Mechanical Behavior of Materials

1、Mechanical Behavior of MaterialsKnow the concepts of mechanical properties of materials.Understand the factors affecting the mechanical properties.Be aware of the basic testing procedures that engineers use to evaluate many of these properties.ObjectiveOutline Mechanical Properties of MaterialsStres

2、s-Strain Diagram & PropertiesBend Test of MaterialsHardness Test of MaterialsImpact Testing of MaterialsFracture Mechanics of MaterialsFatigue of Materials and ApplicationCreep of Materials , Stress Rupture, and Stress CorrosionEvaluation of Creep & Use of Creep DataMechanical Behavior of MaterialsB

3、ehavior and Manufacturing Properties of Materials 2003 Brooks/Cole Publishing / Thomson LearningRepresentative Strengths of Various Categories of MaterialsMaterials Design and SelectionDensity is mass per unit volume of a material, usually expressed in units of g/cm3 or lb/in.3Strength-to-weight rat

4、io is the strength of a material divided by its density; materials with a high strength-to-weight ratio are strong but lightweight.Most common test for determining such mechanical properties as strength, ductility, toughness, elastic modulus, and strain hardening. The test specimen made according to

5、 standard specifications. Most specimens are solid and round, some are flat-sheet. In this test a metal sample is pulled to failure at a constant rate. The load displacement relationship is plotted on a moving chart graph paper, with the signals coming from a load cell fixed at the top of the testin

6、g machine, and an extensometer (strain gauge) attached to the sample.The load displacement data obtained from the chart paper can be converted to engineering stress/strain data, and a plot of engineering stress vs. engineering strain can be constructed.Tension TestMechanical Behavior of MaterialsTen

7、sion Testing Machine Tensile SpecimensMechanical Behavior of MaterialsEngineering Stress Strain Diagram For A High-Strength Aluminum Alloy.(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.A unidirectional force is applied to a specim

8、en in the tensile test by means of the moveable crosshead. The cross-head movement can be performed using screws or a hydraulic mechanismMechanical Behavior of MaterialsMechanical property data obtained from the tensile test are of engineering importance for structural design. These are:modulus of e

9、lasticityyield strength at 0.2 percent offsetultimate tensile strengthpercent elongation at fracturepercent reduction in area at fracture- Stress () = Force or load per unit area of cross-section.- Strain () = Elongation change in dimension per unit length- Youngs modulus (E)= The slope of the linea

10、r part of the stress- strain curve in the elastic region (stress) = E x (strain)or E = (stress)/(strain) psi or paMechanical Behavior of MaterialsSlope of stress strain plot (which is proportional to the elastic modulus) depends on bond strength of metalAdapted from Fig. 6.7, Callister 7e. Mechanica

11、l Behavior of Materials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Comparison of the elastic behavior of steel and aluminum. For a given stress, aluminum deforms elastically three times as much as does steelMechanical Behavior o

12、f MaterialsMechanical Behavior of MaterialsIn industry, components are formed into various shapes by applying external forces to the workpiece using specific tools and dies. A typical operation is rolling of a flat sheet to be processed into a car body. Because deformation in these processes is carr

13、ied out by mechanical means, an understanding of the behavior of materials in response to externally applied forces is important. Forming operations may be carried out at room temperature or at higher temperatures and at a low or a high rate of deformation.The behavior of a manufactured part during

14、its expected service life is an important consideration. For example the wing of an aircraft is subjected to static as well as dynamic forces. If excessive, dynamic forces can lead to cracks and can cause failure of the component.Mechanical Behavior of MaterialsEngineering stress-strain.Elastic rang

15、e in stress-strain. Mechanical Behavior of MaterialsEngineering stress-strain curve, showing various featuresYield stress (Y), Ultimate tensile strength (UTS), and Fracture. 1. Elastic and Plastic, 2. Uniform elongation and Necking. Mechanical Behavior of MaterialsAlloying a metal with other metals

16、or nonmetals and heat treatment can greatly affect the tensile strength and ductility of metals. During the tensile test, after necking of the sample occurs, the engineering stress decreases as the strain increases, leading to a maximum engineering stress in the engineering stress-strain curve. Thus

17、, once necking begins during the tensile test, the true stress is higher than the engineering stress. Engineering stress = P/A0 and Engineering strain =(l-l0)/l0 True stress T = F/Ai = (1+ ) and True strain T =ln (li/l0) = ln (1+ )Mechanical Behavior of MaterialsMechanical Behavior of MaterialsEngin

18、eering stress-strain curves for some metals and alloysChapter 4, mechanical properties of metalsMechanical Behavior of MaterialsChapter 4, mechanical properties of metalsComparison between engineering and tue stress-strain curveMechanical Behavior of MaterialsYield strength is a very important value

19、 in engineering structural design since it is the strength at which a metal or alloy begins to show significant plastic deformation. Since there is no definite point on the stress-strain curve where elastic strain ends and plastic strain begins, the yield strength is chosen to be that at which a fin

20、ite amount of plastic strain has occurred. For American structural design, the yield strength is chosen at 0.2% plastic strain. The ultimate tensile strength (UTS) is the maximum strength reached in the engineering stress-strain curve. If the specimen develops a localized reduction in cross-sectiona

21、l area (necking), the engineering stress will decrease with further strain until fracture.Mechanical Behavior of Materials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license. Determining the 0.2% offset yield strength in gray cast ion,

22、and (b) upper and lower yield point behavior in a low-carbon steelMechanical Behavior of MaterialsResilience, UrAbility of a material to store energy Energy stored best in elastic regionIf we assume a linear stress-strain curve this simplifies toAdapted from Fig. 6.15, Callister 7e.yyr21UesMechanica

23、l Behavior of MaterialsThe area under the elastic region is the elastic strain energy (in.lb./in.3), a measure of the amount of elastic energy that can be stored in each cubic inch of the specimen.For spring steel, MR = 385 in.lb./in.3 or 1355 ./lb. For rubber, MR = 1680 385 in.lb./in.3 or 48,000 ./

24、lb. Rubber can store much more energy per unit volume or weight than can steel. Mechanical Behavior of MaterialsElastic Strain RecoveryAdapted from Fig. 6.17, Callister 7e.1. Initial2. Small load3. UnloadFdMechanical Behavior of MaterialsThe more ductile a metal is, the more the decrease in the stre

25、ss on the stress-strain curve beyond the maximum stress. For high strength aluminum alloy, there is only a small decrease in stress beyond the maximum stress because this material has relatively low ductility.The ultimate tensile strength is not used much in engineering design for ductile alloys sin

26、ce too much plastic deformation takes place before it is reached. However, the ultimate tensile strength can give some indication of the presence of defects. If the metal contains porosity or inclusions, these defects may cause the ultimate tensile strength of the metal to be lower than normal.Mecha

27、nical Behavior of MaterialsDuctility of metals is most commonly expressed as percent elongation and percent reduction in area. The percent elongation and percent reduction in area at fracture is of engineering importance not only as a measure of ductility but also as an index of the quality of the m

28、etal. Percent elongation is the amount of elongation that a tensile specimen under goes during testing provides a value for the ductility of a metal. Percent reduction in area is usually obtained from a tensile test using a specimen 0.50 in (12.7 mm) in diameter. x 100LLLEL%oof-=100 xAAARA%ofo-=Mech

29、anical Behavior of Materials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Localized deformation of a ductile material during a tensile test produces a necked region. The micrograph shows necked region in a fractured sampleMechanic

30、al Behavior of Materials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.The stress-strain behavior of brittle materials compared with that of more ductile materialsMechanical Behavior of MaterialsMechanical Behavior of MaterialsChap

31、ter 4, mechanical properties of metalsToughness: is defined as the total area under the stress strain curve up to fracture (in.lb./in.3). It is a measure of the total amount of energy that can be absorbed prior to fracture. Brittle materials are not tough.Note: It is not possible to make this integr

32、ation unless we have some mathematical function that describes the relationship between stress and strain up to fracture ( = Ee only describes the relationship during elastic deformation, not plastic deformation). Some possible mathematical models will be described in the following section. As an ap

33、proximation, toughness can be estimated as the area under the curve using the combined areas of simple shapes such as rectangles and triangles.Mechanical Behavior of MaterialsGiven the true stress strain curve = Kn , the toughness (the specific energy (in.lb./in3) dissipated up to fracture) can be c

34、alculated by integrating with respect to strain up to the strain at fracture (f) Then using the true stress strain model = Kn Mechanical Behavior of MaterialsExample Problem Mechanical Behavior of Materials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used

35、 herein under license.Figure 6.10 The stress-strain curve for an aluminum alloy from Table 6-1Mechanical Behavior of MaterialsExample Problem Mechanical Behavior of MaterialsExample Problem Youngs Modulus of Aluminum AlloyFrom the data in Example 6.1, calculate the modulus of elasticity of the alumi

36、num alloy. Use the modulus to determine the length after deformation of a bar of initial length of 50 in. Assume that a level of stress of 30,000 psi is applied.Example 6.3 SOLUTIONMechanical Behavior of MaterialsDuctility of an Aluminum AlloyThe aluminum alloy in Example 6.1 has a final length afte

37、r failure of 2.195 in. and a final diameter of 0.398 in. at the fractured surface. Calculate the ductility of this alloy.Example 6.4 SOLUTIONMechanical Behavior of Materials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.The effect

38、of temperance (a) on the stress-strain curve and (b) on the tensile properties of an aluminum alloyMechanical Behavior of MaterialsTrue Stress and True Strain CalculationCompare engineering stress and strain with true stress and strain for the aluminum alloy in Example 6.1 at (a) the maximum load an

39、d (b) fracture. The diameter at maximum load is 0.497 in. and at fracture is 0.398 in.Example 6.5 SOLUTIONMechanical Behavior of MaterialsSOLUTION (Continued)Mechanical Behavior of MaterialsCompression: Many manufacturing processes such as forging, rolling, extrusion, are performed with the work pie

40、ce subjected to compressive forces. Compression test, in which the specimen is subjected to compressive load, gives information useful for these processes. When the results of compression tests and tension tests on ductile metals are compared, the true stress-true strain curves for the two tests coi

41、ncide. This comparability does not hold true for brittle materials, which are generally stronger and more ductile in compression than in tensionMechanical Behavior of Materials. Factor of safety, NOften N isbetween1.2 and 5 Example: Calculate a diameter, d, to ensure that yield does not occur in the

42、 1045 carbon steel rod below. Use a factor of safety of 5.Design or Safety Factors51045 plain carbon steel: sy = 310 MPa TS = 565 MPaF = 220,000NdLod = 0.067 m = 6.7 cmMechanical Behavior of MaterialsBend Test for MaterialsBend Test for Brittle MaterialsBend test - Application of a force to the cent

43、er of a bar that is supported on each end to determine the resistance of the material to a static or slowly applied load.Flexural strength -The stress required to fracture a specimen in a bend test. Flexural modulus - The modulus of elasticity calculated from the results of a bend test, giving the s

44、lope of the stress-deflection curve.Mechanical Behavior of Materials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.The bend test often used for measuring the strength of brittle materials, and (b) the deflection obtained by bending

45、Bend Test for Brittle MaterialsMechanical Behavior of Materials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Stress-deflection curve for Mg0 obtained from a bend testBend Test for Brittle MaterialsMechanical Behavior of MaterialsB

46、ending (Flexure): The Bend test is commonly used for brittle materials. It usually involves a specimen that has a rectangular cross-section. The load is applied vertically, at either one point or two: as a result, these tests are referred to as three-point and four point bend, respectively. The long

47、itudinal stresses in these specimens are tensile at their lower surfaces and compressive at their upper surfaces. The stress at fracture in bending is known as the transverse rupture strength.Bend Test for Brittle MaterialsMechanical Behavior of MaterialsHardness of MaterialsHardness is a measure of

48、 the materials resistance to localized plastic deformation (e.g. dent or scratch). In general, hardness usually implies a resistance to deformation, and for metals the property is a measure of their resistance to permanent or plastic deformation. To a person concerned with the mechanics of materials

49、 testing, hardness is most likely to mean the resistance to indentation.Hardness of MaterialsMechanical Behavior of MaterialsSteel is harder than aluminum, and aluminum is harder than lead. Several methods have been developed to measure the hardness of materials.Hardness of MaterialsMechanical Behav

50、ior of MaterialsHardness and Strength: Studies have shown that (in the same units) the hardness of a cold-worked metal is about three times its yield stress: for annealed metals, it is about five times the yield. A relationship has been established between the ultimate tensile strength (UTS) and the

51、 Brinell hardness (HB) for steels. In SI units, UTS = 3.5*(HB), where UTS is in Mpa. Or UTS = 500*(HB), where UTS is in psi and HB is in kg/mm2, as measured for a load of 3000 kg.Hardness of MaterialsMechanical Behavior of MaterialsHardness-Testing Procedures: The following considerations must be ta

52、ken for hardness test to be meaningful and reliable:The zone of deformation under the indenter must be allowed to develop freely.Indentation should be sufficiently large to give a representative hardness value for the bulk material.Surface preparation is necessary, if conducting Rockwell test and ot

53、her tests, except Brinell test.(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.Hardness of MaterialsMechanical Behavior of MaterialsMechanical Behavior of MaterialsHardness of MaterialsTemperature Effects: Increasing the temperature

54、 generally has the following effects on stress-strain curves:It raises ductility and toughnessIt lowers the yield stress and the modulus of elasticityIt lowers the strain-hardening exponent of most metalsMechanical Behavior of MaterialsRate-of-Deformation (Strain Rate) Effects: Deformation (strain)

55、rate is defined as the speed at which a tension test is being carried out, in units of, say, mm/s.The strain rate is a function of the specimen length. A short specimen elongates proportionately more during the same time period than does a long specimen.(c)2003 Brooks/Cole, a division of Thomson Lea

56、rning, Inc. Thomson Learning is a trademark used herein under license.When a ductile material is pulled in a tensile test, necking begins and voids form starting near the center of the bar by nucleation at grain boundaries or inclusions. As deformation continues a 45 shear lip may form, producing a

57、final cup and cone fractureMechanical Behavior of MaterialsImpact Testing of MaterialsImpact test - Measures the ability of a material to absorb the sudden application of a load without breaking.Impact energy - The energy required to fracture a standard specimen when the load is applied suddenly.Imp

58、act toughness - Energy absorbed by a material, usually notched, during fracture, under the conditions of impact test.Fracture toughness - The resistance of a material to failure in the presence of a flaw.Mechanical Behavior of Materials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomso

59、n Learning is a trademark used herein under license.The impact test: (a) The Charpy and Izod tests, and (b) dimensions of typical specimensMechanical Behavior of MaterialsDuctile to brittle transition temperature (DBTT) - The temperature below which a material behaves in a brittle manner in an impac

60、t test.Notch sensitivity - Measures the effect of a notch, scratch, or other imperfection on a materials properties, such as toughness or fatigue life.Mechanical Behavior of Materials(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.R