Some of the most important mechanical properies of wood products rea listed in Table10.1
Force,expressed on the basis of unit area or volume,is known as a stress(应力). The measure of distortion resluting from applied stress is known as strain.(应变) Figure10.2 illustrates stress and strain in a wood test specimen under compression parallel to the grain. The element catalogies of stress are :tensile stress(拉应 力),compressive stress(压应力),bend stress(弯曲应力),torsinal stress (扭转应力), shear stress(剪切应力) and so on.
If ideal elastic behaviour is attributed to rubber,then a loading – underloading .cycle can be carried out without energy loss,and time is not a factor.For wood and other elastic-plastic materials a stress-strain cycle as in Fig7.27 takes palce.Thermodynamically the area of loop C’D’F’A’repreasents the energy loss during the entire cycle.
The anisotropy od coniferous woods is much more prounced than that of broad-leaved woods:this is sepecially valid for the quadrant z-x,as Fig.7.3
2.3 Influences Affecting the Elastic Properties of Wood (影响木材弹性的 因素)
P
⊿L
L
2 Elastictity ,Plasticity ,and Creep(弹性，塑性，蠕变)
2.1 Hooke’s Law(虎克定 理),Modulus of Elasticity (弹性模量) Hooke’s law states that the strainεis proportional to the stressσ: ε= α.σ where α= ε/σis a compliance ,i.e.,the strain per unit stress.In the technical literature normally the reciprocal value 1/ α=E is used.
(1) Grain Angle
2.4 Plasticity and Creep(塑性和蠕变)
(1) Stress-strain Behaviour Hooke’s law cannot be expected to be valid in a wide range for such comparatively complicated materials as wood and other natural high polymers.The stress-strain diagram is therefore not the same as for an ideal elastic body.
3.2 Effect of Specific Gravity on Strength of Wood(比重对强度的影响)
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 equation :S=K where S is any one of the strength propertise,K is a proportionality constant differing for each stength property,G is the specific gravity ,and n is an exponent that defines the shape of the curve representing the relationship.
E is called the modulus of elasticity(弹性模量) or Young’s modulus.(杨氏模量)
2.2 Rhombie Symmetry of wood (木材的正交对称性),Anisotropic Nature of Wood(各向异性), Systems of Elastic Constants(弹 性体)
(2) Creep and Creep Recovery Wood ,according to the present present experimental results and results ,possesses both elastic as well as plastic properties .
1 Stress(应力) and Strain(应变)
1.1 Definition of Stressand Strain(应力和应变的定义)

tensile stress Ｐ
Ｐ
Ｐ

compressive stress

shee relationship between strain and stress(应力与应变的关系)
Professor WU Yiqiang
Chapter 7
Mechanics Properties of Wood
(木材的力学性能)
Structural applications of wood products are omnipresent in today’s society， and Figure 10.1 shows two such applications.
As has been shown above , wood is an anisotropic material but the trunk of a tree
consists more or less of concentric
cylindrical shells thus imparting a cylindrical symmetry to the wood.The symmetry is reflected in most physical properties of the wood as in of the elastic properties ,the strength values ,the thermal and electrical conductivity values.
Fig 6-6 presents curves for three important strength properties at two levels of moisture content.
The relationship of strength to specific Gravity seen Tab10.2 . The effective of wood in resisting any particular form of applied force is a function not only of the total amount of the wall material,but of the proportions of the cell wall components found in a given piece,and also of the amount of extractives in the cell lumen..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 specfic gravity.This index is general terms as the weight-strength ratio(比强度)
If the stress always acts only in one direction and if the stress-strain cycles are repeated,then the permanent set may beincreased as can be seen from Fig.7.28
3 The Strength of Wood (木材强度)
3.1 Definition of Strengh(木材强度的定义)
The resistance of the body to the applied atress is known as the strength of the material. Since there are a number of defferent kinds of stressws ,the strength of the material must be stated in terms of its compressive , tensile ,shear,or bending stregth.