CHEM-E0120: An Introduction to Wood Properties and Wood Products Material properties II: short-term mechanical properties

Transcription

1 CHEM-E0120: An Introduction to Wood Properties and Wood Products Material properties II: short-term mechanical properties Mark Hughes 9 th October 2017

2 Overview/objectives Mechanical properties Elastic properties of wood and influence of structure Strength and failure in wood Defects and their effect on the properties of wood

3 Mechanical properties Stiffness: resistance to deflection under short-term loading (we will consider deflection under long-term loading, creep, later) Strength: how much force can be sustained before it breaks Toughness: the ability to resist the propagation of cracks (arguably the most important property of an engineering material)

4 Tensile test Performed to determine properties of materials Dog bone shaped specimen deformed usually at constant speed Force measured with load cell Extension measured with and extensometer within the gauge length of the specimen Constant temperature and RH maintained (Source:

5 Stress, strain, stiffness, strength (Source: Wikipedia) Stress: load/crosssectional area Strain: extension/original length Stiffness: Young s modulus, E, stress/strain (in linear elastic region Hooke s law) Strength: stress at ultimate load (tension or compression)

6 Mechanical properties of some materials 14 GPa 210 GPa (Source: J.E. Gordon: Structures )

7 Comparison of mechanical properties Material Specific gravity Young s modulus (GPa) Tensile strength (MPa) Fracture toughness (MPa m 0.5 ) Rubber Concrete Nylon Spruce wood (parallel to grain) (26.7) 80 (133) 6 (10) Spruce wood (perp. to grain) Mild steel (26.7) 400 (51) 140 (18) Fibre reinforced composite (11) 300 (166) 40 (22) (Source: Hull & Clyne, 1996)

8 Comparison of mechanical properties Material Specific gravity Young s modulus (GPa) Tensile strength (MPa) Fracture toughness (MPa m 0.5 ) Rubber Concrete Nylon Spruce wood (parallel to grain) (26.7) 80 (133) 6 (10) Spruce wood (perp. to grain) Mild steel (26.7) 400 (51) 140 (18) Fibre reinforced composite (11) 300 (166) 40 (22) Note: Figures in parentheses are specific values, i.e. value divided by specific gravity (Source: Hull & Clyne, 1996)

9 Directionality of wood properties All wood properties are very dependent on the orientation, however, The mechanical properties, in particular, show strong dependence upon the direction of measurement Orthotropic material properties different in 3 mutually perpendicular directions (Source:

10 Properties along & across the gain (Source: Desch and Dinwoodie, 1981)

11 Wood as an orthotropic material Wood is orthotropic its properties are different in each of the three mutually perpendicular directions longitudinal, radial and tangential The cell wall is also orthotropic think structure! Species Density (kg /m 3 ) MC (%) E L (N/mm 2 ) E R (N/mm 2 ) E T (N/mm 2 ) Birch Ash Norway spruce Scots pine (Adapted from Dinwoodie 2000)

12 Structure! Why is this? Longitudinal ( along the grain ) strength and stiffness is governed principally by the axial properties of the fibre (which is in turn dependent the structure of the cell wall, in particular the microfibril angle in the S2 layer) This is because most of the cells run along the length of the tree (90%) and only about 10% run radially Transverse properties dominated by transverse properties of the cell wall and the middle lamella

13 Cell wall as a laminate under off-axis (Dinwoodie, 2000) loading

14 Cell wall as a laminate under off-axis loading

15 S2 layer and wood properties Since ~85% of secondary wall is consists of the S 2 later, mechanical and other properties are dominated by the winding angle of this layer

16 Relationship between density and elastic properties Higher wood density indicates a greater amount of cell wall material Greater stiffness and strength therefore if there is greater density However, microstructure (anatomy) will also have an effect

17 Density and properties (Source: Dinwoodie, 2000) In general mechanical properties (strength and stiffness) increase with increasing density

18 Strength and failure of wood

19 Load-deflection Strength But what happens beyond here? Strength And here? What is physically happening beyond the linear-elastic region? (Source: Dinwoodie, 2000)

20 Non-linear behaviour Non linear behaviour can be attributed to a number of causes. It may be micro-structural damage, viscoelasticity or plastic flow of the cell wall polymers It is important as some forms of micro-structural damage are associated with large changes in the properties of wood, even though the damage may not be (clearly) visible Again we can look at the analogy with fibre reinforced composite materials

21 Damage and defects in wood and its implications

22 Axial compression failure Failure in compression dictated by the properties of the matrix Note: Compression wood in softwoods is characterised by higher amounts of lignin (matrix) and higher microfibrillar angles in the S2 layer (Adapted from: Wardop & Dadswell, 1947)

23 Micro-compressive damage In solid wood, microscopic damage to individual cells caused by mechanical overloading in compression can dramatically affect its mechanical properties (Dinwoodie, 1978) This damage takes the form of small creases in the cell wall known variously as slip planes, kink bands or microcompressions These features can be observed by polarised light microscopy and appear in the form of a bright X traversing the double cell wall Micro-compressive damage in the cell wall of wood (Source: Dinwoodie, 1968)

24 Micro-compressive damage Under certain circumstances these creases are visible to the naked eye and are readily observable under a microscope using low angle incident light In wood, these micro-compressive defects can arise as a result of growth or mechanical stresses within the tree ( brittleheart ) Micro-compressions may also result from compressive stresses in the wood induced during harvesting, conversion or whilst in service

25 Micro-compressive defects What influence do microcompressive defects have on the properties of wood? Wood fibres containing micro-compressive defects have shown failure loads which are reduced by around 46%, as well as decreased stiffness (Dinwoodie, 1978)

26 Influence on toughness The most significant effect observed in wood having undergone compression failure is a reduction in impact properties (a measure of the materials toughness ) Failure is invariably noted to follow the line of the compression crease Micro-compressions formed during cyclic loading have been shown to propagate a line of compression creases in wood which can subsequently lead to crack formation and ultimately to failure

27 Factors affecting strength Anisotropy! Strength is dependent upon the direction in which it is measured Biggest difference seen in tension (Source: Dinwoodie 2000)

28 Relationship between the strength of a laminate and loading angle (Source: Hull & Clyne 1996)

29 Effect of grain angle on strength (Source: Dinwoodie 2000)

30 Grain angle Where is variation in grain angle frequently seen?

31 Grain angle Where is variation in grain angle frequently seen?

32 Grain angle Where is variation in grain angle frequently seen?

33 Grain angle Where is variation in grain angle frequently seen?

34 Effect of knots (Source: Dinwoodie 2000)

35 Measurement of properties in practice Tension tests (Young s modulus/tensile strength) Compression tests (parallel/perpendicular to grain) Static 3 or 4 point bending (Modulus of Elasticity MOE and Modulus of Rupture MOR) Impact tests (measure of toughness) Hardness

36 Tensile (Source: 3-pt. flexure (Source:

37 Summary Wood and the wood cell wall can again be thought of as being analogous to a fibre reinforced composite Like the elastic properties of wood, the strength properties are highly anisotropic Non-linear behaviour indicates micro structural failure Defects of one form or another can significantly influence the strength properties

38 Literature J.E. Gordon: The New Science of Strong Materials: Or Why You Don't Fall Through the Floor (Penguin Science) J.E. Gordon: Structures: Or Why Things Don't Fall Down (Penguin Science) Desch H.E. and Dinwoodie, J.M. (1981): Timber: Its structure, properties and utilisation, 6th Edition, Macmillen Dinwoodie, J.M. (2000). Timber: Its nature and behaviour D. Hull & T.W. Clyne (1996): An Introduction to Composite Materials, Cambridge University Press

39 References & further reading Argon, A.S. (1972). Fracture of Composites. Academic: New York Dinwoodie, J.M. (1968). Failure in Timber. Part 1. Microscopic Changes in Cell-Wall Structure Associated with Compression Failure. J. Inst. Wood Sci., 21: Dinwoodie, J.M. (1974). Failure in Timber. Part 2: The Angle of Shear Through the Cell Wall During Longitudinal Compression Stressing. Wood Sci. and Technol., 8: Dinwoodie, J.M. (1976). Causes of Brashness in Timber. In: Wood Structure in Biological and Technological Research. Eds. Baas, P., Bolton, A.J. and Catling, D. Leiden Botanical Series No. 3, pp Dinwoodie, J.M. (1978). Failure in Timber Part 3: The Effect of Longitudinal Compression on Some Mechanical Properties. Wood Sci. Technol., 12: Kettunen P.O. (2006). Wood: Structure and properties. Trans Tech Publications Inc, Enfield, USA Wardop, A.B. and Dadswell, H.E. (1947). Contributions to the Study of the Cell Wall. 5. The Occurrence, structure and Properties of Certain Cell Wall Deformations. Commonwealth of Australia C.S.I.R. Bulletin No. 221