1、Wood ScienceProfessor WU YiqiangFirst-class Course in Hunan ProvinceKey National Discipline on Wood Science and Technology Central South University of Forestry&TechnologyChapter 7 Mechanics Properties of Wood(木材旳力学性能木材旳力学性能)Structural applications of wood products are omnipresent in todays society，a
2、nd Figure 10.1 shows two such applications.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(应力应力).The measure of distortion resluting from applied stress is known as strain.(应变应变)Figure10
3、.2 illustrates stress and strain in a wood test specimen under compression parallel to the grain.The element catalogies of stress are:tensile stress(拉应力拉应力),compressive stress(压应力压应力),bend stress(弯曲应力弯曲应力),torsinal stress(扭转应力扭转应力),shear stress(剪切应力剪切应力)and so on.1 Stress(应力应力)and Strain(应变应变)1.1 De
4、finition of StressandStrain(应力和应变旳定义应力和应变旳定义)nshear stressntensile stressncompressive stressLLP1.2 The relationship between strain and stress(应力与应变旳关系)2 Elastictity,Plasticity,and Creep(弹性，塑性，蠕变弹性，塑性，蠕变)2.1 Hookes Law(虎克定理),Modulus of Elasticity(弹性模量)Hookes law states that the strainis proportional
5、to the stress:=.where=/is a compliance,i.e.,the strain per unit stress.In the technical literature normally the reciprocal value 1/=E is used.2.2 Rhombie Symmetry of wood(木材旳正交对称性木材旳正交对称性),Anisotropic Nature of Wood(各向异性各向异性),Systems of Elastic Constants(弹性体弹性体)As has been shown above,wood is an ani
6、sotropic material but the trunk of a tree consists more or less of concentric E is called the modulus of elasticity(弹性模量)or Youngs modulus.(杨氏模量)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
7、 properties,the strength values,the thermal and electrical conductivity values.2.3 Influences Affecting the Elastic Properties of Wood(影响木材弹性旳影响木材弹性旳原因原因)(1)Grain Angle2.4 Plasticity and Creep(塑性和蠕变塑性和蠕变)(1)Stress-strain Behaviour Hookes law cannot be expected to be valid in a wide range for such co
8、mparatively complicated materials as wood and other natural high polymers.The stress-strain diagram is therefore not the same as for an ideal elastic body.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 fac
9、tor.For wood and other elastic-plastic materials a stress-strain cycle as in Fig7.27 takes palce.Thermodynamically the area of loop CDFArepreasents the energy loss during the entire cycle.If the stress always acts only in one direction and if the stress-strain cycles are repeated,then the permanent
10、set may beincreased as can be seen from Fig.7.28(2)Creep and Creep Recovery Wood,according to the present present experimental results and results ,possesses both elastic as well as plastic properties.As can be seen from Fig7.30,if we apply a stress at zero-time there is an instaneaous elastic defor
11、mation OA.This is followed by a retarted creep AB ender constant stress.On removal of the stress at time an instaneaous elastic recovery takes place,followed by a retarded partial creep recovery from to D at time.After that ,further recovery is so insignificant that it can be neglected.Thus DE repre
12、sents the permanent deformation left at the end of the loading-unloading cycle.3 The Strength of Wood(木材强度木材强度)3.1 Definition of Strengh(木材强度旳定义木材强度旳定义)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,th
13、e strength of the material must be stated in terms of its compressive,tensile,shear,or bending stregth.3.2 Effect of Specific Gravity on Strength of Wood(比重对强度旳影响比重对强度旳影响)The specific gravity of wood,because it is a measure of the relative amount of solid cell wall material,is the best index that ex
14、ists 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
15、 property,G is the specific gravity,and n is an exponent that defines the shape of the curve representing the relationship.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
16、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 s
17、tress 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)In Comparison with other structhral materials,the weight-strength ratio for wood is very favorable for some applications.Wood has a h
18、igh index of rigidity in comparison with solid structural materials and is well suited for ues in situations that require elastic stability.Besides,wood suffers in compasion with metals for uses that require high shear and compression resistance,because the very distribution of wood substance that i
19、ncreases the rigidity in bending reduces the shear and compression efficiency.3.3 Effect of Moisture Content on Strength of Wood(含水率含水率对强度旳影响对强度旳影响)Most of the strength properties and elastic characteristics of wood vary inversely with the moisture content of the wood below the fiber saturation poin
20、t.In effect,the changes in moisture affect specific gravity values,as has been pointed out earlier,and result in changes in strength.These relationships for three different strength properties are plotted as curves in Fig6-7.3.4 Effect of Time on Strength of Wood(时间对强度旳影响时间对强度旳影响)，Deformation in Woo
21、d（木材旳形变）（木材旳形变）,Relaxation(松弛松弛)Deformation in wood under stress is the result of two independent components operating simultaneously.The elastic behaviour（弹性变形）（弹性变形）of wood results from the presence and behaviour of the cellulose microfibrils which exhibit elastic response to load application and
22、removal;i.e.,the deformationn is fully recoverable when the load is removed.The second component of deformatioon is the plastic deformation（塑性变形）（塑性变形）begins with the first application of load and increases with time.Recovery of plastic deformation in wood is slow and eventually amounts to only abou
23、ot half of the total deformation.A piece of wood maintained at a constant deformation will show a decreasing magnitude of stress resistance to the deformation with increasing time.This phenomenon of decreasing stress resistance of the wood as the result of plastic flow(塑性流动塑性流动)is known as relaxatio
24、n(松弛松弛).3.5 Tensile Strength(拉伸强度拉伸强度)Nevertheless the tensile strength of wood parallel to the grain is extremely high and may reach,for some species in the air-dry condition,a maximum of 3,000 kP/.The tensile strength of separated wood fibers is even higher and may vary between abouot 2,000 and 13
25、,000.kP/Determination of Tensile Strength along the Grain(顺纹方向旳拉伸强度测量).As the Tensile Strength along the Grain is much greater than that the compressive strength perpendicular to the grain and the shearing strength,it is difficult to carry out satisfactory tests in tension parallel to the grain.The
26、measurement are as follows:3.6 Compressive strength(压缩强度压缩强度)The maximum Compressive strength plays an important role in the utilization of wood as a building and construction material.The tests of short wood columns or even odf cubes,are easily carried out.The maximum crushing strength parallel to
27、the grain reaches,on the average,for air-dry woods only about 50%of the tensile strength along the grain.3.7 Bending Strength(弯曲强度弯曲强度)The difference between the tensile strength and the crushing strength of wood determines thecharacteristics behavior of a wood beam in bending.Fig7.103 shows that on
28、 the average the ratio of bending strength to crushing strength amounts to 1.75 for common European pinewood in the air-dry condition.BAUMAMN(1922)has used the stress-strain diagrams(Fig7.104)and plotted stresses belonging to the different moments of force(Fig7.105)Testing of small wood beans under static center loading3.8 Shear Strength of Wood(木材抗剪强度木材抗剪强度)The specimens for shear strength tests
