wood Solutions (2013): Laminated Veneer Lumber (LVL). Avaible at: http://www.woodsolutions.com.au/Wood-Product-Categories/Laminated-Veneer-Lumber-LVL [Accessed 2014-03-15] Department of Natural Resources (2013): The Physical and Mechanical Properties of Wood. Department of Natural Resources, Nova Scotia, Canada. Available at: http://woodlot.novascotia.ca/content/lesson-two-physical-and-mechanical-properties-wood [Accessed 2014-02-11]

Appendices

Appendix A: The scheme of a MTS Frame machine 320.

Appendix B1: Plots for pressure variation.

(a)

(b)

Figure B1.1: Pressure variation at 0° fiber direction for PW (a) and PN (b).

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Co

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(a)

(b)

(c)

Figure B1.2: Pressure variation at 90° fiber direction for LVL (a), PW (b) and PN (c).

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Appendix B2: Plots for moisture content variation

(a) LVL (b) PW (c) PN

Figure B2.1: Moisture content variation for 0° fiber direction, 0.3 MPa contact pressure.

(a) LVL (b) PW (c) PN

Figure B2.2: Moisture content variation for 0° fiber direction, 5 MPa contact pressure (no data for normal moisture).

0

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Co

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Moisturedry wet dry wet dry wet

dry wet dry wet normal wet dry normal normal

(a) LVL (b) PW (c) PN

Figure B2.3: Moisture content variation for 90° fiber direction, 0.3 MPa contact pressure.

(a) LVL (b) PW (c) PN

Figure B2.4: Moisture content variation for 90° fiber direction, 1 MPa contact pressure.

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normal normal normal

normal normal normal dry dry dry

dry dry dry wet wet wet

wet wet wet

(a) LVL (b) PW (c) PN

Figure B2.5: Moisture content variation for 90° fiber direction, 2.5 MPa contact pressure.

0

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normal normal normal dry dry dry wet wet wet

Appendix B3: Plots for fiber direction variation

(a)

(b)

(c)

Figure B3.1: Fiber direction variation, 0.3 MPa contact pressure for LVL (a), PW (b) and PN (c).

0,00

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-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90

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Angle [°]

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Co

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Angle [°]

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[-]

Angle [°]

(a)

(b)

(c)

Figure B3.2: Fiber direction variation, high contact pressure (list of pressures in Table 3.3) for LVL (a), PW (b) and PN (c).

0,00

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-90 -75 -60 -45 -30 -15 0 15 30 45 60 75 90

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Angle [°]

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Angle [°]

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Angle [°]

Appendix B4: Plots for plate roughness variation

(a) -45° (b) 0°

(c) 45° (d) 90°

Figure B4.1: Plate roughness variation, 0.3 MPa contact pressure for LVL.

0,00

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Roughness

smooth rough smooth rough

smooth smooth rough rough

Appendix B5: Plots for load rate variation

(a) LVL (b) PW (c) PN

Figure B5.1: Load rate variation, 90° fiber direction, 0.3 MPa contact pressure.

(a) LVL (b) PW (c) PN

Figure B5.2: Load rate variation, 90° fiber direction, 5 MPa contact pressure.

0,00

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Load rate [mm/min]

1 10 100 1 10 100 1 10 100

1 10 100 1 10 100 1 10 100

Appendix C1: Statistical values for experiments with different contact pressures

Table C1.1: Pressure variation for 0° fiber direction. Wood LVL Wide Narrow

x R s2 d x R s2 d x R s2 d

0.3 MPa 0.251 0.0479 0.00024 0.0126 0.295 0.0540 0.00033 0.0153 0.244 0.0678 0.00035 0.0134

1 MPa 0.197 0.0694 0.00040 0.0138 0.224 0.0486 0.00021 0.0115 0.203 0.0494 0.00027 0.0127

10 MPa 0.172 0.0439 0.00026 0.0135 0.199 0.0426 0.00019 0.0109 0.199 0.0353 0.00021 0.0124

30 MPa 0.166 0.0585 0.00030 0.0133

Table C1.2: Pressure variation for 90° fiber direction. Wood LVL Wide Narrow

x R s2 d x R s2 d x R s2 d

0.3 MPa 0.189 0.0580 0.00052 0.0159 0.242 0.1706 0.00418 0.0459 0.203 0.0305 0.00012 0.0073

0.6 MPa 0.178 0.0401 0.00022 0.0106 0.194 0.0323 0.00061 0.0188 0.170 0.0376 0.00023 0.0116

1 MPa 0.168 0.0276 0.00013 0.0088 0.225 0.0446 0.00030 0.0135 0.183 0.0629 0.00055 0.0154

2.5 MPa 0.165 0.0279 0.00011 0.0071 0.220 0.0286 0.00013 0.0089 0.179 0.0662 0.00069 0.0189

5 MPa 0.179 0.0340 0.00024 0.0128 0.219 0.0506 0.00035 0.0131 0.157 0.0168 0.00004 0.0042

7.5 MPa 0.161 0.0600 0.00041 0.0159 0.203 0.0557 0.00063 0.0215 0.162 0.1031 0.00095 0.0242

8.5 MPa 0.170 0.0347 0.00020 0.0092 0.175 0.0174 0.00006 0.0061 0.178 0.0364 0.00034 0.0131

10 MPa 0.169 0.0201 0.00020 0.0100

Appendix C2: Statistical values for experiments with different moisture contents

Table C2.1: Statistical values for experiments with different moisture content for LVL. Angle 0° 90°

Pressure 0.3 MPa 5 MPa 0.3 MPa 1 MPa 2.5 MPa

x R s2 d x R s2 d x R s2 d x R s2 d x R s2 d

Dry 0.185 0.0674 0.02979 0.0223 0.117 0.0106 0.00002 0.0031 0.180 0.0248 0.00011 0.0089 0.127 0.0135 0.00004 0.0049 0.122 0.0089 0.00004 0.0048

Normal 0.251 0.0479 0.00024 0.0126 0.189 0.0580 0.00052 0.0159 0.168 0.0276 0.00013 0.0088 0.165 0.0279 0.00011 0.0071

Wet 0.279 0.0838 0.00049 0.0182 0.257 0.0640 0.00067 0.0213 0.315 0.0277 0.00014 0.0093 0.247 0.0888 0.00135 0.0303 0.338 0.0321 0.00014 0.0080

Table C2.2: Statistical values for experiments with different moisture content for PW. Angle 0° 90°

Pressure 0.3 MPa 5 MPa 0.3 MPa 1 MPa 2.5 MPa

x R s2 d x R s2 d x R s2 d x R s2 d x R s2 d

Dry 0.172 0.0526 0.00044 0.0167 0.143 0.0170 0.00006 0.0066 0.171 0.0534 0.00045 0.0155 0.136 0.0149 0.00005 0.0064 0.123 0.0157 0.00004 0.0046

Normal 0.295 0.0540 0.00033 0.0153 0.242 0.1706 0.00418 0.0459 0.225 0.0446 0.00030 0.0135 0.164 0.0480 0.00040 0.0142

Wet 0.343 0.0724 0.00095 0.0257 0.279 0.0591 0.00053 0.0174 0.333 0.1405 0.00309 0.0442 0.279 0.1482 0.00372 0.0485 0.215 0.1420 0.00297 0.0395

Table C2.3: Statistical values for experiments with different moisture content for PN. Angle 0° 90°

Pressure 0.3 MPa 5 MPa 0.3 MPa 1 MPa 2.5 MPa

x R s2 d x R s2 d x R s2 d x R s2 d x R s2 d

Dry 0.181 0.0273 0.00010 0.0073 0.150 0.0544 0.00050 0.0157 0.159 0.0357 0.00023 0.0128 0.154 0.0199 0.00008 0.0068 0.134 0.0324 0.00015 0.0093

Normal 0.244 0.0678 0.00035 0.0134 0.199 0.0353 0.00021 0.0124 0.203 0.0305 0.00035 0.0073 0.183 0.0629 0.00055 0.0154 0.179 0.0662 0.00069 0.0189

Wet 0.253 0.0180 0.00004 0.0045 0.263 0.0970 0.00125 0.0248 0.321 0.0979 0.00186 0.0362 0.306 0.1765 0.00462 0.3061 0.275 0.1702 0.00422 0.0492

Appendix C3: Statistical values for experiments with different fiber directions

Table C3.1: Fiber direction variation for low pressure (0.3 MPa). Wood LVL Wide Narrow

x R s2 d x R s2 d x R s2 d

-90° 0.189 0.0580 0.00052 0.0159 0.232 0.0930 0.00152 0.0325 0.203 0.0305 0.00012 0.0073

-75° 0.190 0.0417 0.00023 0.0106 0.197 0.0365 0.00022 0.0114 0.202 0.0517 0.00060 0.0181

-60° 0.162 0.0540 0.00040 0.0143 0.186 0.0270 0.00013 0.0077 0.173 0.0398 0.00028 0.0113

-45° 0.155 0.0397 0.00027 0.0115 0.199 0.0487 0.00046 0.0172 0.197 0.0511 0.00034 0.0122

-30° 0.170 0.0422 0.00035 0.0155 0.189 0.0344 0.00020 0.0098

-15° 0.202 0.0727 0.00083 0.0213 0.197 0.0328 0.00020 0.0119 0.189 0.0301 0.00014 0.0092

0° 0.251 0.0479 0.00024 0.0126 0.295 0.0540 0.00033 0.0153 0.244 0.0678 0.00035 0.0134

15° 0.162 0.0351 0.00021 0.0120 0.211 0.0300 0.00016 0.0098 0.217 0.0244 0.00012 0.0094

30° 0.147 0.0557 0.00043 0.0144 0.197 0.0841 0.00091 0.0200

45° 0.157 0.1005 0.00149 0.0299 0.219 0.0339 0.00020 0.0100 0.195 0.0774 0.00080 0.0199

60° 0.170 0.0641 0.00076 0.0215 0.199 0.0234 0.00011 0.0077 0.179 0.0129 0.00003 0.0041

75° 0.184 0.0701 0.00078 0.1835 0.182 0.0203 0.00008 0.0070 0.164 0.0233 0.00010 0.0082

90° 0.189 0.0580 0.00052 0.0159 0.232 0.0930 0.00152 0.0325 0.203 0.0305 0.00012 0.0073

Table C3.2: Fiber direction variation for high pressure (see list of pressures in Table 3.3). Wood LVL Wide Narrow

x R s2 d x R s2 d x R s2 d

-90° 0.179 0.0340 0.00024 0.0128 0.219 0.0506 0.00035 0.0131 0.157 0.0168 0.00004 0.0042

-75° 0.143 0.0312 0.00014 0.0089 0.153 0.0347 0.00018 0.0101 0.130 0.0398 0.00023 0.0108

-60° 0.128 0.0582 0.00047 0.0161 0.176 0.0531 0.00048 0.0141 0.143 0.0125 0.00003 0.0041

-45° 0.146 0.0405 0.00023 0.0102 0.154 0.0386 0.00036 0.0160 0.123 0.0146 0.00003 0.0043

-30° 0.144 0.0347 0.00017 0.0098 0.174 0.0429 0.00027 0.0109 0.135 0.0197 0.00006 0.0058

-15° 0.167 0.0197 0.00005 0.0051 0.150 0.0294 0.00015 0.0086

0° 0.166 0.0585 0.00030 0.0133 0.197 0.0426 0.00019 0.0109 0.199 0.0353 0.00021 0.0124

15° 0.151 0.0174 0.00005 0.0049 0.185 0.0427 0.00025 0.0112 0.192 0.0415 0.00037 0.0167

30° 0.178 0.0523 0.00041 0.0159 0.182 0.0197 0.00005 0.0053 0.181 0.0438 0.00033 0.0143

45° 0.186 0.0411 0.00025 0.0116 0.186 0.0481 0.00056 0.0197 0.157 0.0173 0.00005 0.0050

60° 0.158 0.0676 0.00083 0.0206 0.195 0.0257 0.00013 0.0081 0.183 0.0179 0.00006 0.0055

75° 0.153 0.0221 0.00011 0.0091 0.157 0.0350 0.00020 0.0113 0.152 0.0664 0.00083 0.0237

90° 0.179 0.0340 0.00024 0.0128 0.219 0.0506 0.00035 0.0131 0.157 0.0168 0.00004 0.0042

Appendix C4: Statistical values for experiments with different sliding plate roughness

Table C4.1: Roughness variation for LVL. Angle 0° 90° -45° 45°

Pressure 0.3 MPa 0.3 MPa 1 MPa 0.3 MPa 0.3 MPa

x R s2 d x R s2 d x R s2 d x R s2 d x R s2 d

Smooth 0.251 0.0479 0.00024 0.0126 0.189 0.0580 0.00052 0.0159 0.168 0.0276 0.00013 0.0088 0.155 0.0397 0.00027 0.0115 0.157 0.1005 0.00149 0.0299

Rough 0.582 0.1331 0.00309 0.0440 0.554 0.0948 0.00129 0.0260 0.550 0.0738 0.00102 0.0267 0.626 0.1697 0.00325 0.0442 0.609 0.1894 0.00349 0.0424

Appendix C5: Statistical values for experiments with different loading rate

Table C5.1: Loading rate variation for LVL. Angle 0° 90°

Pressure 0.3 MPa 0.3 MPa 5 MPa

x R s2 d x R s2 d x R s2 d

1 mm/min 0.189 0.0854 0.00143 0.0321 0.190 0.0703 0.00087 0.0219 0.139 0.0256 0.00011 0.0083

10 mm/min 0.251 0.0479 0.00024 0.0126 0.189 0.0580 0.00052 0.0159 0.179 0.0340 0.00024 0.0128

100 mm/min 0.561 0.0951 0.00119 0.0290 0.526 0.1566 0.00358 0.0452 0.171 0.0162 0.00005 0.0057

Table C5.2: Loading rate variation for pine wood. Wood Pine Wide Pine Narrow

Angle 90° 90°

Pressure 0.3 MPa 5 MPa 0.3 MPa 5 MPa

x R s2 d x R s2 d x R s2 d x R s2 d

1 mm/min 0.175 0.0786 0.00148 0.0335 0.156 0.0530 0.00042 0.0143 0.180 0.0977 0.00153 0.0315 0.133 0.0558 0.00043 0.0152

10 mm/min 0.242 0.1706 0.00418 0.0459 0.219 0.0506 0.00035 0.0131 0.203 0.0305 0.00012 0.0073 0.157 0.0168 0.00004 0.0042

100 mm/min 0.465 0.1315 0.00236 0.0342

Faculty of Technology 351 95 Växjö, Sweden Telephone: +46 772-28 80 00, fax +46 470-832 17 (Växjö)