机械工程导论（双语）课件

1、mechanical engineering,An introduction to,1,Lecturer: Liu Ju-rong,CHAPTER 5 Materials and Stress,2,Vocabulary,3,shear stress 剪应力 stress-strain 应力-应变 aluminum 铝 elastic modulus 弹性模数 plastic behavior n. 塑性 round rod 圆钢 shrink 收缩 Poisson contraction(泊松收缩 gauge 测量 rubber band 橡皮筋 spring back vi. 弹回 hypo

2、thetical 假想的 chassis 底盘 low-carbon steel 低碳钢 stiffness 刚度 ductility 延展性;塑性 brittle materials 脆性材料 cast iron 铸铁;生铁 concrete 混凝土,Vocabulary,4,ceramics 陶器 glass 玻璃 low-strain 小应变 permanent set 永久变形;残余变形 proportional limit 比例极限 pipe fittings 管接头 bushings 衬套 annealed 退火 tempered 回火 heat treatment n. 热处理

3、quenching n. 淬火 artificial aging n. 人工时效;人工老化 hinge pin n. 铰销,1 OVERVIEW,5,As part of their responsibilities, mechanical engineers must design hardware so that it doesnt break,Knowing those forces alone, however, is not sufficient for an engineer to predict whether a certain mechanical component wil

4、l be strong enough and not fail in the task at hand,By fail or failure we mean that the hardware will not break, stretch, or bend so much as to become nonusable,In short, engineers are able to combine their knowledge of forces, materials, shapes, and dimensions in order to learn from past failures a

5、nd to better design new hardware,1 OVERVIEW,6,In this chapter, we will discuss the properties of engineering materials and examine the various types of stresses that can develop within them,Tension, compression, and shear stress are quantifies that engineers calculate when they relate the dimensions

6、 of a mechanical component to the forces acting on it,Those stresses are then compared with the materials physical properties in order to determine whether or not failure is expected to occur,Engineers generally conduct this type of force, stress, and fail analysis early in a products design cycle a

7、s choices are made for dimensions, materials, and operating conditions,1 OVERVIEW,7,FINGURE 5.1 A broken crankshaft from a single-cylinder internal combustion engine,1 OVERVIEW- The ability to be available in this chapter,8,Identify circumstances where a machine component is loaded in tension, compr

8、ession, or shear, and calculate the stress that is present,Draw stress-strain curves for and steel, and determine the elastic modulus and yield strength from such,Understand the differences between elastic and plastic behavior,Apply a factor of safety to components that are subjected to tension or s

9、hear stress,2 TENSION AND COMPRESSION,9,the rod does not take on a permanent set after F has been applied, the stretching is said to occur elastically,the force could have been large enough that the rod would have been plastically deformed, meaning that when the force was applied, and then removed,

10、the rod would end up being longer than it was initially,The diameter of the rod shrinks by a small amount due to the effect known as Poisson contraction,2 TENSION AND COMPRESSION,10,FINGURE 5.3 description of tensile stress in a rod. (a) The stretched rod with end load, (b) a section sliced from the

11、 rod exposing the internal force F, (c) the stress distributed over the cross section, and (d) tensile stresses acting on the ends of any element sliced from the rod,2 TENSION AND COMPRESSION,11,imagine slicing through the bar at some point inward of the right-hand end,When the stress tends to lengt

12、hen the rod, it is called tension, and 0,when the rod would be Compression shortened, the stress is called compression, and 0,F/A,2 TENSION AND COMPRESSION,12,The stretch or elongation L is one way to describe how the rod lengthens when F is applied, but it is not the only way, nor is it necessarily

13、 the best way,Strain is defined as the amount of stretching that occurs per unit of length. Strain (the lowercase Greek character epsilon) is scaled to the rods original length and calculated from the expression = L/L,3 RESPONSE OF ENGINEERING MATERIALS,13,Figure 5.7 shows an idealized stress-strain

14、 curve for a material such as ductile(韧性;可塑 ) low-carbon steel(低碳钢,Spring constant force F and elongation L are proportional according to F = k L. The parameter k is called the stiffness or spring constant,This observation is the Hooke s law basis of the concept called Hooke s law, which states that

15、 force and deflection are proportional,3 RESPONSE OF ENGINEERING MATERIALS,14,FINGURE 5.76 Each rod has similar stress-strain behavior,3 RESPONSE OF ENGINEERING MATERIALS,15,FINGURE 5.7 Idealized stress-strain curve for a ductile low-carbon steel,3 RESPONSE OF ENGINEERING MATERIALS,16,The stress-str

16、ain diagram for a ductile material is broken down into two regions: the low-strain elastic region (where no permanent set remains regions after a load has been applied and removed) and the higher-strain plastic region (where the load is large enough that, upon removal, the material is permanently el

17、ongated,Structural steels ductile material and some aluminum alloys are examples of ductile metals, and they are often used in machine components because when they become overloaded, they tend to noticeably stretch or bend before they break,Brittle materials such as cast iron, concrete, ceramics, an

18、d glass break suddenly and without much prior warning when they are overloaded,3 RESPONSE OF ENGINEERING MATERIALS,17,The material property that quantifies changes in the cross section is called the Poisson ratio, v (the lowercase Greek character nu). v=diameter / length = -(d/d)/ (L/L) d = -vd L /L

19、,For strains below the proportional limit A in Figure 5.7, stress and strain are linearly related according to = E,The elastic modulus is a material property, and it is measured simply as the slope of the stress-strain curve in its straight-line region. L=L=(/E)L=(F/A)/E)L=FL/EA F = kL where k = EA/

20、L Esteel 210 GPa 30 Mpsi Ealuminum 70 GPa 10 Mpsi,3 RESPONSE OF ENGINEERING MATERIALS,18,For that reason, the onset of yielding is often taken by engineers as an indication of failure. The value of stress in region B-C defines a quantity called the yield strength, Sy,Returning to our discussion of t

21、he stress-strain behavior in Figure 5.7, point B is called the elastic limit,Yielding occurs in the region between B and C; for small changes in stress, the rod experiences a large change in strain,As the load is increased even further beyond Ultimate strength point C, grows to the ultimate tensile

22、strength Su at point D,19,As the force F is applied, the material tends to be sliced, sheared, or cut along the two edges marked as shear planes,The two forces V are called shear forces, and they are oriented parallel to the shear planes. The shear forces are, in fact, distributed within the materia

23、l over the entire cross section so that V is the resultant of the shear stress,4 SHEAR,=V/A,20,4 SHEAR,FINGURE 5.11 Shear force V and sheat stresses act on an elastic block that is being pressed between two rigid supports,21,4 SHEAR,FINGURE 5.12 Adhesively bonded connections that are placed in (a) s

24、ingle shear with V=F and (b) double shear with V=F/2. The adhesive layers are indicated by the heavily weighted lines,5 FACTOR OF SAFETY,22,Failure due to ductile yielding is predicted if Sy. Engineers define the tensile factor of safety as ntension=sy/ If n 1, this viewpoint predicts that the component will not yield, and if n 1, fail is expected to occur. As developed in more advanced treatments of stress analysis, one theory for material failure relates the yield strength in shear to the value in tension according to Ssy=Sy/2 To evaluate the possibility of