Chapter 10 Mechanical properties of wood 木材力学性能

1. Chapter 10 Mechanical properties of wood木材力学性能 Ⅰ. Definition of terms Ⅱ. Effect of specific gravity on strengthen of wood Ⅲ. Effect of moisture content on strength of wood Ⅳ. Anisotropic behavior of wood Ⅴ. Nondestructive stress determination in lumber Ⅵ. Time-dependent properties of wood Ⅶ. Degradative changes in strength of wood

2. Ⅰ. Definition of terms /术语定义 The mechanical properties of wood are an expression of its behavior under applied forces. • stress (应力 σ): force on unit area or volume There are compressive stress, tensile stress, shear stress, bending stress. • strain(应变ε):deformation per unit length, area or volume Each different type of stress produces a corresponding strain. • modulus of elasticity (弹性模量 E): the proportionality constant between stress and strain, . MOE = σ/ ε. Usually,The modulus of elasticity for compressive and tensile tresses is known as Yong’s-modulus (Y), and the modulus of bending elasticity is commonly indicated as E. • modulus of rupture (断裂模量 R)：the stress required to cause failure R = σmax • proportional limit(比例极限 σp): the maximum stress beyond which the σ/ ε ratio doesn’t keep constant.

3. The elastic behaviorof wood is illustrated by the straight-line portion of the curve for load and deformation, as shown in the figure. The area under the straight-line portion of the curve represents the potential energy, or recoverable work, and is a measure of the resilience of the material. The steepness of the slope of the elastic line is a measure of the magnitude of the elastic modulus; i.e., the steeper the slope, the greater the modulus.

4. Ⅱ. Effect of specific gravity on strength of wood The specific gravity of wood, because it is a measure of the relative amount of solid cell wall material, is the best index that exists for predicting the strength properties of wood. In general terms, without regard to the kind of wood, the relationship between specific gravity and strength can be expressed by the following equation: S = K (G) n S is any one of the strength properties K is a constant differing for each strength property G is the specific gravity n is an exponent that defines the shape of the curve representing the relationship.

5. Conception of specific strength A measure of the efficiency of the wood to resist stress is given by an index called the specific strength, which is the ratio of strength to specific gravity. This index is often referred to in general terms as the weight-strength ratio. In comparison with other structural materials the weight-strength ratio for wood is very favorable for some applications. • Superior as bending members The dispersal of the cell wall material as thin shells has an important effect on the flexural rigidity of wood. For this reason wood is well suited for long beams and columns or stressed skin construction. 3. Inferior in compression and shear The dispersal of the cell wall material as thin shells reduces the efficiency of shear along the grain and compression across the grain. So wood is inferior to metals in comparison and shear.

6. Ⅲ. Effect of moisture content on strength of wood Most of the strength properties and elastic characteristics of wood vary inversely with the moisture content below the fiber saturation point, and keep constant above the fiber saturation point, as shown in the following figure.

7. Ⅳ. Anisotropic behavior of wood /木材的各向异性 1. Conception A material, which has different physical properties in the directions of the various structural axes, is said to be anisotropic. Wood is anisotropic in mechanical strength. 2. C‖> C⊥ Wood is 4 to 12 times stronger in compression parallel to the grain than it is perpendicular to the grain. 3. σr > σt Many kinds of mechanical properties of wood also vary somewhat between the radial and tangential axes because of the orientation of the rays in the radial direction. Usually, σr > σt, especially for wood with large wood rays.

8. Ⅴ. Nondestructive stress determination in lumber木材非破坏性应力测定 • The Principle • There are very strong correlations between modulus of rupture and elasticity in bending, maximum compressive strength parallel to the grain and Yong’s modulus in compression, and also between maximum ensile strength and modulus of elasticity in tension. • The determination of elasticity in pieces of wood of structural size is quite simple and can be performed without damage of the wood using either static or vibrational methods.

9. Practical use This theory is used to determine stress grades in dimension lumber. The most common systems employ equipment, which bends the lumber slightly as the piece passes through a series of rollers. The load and deflection at the rolls is measured electrically; a small computer calculates the elasticity and converts it to bending strength. The effects of all the factors influencing the allowable strength and elasticity are automatically integrated and accounted for in the stress value which the machine stamps on the piece.

10. Ⅵ. Time-dependent properties of wood 木材的依时性能 Deformation in wood under stress is the result of two independent components operating simultaneously. • The first component is the elastic deformation，which occurs as the result of elastic response to load of the cellulose microfibils. • The second component is the plastic deformation of wood with time, which occurs as the result of the flow properties of the lignin fraction of the cell walls under load.

11. 1. Creep Under a constant magnitude of load, wood deforms plastically in direct relation to the duration of the load application. Keep σ constant, ε↑with time. e.g.: a book shelf with heavy books. 2. Relaxation Under a constant deformation, wood shows a decreasing magnitude of stress to the deformation with increasing time. Keep ε constant, σ↓with time. e.g.: the veneer load in the hot press under pressure.

12. Ⅶ. Degradative changes in strength of wood木材强度的降等 Wood in service can be subjected to a wide range of conditions, which may result in degradative chemical changes in the wood. The most important of the degradative reactions affect the cellulose and depend on a number of interrelated factors, such as: • Temperature; • Length of time of exposure to the temperature; • Moisture content of the wood; • pH of the system in which wood is maintained.

13. 1. Immediate temperature response of wood • Cases with short time effect of temperature The short-term response in strength of wood to changes in temperature is degradative only for extremely low temperature at high moisture contents and for temperatures at or above the ignition point of wood. • The order of decreasing influence of temperature Both the strength and elastic properties of wood vary inversely with temperature at a given moisture content. The various mechanical properties are affected to different degree. C‖> MOR > shear > bending > tensioning • Practical implication — Steam treating of bolts for veneer peeling — Steam treating lumber for bending

14. 2. Permanent changes in wood with temperature and time Wood which is heated in the temperature range between 65oC and the ignition point for any appreciable length of time, and subsequently tested at room temperatures, will show a permanent loss of strength and elastic properties. These changes in the mechanical properties of wood so treated are the result of hydrolysis of the cellulose.

15. 3. Anaerobic decomposition of wood Wood that is completely buried under conditions which prevent free interchange of atmospheric gases, will hydrolyze slowly. Estimates of the time required to produce a 50 percent reduction in the cellulose content in wood completely submerged in water are 1500 to 2000 years for softwoods and 200 to 420 years for hardwoods.

16. 4. Degradation of wood by chemicals Wood is remarkably resistant to degradation when used in contact with acids, but it is less resistant to basic solutions. For example, a group of hardwoods and conifers after soaking in 2 percent HCl for 32 days showed only minor loss in modulus of rupture, while Soaking in 2 percent NaOH for 32 days at 20℃ resulted in 50 percent or more loss in bending strength.

17. Reflection and practice: • Conceptions of stress, strain, modulus of elasticity, modulus of rupture and proportional limit? • Why wood is a more suitable material used for beams than other construction materials? • Why wood usually has higher strength parallel to grain than perpendicular to grain? • What kind of wood will have higher radial strength than tangential strength? • The mechanism of nondestructive stress determination? • Conceptions of wood creep and relaxation? • What is specific strength?