工程力学专业英语翻译ppt课件.pptx

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1、在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确Translation The concepts of stress and strain can be illustrated in an elementary way by considering the extension of a prismatic bar.As shown in Fig.1,a prismatic bar is one that has constant cross section throughout its length and a straight ax

2、is.In this illustration the bar is assumed to be loaded at its ends by axial forces P that produce a uniform stretching,or tension,of the bar.应应力力和和应应变变的的概概念念可可以以通通过过考考虑虑一一根根矩矩形形梁梁的的拉拉伸伸的的简简单单方方法法来来举举例例说说明明。如如图图1所所示示，这这根根矩矩形形梁梁可可以以看看作作是是由由遍遍及及长长度度方方向向的的连连续续横横截截面面所所组组成成，这这些些横横截截面面垂垂直直于于它它的的轴轴向向。在在这这个

3、个例例子子中中，这这根根矩矩形形梁梁被被假假定定在在它它两两端端施施加加了了一一对对使使它它发发生生均均匀匀拉拉伸的轴向力伸的轴向力P。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 By making an artificial cut(section mm)through the bar at right angles to its axis,we can isolate part of the bar as a free body see Fig.1(b).At the left-hand end the tensile fo

4、rce P is applied,and at the other end there are forces representing the action of the removed portion of the bar upon the part that remains.These forces will be continuously distributed over the part cross section,analogous to the continuous distribution of hydrostatic pressure over a submerged surf

5、ace.假假设设在在梁梁的的轴轴向向上上做做一一个个垂垂直直截截面面（截截面面mm），可可以以分分离离出出一一部部分分自自由由的的梁梁见见图图1(b)。在在该该梁梁的的左左端端，有有拉拉力力P，而而在在另另一一端端有有相相应应的的力力可可以以替替代代梁梁的的分分离离部部分分对对它它的的作作用用。这这些些力力连连续续分分布在横截面上，类似于在水平面下的静水压力的连续分布。布在横截面上，类似于在水平面下的静水压力的连续分布。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 The intensity of force,that is,th

6、e force per unit area,is called the stress and is commonly denoted by the Greek letter.Assuming that the stress has a uniform distribution over the cross section see Fig.1(b),we can readily see that its resultant is equal to the intensity times the cross-sectional area A of the bar.Furthermore,from

7、the equilibrium of the body shown in Fig.1(b),we can also see that this resultant must be equal in magnitude and opposite in direction to the force P.Hence,we obtain =P/A.(1)力力的的强强度度，也也就就是是说说单单位位面面积积上上的的力力，被被称称为为应应力力，通通常常用用希希腊腊字字母母来来表表示示。假假定定应应力力在在横横截截面面上上均均匀匀分分布布见见图图1(b)，那那么么我我们们可可以以很很容容易易的的看看出出它它的

8、的合合力力等等于于强强度度乘乘以以梁梁的的横横截截面面积积A。而而且且，从从图图1上上显显示示的的物物体体的的平平衡衡来来看看，我我们们可可以以发发现现这这个个合合力力是是跟跟拉拉力力P在在数数值值上上相相等等，方方向向相相反反的的。因因此此，我我们们得得到到方方程（程（1）=P/A。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 Eq.(1)can be regarded as the equation for the uniform stress in a prismatic bar.This equation shown th

9、at stress has units of force divided by area.When the bar is being stretched by the force P,as shown in the figure,the resulting stress is a tensile stress;if the forces are reversed in direction,causing the bar to be compressed,they are called compressive stress.方方程程(1)用用于于求求解解在在梁梁中中均均匀匀分分布布的的应应力力问

10、问题题。它它表表示示了了应应力力的的单单位位是是力力除除以以面面积积。正正如如我我们们在在图图1中中所所看看到到的的，当当梁梁被被力力P拉拉伸伸的的时时候候，生生成成的的应应力力是是拉拉应应力力；如如果果力力的的方方向向被被颠颠倒倒，导导致致梁梁被压缩时，产生的应力被称为压应力。被压缩时，产生的应力被称为压应力。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 A necessary condition for Eq.(1)to be valid is that the stress must be uniform over the

11、cross section of the bar.This condition will be realized if the axial force P acts through the centroid of the cross section.When the load P does not act at the centroid,bending of the bar will result,and a more complicated analysis is necessary.At present,however,it is assumed that all axial forces

12、 are applied at the centroid of the cross section unless specifically stated to the contrary.Also,unless stated otherwise,it is generally assumed that the weight of the object itself is neglected,as was done when discussing the bar in Fig.1.方方程程(1)成成立立的的必必要要条条件件是是应应力力在在梁梁的的横横截截面面上上是是均均匀匀分分布布的的。如如果果轴

13、轴向向力力P通通过过横横截截面面的的形形心心，那那么么这这个个条条件件是是可可以以实实现现的的。如如果果轴轴向向力力P不不通通过过横横截截面面的的形形心心，则则会会导导致致梁梁的的弯弯曲曲，必必须须经经过过更更复复杂杂的的分分析析。然然而而，目目前前除除非非特特定定说说明明，都都假假定定所所有有的的轴轴向向力力都都通通过过横横截截面面的的形形心心。同同样样，除除非非是是另另外外说说明明，一一般般我我们们不考虑物体自重，正如我们在图不考虑物体自重，正如我们在图1中讨论的梁一样。中讨论的梁一样。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很

14、明确 The total elongation of a bar carrying an axial force will be denoted by the Greek letter see Fig.1(a),and the elongation per unit length,or strain,is then determined by the equation=/L(2).Where L is the total length of the bar.Note that the strain is a non-dimensional quantity.It can be obtained

15、 accurately from Eq.(2)as long as the strain is uniform throughout the length of the bar.If the bar is in tension,the strain is a tensile strain,representing an elongation or stretching of the material;if the bar is in compression,the strain is a compressive strain,which means that adjacent cross se

16、ction of the bar move closer to one another.在在轴轴向向力力作作用用下下，梁梁的的总总伸伸长长用用希希腊腊字字母母来来表表示示见见图图1(a)，单单位位伸伸长长量量或或者者说说应应变变将将由由方方程程(2)决决定定，这这里里L是是指指梁梁的的总总长长度度。注注意意，这这里里应应变变是是一一个个无无量量纲纲量量，只只要要应应变变在在梁梁的的长长度度上上各各处处是是均均匀匀的的，那那么么它它可可以以通通过过方方程程(2)精精确确获获得得。如如果果梁梁被被拉拉伸伸，那那么么得得到到拉拉应应变变，表表现现为为材材料料的的延延长长或或者者拉拉伸伸；如如果果梁梁

17、被被压压缩缩，那么得到压应变，意味着梁的横截面将彼此更加靠近。那么得到压应变，意味着梁的横截面将彼此更加靠近。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 When a material exhibits a linear relationship between stress and strain,it is said to be linear elastic.This is an extremely important property of many solid materials,including most metals

18、,plastics,wood,concrete,and ceramics.The linear relationship between stress and strain for a bar in tension can be expressed by the simple equation=E(3)in which E is a constant of proportionality known as the modulus of elasticity for the material.当当一一种种材材料料的的应应力力与与应应变变表表现现出出线线性性关关系系时时，我我们们称称这这种种材材料

19、料为为线线弹弹性性材材料料。这这是是许许多多固固体体材材料料的的一一个个极极其其重重要要的的性性质质，这这些些材材料料包包括括大大多多数数金金属属，塑塑料料，木木材材，混混凝凝土土和和陶陶瓷瓷。对对于于被被拉拉伸伸的的梁梁来来说说，这这种种应应力力与与应应变变之之间间的的线线性性关关系系可可以以用用简简单单方方程程(3)=E 来来表表示示，这这里里E是是一一个个已已知知的的比比例例常常数数，即即该该材材料料的的弹弹性性模模量。量。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 Note that E has the same uni

20、ts as stress.The modulus of elasticity is sometimes called Youngs modulus,after the English scientist Thomas Young(1773-1829)who studied the elastic behavior of bars.For most materials the modulus of elasticity in compression is the same as in tension.注注意意，弹弹性性模模量量的的单单位位跟跟应应力力的的单单位位相相同同。在在研研究究梁梁的的弹弹

21、性性行行为为的的英英国国科科学学家家Thomas Young(1773-1829)出出现现之之后后，弹弹性性模模量量有有时时也也被被称称为为杨杨氏氏模模量量。对对大大多多数数材材料料而而言言，压压缩缩和和拉拉伸伸时时的的弹性模量是一样的。弹性模量是一样的。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确Translation The relationship between stress and strain in a particular material is determined by means of a tensile te

22、st.A specimen of the material,usually in the form of a round bar,is placed in a testing machine and subjected to tension.The force on the bar and the elongation of the bar are measured as the load is increased.The stress in the bar is found by dividing the force by the cross-sectional area,and the s

23、train is found by dividing the elongation by the length along which the elongation occurs.In this manner a complete stress-strain diagram can be obtained for the material.一一种种材材料料的的应应力力-应应变变关关系系可可以以通通过过一一个个拉拉伸伸测测试试来来确确定定。材材料料的的样样品品通通常常做做成成圆圆棒棒状状，放放置置在在测测试试仪仪器器上上然然后后施施加加拉拉力力。随随着着载载荷荷的的增增加加，圆圆棒棒受受的的力力

24、和和伸伸长长量量可可以以被被测测定定。圆圆棒棒的的应应力力可可以以通通过过力力除除以以横横截截面面积积得得到到，应应变变则则通通过过伸伸长长量量除除以以圆圆棒棒的的长长度度得到。这样，我们就得到了这种材料完整的应力得到。这样，我们就得到了这种材料完整的应力-应变图表。应变图表。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 The typical shape of the stress-strain diagram for structural steel is shown in Fig.1,where the axial stra

25、ins are plotted on the horizontal axis and the corresponding stresses are given by the ordinates to the curve OABCDE.From O to A the stress and the strain are directly proportional to one another and the diagram is linear.Beyond point A the linear relationship between stress and strain no longer exi

26、sts,hence the stress at A is called the proportional limit.图图1是是结结构构钢钢的的经经典典应应力力-应应变变图图，图图中中横横坐坐标标表表示示轴轴向向的的应应变变，跟跟通通过过纵纵坐坐标标表表示示的的相相应应的的应应力力一一起起形形成成曲曲线线OABCDE。从从O到到A这这一一段段，应应力力和和应应变变彼彼此此成成正正比比关关系系，图图形形是是成成线线性性的的。超超过过A点点以以后后，应应力力和和应应变变的的线线性性关关系系不不再再存存在在，因因此此A点点的的应应力力被被称为比例极限。称为比例极限。在整堂课的教学中，刘教师总是让学生

27、带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 With an increase in loading,the strain increases more rapidly than the stress,until at point B a considerable elongation begins to occur with no appreciable increase in the tensile force.This phenomenon is known as yielding of the material,and the stress at poin

28、t B is called the yield point or yield stress.In the region BC the material is said to have become plastic,and the bar may actually elongate plastically by an amount which is 10 or 15 times the elongation which occurs up to the proportional limit.随随着着载载荷荷的的增增加加，应应变变的的增增长长比比应应力力更更快快，直直到到B点点一一个个显显著著的的

29、伸伸长长开开始始出出现现，而而拉拉力力的的增增加加并并不不明明显显。这这就就是是众众所所周周知知的的材材料料的的屈屈服服现现象象，B点点的的应应力力被被称称为为屈屈服服点点或或者者屈屈服服应应力力。在在区区域域BC中中，材材料料被被认认为为是是塑塑性性的的，事事实实上上杆杆的的塑塑性性伸伸长长是是达达到到比比例例极限时伸长的极限时伸长的10到到15倍。倍。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 At point C the material begins to strain harden and to offer addit

30、ional resistance to increase in load.Thus,with further elongation the stress increases,and it reaches its maximum value,or ultimate stress,at point D.Beyond this point further stretching of the bar is accompanied by a reduction in the load,and fracture of the specimen finally occurs at point E on th

31、e diagram.材材料料在在C点点出出现现应应变变强强化化，对对载载荷荷的的增增加加产产生生了了额额外外的的阻阻力力。这这样样，随随着着进进一一步步的的伸伸长长，应应力力随随之之增增加加，直直到到D点点，应应力力达达到到最最大大值值即即极极限限应应力力。超超过过D点点以以后后，进进一一步步的的伸伸长长伴伴随随着着载载荷荷的的减少，最后在图表的减少，最后在图表的E点处，样品发生断裂。点处，样品发生断裂。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 During elongation of the bar a lateral co

32、ntraction occurs,resulting in a decrease in the cross-sectional area of the bar.This phenomenon has no effect on the stress-strain diagram up to about point C,but beyond that point the decease in area will have a noticeable effect upon the calculated value of stress.A pronounced necking of the bar o

33、ccurs(see Fig.2),and if the actual cross-sectional area at the narrow part of the neck is used in calculating,it will be found that the true stress-strain curve follows the dashed line CE.Whereas the total load the bar can carry does indeed diminish after the ultimate stress is reached(line DE),this

34、 reduction is due to the decrease in area and not to a loss in strength of the material itself.在在伸伸长长过过程程中中，圆圆棒棒发发生生了了一一个个横横向向的的收收缩缩，导导致致了了圆圆棒棒横横截截面面积积的的减减少少。直直到到C点点为为止止，这这个个现现象象对对应应力力-应应变变图图都都没没有有影影响响，但但是是超超出出C点点以以后后，面面积积的的减减少少对对应应力力的的计计算算有有着着显显著著的的影影响响。圆圆棒棒发发生生了了一一个个明明显显的的颈颈缩缩(见见图图2)，如如果果在在计计算算时时使

35、使用用颈颈缩缩处处狭狭窄窄的的真真实实横横截截面面积积，我我们们发发现现真真实实应应力力-应应变变曲曲线线将将沿沿着着虚虚线线CE进进行行。然然而而，当当达达到到极极限限应应力力时时(线线段段DE)，圆圆棒棒的的总总载载荷荷可可能能真真正正的的减减少少，这个减少归功于面积的减少而不是材料本身强度的损失。这个减少归功于面积的减少而不是材料本身强度的损失。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 The material actually withstands an increase in stress up to the poi

36、nt of failure.For most practical purposes,however,the conventional stress-strain curve OABCDE,based upon the original cross-sectional area of the specimen,provides satisfactory information for design purposes.直直到到失失效效点点为为止止，材材料料一一直直承承受受应应力力的的增增加加。然然而而，对对大大多多数数实实际际目目的的而而言言，建建立立在在样样品品的的原原始始横横截截面面积积上上的

37、的传传统统应应力力-应应变曲线变曲线OABCDE给设计用途提供了令人满意的信息。给设计用途提供了令人满意的信息。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 The diagram in Fig.1 has been drawn to show the general characteristics of the stress-strain curve.There is an initial region on the stress-strain curve in which the material behaves both e

38、lastically and linearly.The region from O to A on the stress-strain diagram for steel is an example.The presence of a pronounced yield point followed by large plastic strains is somewhat unique to steel,which is the most common structural metal in use today.Aluminium alloys exhibit a more gradual tr

39、ansition from the linear to the nonlinear region.图图1显显示示了了应应力力-应应变变曲曲线线的的一一般般特特征征，在在这这个个曲曲线线上上有有一一个个体体现现材材料料的的弹弹性性和和线线性性的的初初始始区区域域。钢钢的的应应力力-应应变变曲曲线线上上从从O点点到到A点点的的这这个个区区域域就就是是一一个个例例子子。紧紧随随大大塑塑性性应应变变之之后后的的显显著著的的屈屈服服点点现现象象是是目目前前最最常常用用的的结结构构金金属属钢钢的的一一点点独独特特的的性性质质。铝铝合金则展示出了从线性到非线性区域的更平缓的转变。合金则展示出了从线性到非线性

40、区域的更平缓的转变。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 Both steel and many aluminium alloys will undergo large strain before failure and are therefore classified as ductile.On the other hand,materials that are brittle fail at relatively low values of strain.Examples include ceramics,cast i

41、ron,concrete,certain metallic alloys,and glass.钢钢和和许许多多铝铝合合金金在在失失效效之之前前都都会会出出现现大大的的应应变变，因因此此可可以以被被分分类类为为韧韧性性材材料料。另另一一方方面面，许许多多材材料料在在相相当当小小的的应应变变时时也也会会出出现破裂失效，例如陶瓷，铸铁，混凝土，某些金属合金和玻璃。现破裂失效，例如陶瓷，铸铁，混凝土，某些金属合金和玻璃。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确Translation When a structure is subjec

42、ted to dynamic loading,the whole or part of it is accelerated with the result that inertia forces are introduced.Due to the influence of inertia forces,the stresses vary during and after loading so that a particular state of stress exists only at a corresponding instant during the process.In many ca

43、ses,however,when the loads are gradually applied or change slowly,the dynamic effect is insignificant and can be neglected.With suddenly applied loads the effect of inertia forces must be taken into account and in extreme cases such as impact or resonance vibration,the dynamic effect predominates.当当

44、一一个个结结构构被被施施加加动动载载荷荷时时，它它的的整整体体或或者者部部分分会会由由于于惯惯性性力力的的引引入入而而被被加加速速。由由于于惯惯性性力力的的影影响响，在在加加载载过过程程中中和和加加载载后后的的应应力力变变化化很很大大，以以至至于于在在过过程程中中每每一一个个瞬瞬间间只只对对应应一一个个特特别别的的应应力力状状态态。然然而而，在在许许多多情情况况下下，当当载载荷荷是是缓缓慢慢增增加加或或者者变变化化很很小小时时，动动态态影影响响是是无无关关紧紧要要的的或或者者是是可可以以被被忽忽略略的的。但但是是在在突突然然加加载载时时，惯惯性性力力的的影影响响必必须须被被考考虑虑，而而且且在

45、在一一些些特特殊殊的的情情况况下下例例如如冲冲击击或或者者共共振时，动态作用是主要的影响因素。振时，动态作用是主要的影响因素。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 As mentioned previously,the dynamic effect,i.e.,the influence of inertia forces on the process of stress development in a body,depends on the dynamic loading conditions.Three groups

46、of typical phenomena can be distinguished.There are(1)quasi-static states of stress,(2)vibrations,and(3)stress waves.The limits between these groups are not clearly defined,however,and frequently the phenomena associated with more than one groups can occur in the same dynamic event.正正如如前前面面所所提提到到的的一

47、一样样，动动力力学学的的影影响响也也就就是是在在一一个个物物体体中中，根根据据动动态态加加载载的的条条件件，惯惯性性力力对对应应力力发发展展过过程程的的影影响响。这这些些影影响响可可以以区区分分为为三三种种典典型型的的现现象象。它它们们是是应应力力的的准准静静态态状状态态，振振动动和和应应力力波波。然然而而，三三种种现现象象的的界界限限并并没没有有被被清清楚楚的的说说明明，因因此此常常常常在同一个动力学事件中有超过一种以上的现象发生。在同一个动力学事件中有超过一种以上的现象发生。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 The

48、 dynamic response of a body depends not only on the magnitude of the forces acting but also,to a decisive extent,on their rate of change.Thus,while stress waves are produced by the change of forces,the frequency of these waves is determined by their rate of change.If the change of forces is due to t

49、he impact of a striking body,this means that the response of the body struck depends on the time of contact between the two bodies.一一个个物物体体的的动动态态响响应应不不但但跟跟作作用用力力的的数数值值有有关关，起起决决定定因因素素的的还还是是这这些些力力变变化化的的频频率率。因因此此，当当力力的的变变化化产产生生应应力力波波时时，这这些些波波的的频频率率已已经经由由产产生生它它们们的的力力的的变变化化率率所所确确定定了了。如如果果这这些些力力的的变变化化是是由由

50、于于一一个个物物体体的的冲冲击击，那那就就意意味味着着这这个个被被撞撞击击物物体体的的响响应决定于这两个物体的接触时间。应决定于这两个物体的接触时间。在整堂课的教学中，刘教师总是让学生带着问题来学习，而问题的设置具有一定的梯度，由浅入深，所提出的问题也很明确 When the forces acting on a body change slowly so that the frequency is very low,the length of the wave is usually great compared with the dimensions of the body.In such