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1、第二章 拉伸、压缩与剪切一 、 轴向拉伸的概念和实例、Examples(p50-51： 1.2-5、 1.2-7、 1.2-9、 1.2- 10、 1.2-12)：apes ： 、 、 、 、 ：Thelineofactionoftheresultantoftheexternal forces coincides with the axis of thebar.the bar either elongates or shortensalong the axis of the bar.二、截面上的内力和轴力Axial forcesExtension Compression Compression
2、Exam 2-1(p49, 1.2-3) calculation of internal forcesDiagram for axial forcesDescribing distribution of axial forces along the axis of the bar distribution of axial the Plane assumption and its consequences consequencesObservationAssumptionAfter deformation, the plane crosssection remains plane, and i
3、s perpendicular to the axis of the bar. Calculation Ex： 1-2(p10) +(p49, 1.2-3)Straight bar with a varying cross section：( )N(Ex. Piecewise tt()xAx =constant cross section)varying axial loads along the axis：( )xN()()xAx =(Ex2.3-8, p161)The difference between the stresses caused byill il ld i iiifista
4、ticallyequivalent load systems s insignificantat distances greater than the largest dimension ofthe area over which the loads are acting.St. Venants Principle三、材料拉伸的力学性能Stress-strain diagram for mild steels (low-carbon)HW reading 1 3： .HW： Reading 1.4, 1.5elastic stage stageHardening stagelocal defo
5、rmation stageyield stageneckingYoungs ModulusstageUnloading and working hardening：ggClassification: high value of (): ductile material;low value of (): brittle material (p18)percent of elongationpercent reduction in area二、低碳钢的压缩力学性能三、脆性材料拉伸（压缩）的力学性能stress-strain diagram: slightly curved （ Fig.1-16，
6、p19）no yielding and necking, small strain before break, small percentage of elongationUltimate strength: the only parameter to characterize strength四、失效、安全系数和强度计算Estimationallowable stress (p40)of Strength：F.S.： nb(ns)1(p39)coefficient for safetyDesign of cross sections(1.8, p44)Determination of all
7、owable loads(p42)E1-7(a)&(b)Verification of strengthHW： Reading 1.7 & 1.8 (p39-), 1.7-6, 五、轴向拉伸或压缩的变形Force-elongation relation for straight bars (two-force bars)：Hookes Law：Youngs ModulusAxial rigidityEx (p82, E2-4)Lateral strain, Poissons ratioPoissons ratioHW： 2.2-6, 2.2-2, 2.2-10, 2.3-7, 2.3-13六、
8、轴向拉伸或压缩的变形能Deformation energy (Strain Energy) energy (Strain The external workLi l tinear e asticStrain-Energy Densitypiecewise constant axial forces： varying cross section and constant forces：=niiAiEiLiNU122( ) ( )=ldxxNU2)(cross axial forcesHW： 2.7-5, 2.7-9xAxE02 Ex. 2-12(p122), 2-13(p124), 2-14(p
9、126) 七、拉伸、压缩静不定问题Dfiii Defin t on：A structure is statically indeterminate when h i ilib i iffit e stat c equ r um equations are not suff cient to determine the internal forces and reactions on that structure . Ex： Fig.2-16(p85)Steps of solving the problem： of solving problem：1、 static equilibrium eq
10、uation2 compatibilityHW: ReadingReading EE22-55, 22-66(p(p8787-9090)、3、 physical equation4、 solvingMi fit & PtiE2 9(Fi 2 29( 103) s s Prestra ns：- g. - p Thermal stress： E2-7(Fig. 2-23 (p96)HW： 2.4-2, 2.4-10, 2.5-2, 2.5-7, 2.5-16, 2.5-21八、应力集中的概念Any of such discontinuities causes changes of thestres
11、s distribution in the vicinity of the discontinuity. in the discontinuity Such discontinuities or stress raisers which cause a local increase of stress is referred to stress concentration.Stress-Concentration Factors (p141) ：Sensitivity to Stress-Concentration： p144,Ductile Materials： a stress conce
12、ntration can often be ignored (the material at the：point of maximum stress will yield and plastic flow will occur so that the intensity ofthe stress concentration is reduced).Brittle Materials： a stress concentration will remain up to the point of fracture.HW: ReadingReading 22.1010(p(p139139-144144
13、)九、剪切和挤压的实用计算Surface of Shearpractical calculation:average shear t fths ress area of the cross sectioncondition for strength：allowable shear stress shear Design of cross sectionsDetermination of allowable loadsVerification of strengthBiBearingbearing stressThe height of contact area multiplyingstres
14、sarea multiplying the diameter of the cylinderpractical calculation:bearing stress stresszthe bearing area is the contact areazthe bearing area is the height of contact area ltili th di t fth lidmultiply ng the diame er o the cylinderallowableCondition for strength：bearing stressEx: E1-4、 E1-5(Fig.1-30)HW： 1.6-6
