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©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Chapter 2: MECHANICAL PROPERTIES OF MATERIALS 1.MECHANICAL PROPERTIES.

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Presentation on theme: "©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Chapter 2: MECHANICAL PROPERTIES OF MATERIALS 1.MECHANICAL PROPERTIES."— Presentation transcript:

1 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Chapter 2: MECHANICAL PROPERTIES OF MATERIALS 1.MECHANICAL PROPERTIES  Physical and mechanical properties 2.STATIC STRESS  Tension, Compression and Shear  Stress ‑ Strain Relationships 3.HARDNESS AND TESTING 4.DYNAMIC STRESS  Fatigue, Creep, etc

2 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Mechanical Properties in Design and Manufacturing  Physical properties: A general method to differentiate materials  e.g: Density, melting point, optical property (transparency or opaque), electrical conductivity  Mechanical properties: Determine a material’s behavior when subjected to mechanical stresses/ load or forces  Properties include elastic modulus, ductility, hardness, and various measures of strength  Dilemma: mechanical properties desirable to the designer, such as high strength, usually make manufacturing more difficult  The manufacturing engineer should appreciate the design viewpoint  And the designer should be aware of the manufacturing viewpoint

3 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Stress ‑ Strain Relationships  Two types of stress/ load: static and dynamics  Static stress is when forces that are applied to material are constant.  Static stresses to which materials can be subjected: 1.Tensile - tend to stretch the material 2.Compressive - tend to squeeze material 3.Shear – tend to slide material  Stress ‑ strain curve - basic relationship that describes mechanical properties for materials

4 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Tensile Test Most common test for studying stress ‑ strain relationship, especially metals In the test, a force pulls the material, elongating it and reducing its diameter Figure 2.1 Tensile test: (a) tensile force applied in (1) and (2) resulting elongation of material

5 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Tensile Test Specimen ASTM (American Society for Testing and Materials) specifies preparation of test specimen Figure 2.2 Tensile test: (b) typical test specimen

6 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Tensile Test Setup

7 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Tensile Test Sequence Figure 2.3 Typical progress of a tensile test: (1) beginning of test, no load; (2) uniform elongation and reduction of cross ‑ sectional area; (3) continued elongation, maximum load reached; (4) necking begins, load begins to decrease; and (5) fracture. If pieces are put back together as in (6), final length can be measured.

8 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Engineering Stress Defined as force divided by original area: where  e = engineering stress (tegasan kejuruteraan) N/mm 2 ), F = applied force (N), and A o = original area of test specimen (mm 2 )

9 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Engineering Strain Defined at any point in the test as where e = engineering strain (terikan kejuruteraan) (dimensionless); L = length at any point during elongation (mm); L o = original gage length (mm)

10 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Typical Engineering Stress-Strain Plot Figure 2.4 Typical engineering stress ‑ strain plot in a tensile test of a metal.

11 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Tensile-test Specimen and Machine Figure 2.5 (a) A standard tensile-test specimen before and after pulling, showing original and final gage lengths. (b) A tensile-test sequence showing different stages in the elongation of the specimen.

12 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Yield Point in Stress ‑ Strain Curve  As stress increases, a point in the linear relationship is finally reached when the material begins to yield  Yield point Y can be identified by the change in slope at the upper end of the linear region  Y = a strength property  Other names for yield point = yield strength, yield stress, and elastic limit

13 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Two Regions of Stress ‑ Strain Curve The two regions indicate two distinct forms of behavior: 1.Elastic region 2.Plastic region

14 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Elastic Region in Stress ‑ Strain Curve  Relationship between stress and strain is linear  Material returns to its original length when stress is removed Hooke's Law:  e = E e where E = modulus of elasticity  E is a measure of the inherent stiffness of a material  Its value differs for different materials

15 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Plastic Region in Stress ‑ Strain Curve  Yield point marks the beginning of plastic deformation  The stress-strain relationship is no longer guided by Hooke's Law  As load is increased beyond Y, elongation proceeds at a much faster rate than before, causing the slope of the curve to change dramatically

16 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Tensile Strength in Stress ‑ Strain Curve  Elongation is accompanied by a uniform reduction in cross ‑ sectional area, consistent with maintaining constant volume  Finally, the applied load F reaches a maximum value, and engineering stress at this point is called the tensile strength TS (a.k.a. ultimate tensile strength) TS (UTS) =

17 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Ductility in Tensile Test Ability of a material to plastically deform without fracture  Ductility measure = elongation EL where EL = elongation; L f = specimen length at fracture; and L o = original specimen length L f is measured as the distance between gage marks after two pieces of specimen are put back together

18 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e EXAMPLE A tensile test uses a test specimen that has a gauge length of 50 mm and an area = 200mm 2. During the test, the specimen yields under a load of 98 kN. The corresponding gage length = mm. This is the 0.2 % yield point. The maximum load = 168,000 N is reached at a gauge length = 64.2 mm. Determine: (a) yield strength Y, (b) percentage elongation (c) modulus of elasticity E, and (d) tensile strength TS.

19 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e True Stress Stress value obtained by dividing the instantaneous area into applied load where  = true stress; F = force; and A = actual (instantaneous) area resisting the load

20 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e True Strain Provides a more realistic assessment of "instantaneous" elongation per unit length

21 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e True Stress-Strain Curve Figure 2.6 ‑ True stress ‑ strain curve for the previous engineering stress ‑ strain plot in Figure 3.3.

22 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Compression Test Applies a load that squeezes the ends of a cylindrical specimen between two platens As the specimen is compressed, its height is reduced and its cross- sectional area is increased. Figure 2.7 Compression test: (a) compression force applied to test piece in (1) and (2) resulting change in height.

23 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Compression Test Setup

24 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Engineering Stress in Compression As the specimen is compressed, its height is reduced and cross ‑ sectional area is increased  e = - where A o = original area of the specimen

25 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Engineering Strain in Compression Engineering strain is defined Since height is reduced during compression, value of e is negative (the negative sign is usually ignored when expressing compression strain)

26 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Shear Properties Application of stresses in opposite directions on either side of a thin element Figure 2.8 Shear (a) stress and (b) strain.

27 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Shear Stress and Strain Shear stress defined as where F = applied force; and A = area over which deflection occurs. Shear strain defined as where  = deflection element; and b = distance over which deflection occurs

28 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Torsion Stress-Strain Curve Figure 2.9 Typical shear stress ‑ strain curve from a torsion test.

29 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Shear Elastic Stress ‑ Strain Relationship In the elastic region, the relationship is defined as where G = shear modulus, or shear modulus of elasticity For most materials, G  0.4E, where E = elastic modulus

30 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Testing of Brittle Materials  Hard brittle materials (e.g., ceramics) possess elasticity but little or no plasticity  Often tested by a bending test (also called flexure test)  Specimen of rectangular cross ‑ section is positioned between two supports, and a load is applied at its center

31 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Bending Test Figure 2.10 Bending of a rectangular cross ‑ section results in both tensile and compressive stresses in the material: (1) initial loading; (2) highly stressed and strained specimen; and (3) bent part.

32 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Bend-test Methods Figure 2.11 Two bend-test methods for brittle materials: (a) three-point bending; (b) four-point bending. The areas on the beams represent the bending-movement diagrams, described in texts on mechanics of solids. Note the region of constant maximum bending movement in (b); by contrast, the maximum bending moment occurs only at the center of the specimen in (a).

33 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Hardness  It is defined as resistance to permanent indentation  Good hardness generally means material is resistant to scratching and wear  Most tooling used in manufacturing must be hard for scratch and wear resistance

34 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Hardness Tests  Commonly used for assessing material properties because they are quick and convenient  Variety of testing methods are appropriate due to differences in hardness among different materials  Most well ‑ known hardness tests are Brinell and Rockwell  Other test methods are also available, such as Vickers, Knoop, Scleroscope, and durometer

35 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Widely used for testing metals and nonmetals of low to medium hardness A hard ball is pressed into specimen surface with a load of 500, 1500, or 3000 kg Figure 2.12 Hardness testing methods: (a) Brinell Brinell Hardness Test

36 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Brinell Hardness Number Brinell Hardness Number (BHN) = Load divided into indentation surface area where HB = Brinell Hardness Number (BHN), F = indentation load, kg; D b = diameter of ball, mm, and D i = diameter of indentation, mm

37 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e EXAMPLE In a Brinell hardness test, a 1500 kg load is pressed into a specimen using a 10 mm diameter hardened steel ball. The resulting indentation has a diameter of 3.2 mm. Determine the Brinell hardness number for the metal

38 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Rockwell Hardness Test  Another widely used test  A cone shaped indenter is pressed into specimen using a minor load of 10 kg, thus seating indenter in material  Then, a major load of 150 kg is applied, causing indenter to penetrate beyond its initial position  Additional penetration distance d is converted into a Rockwell hardness reading by the testing machine

39 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Rockwell Hardness Test Figure 2.13 Hardness testing methods: (b) Rockwell: (1) initial minor load and (2) major load.

40 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Table 2.1: Common Rockwell Hardness Scales Rockwell Scale Hardness Symbol IndenterLoad (kg) Typical Material Tested AHRACone60Carbides, ceramic BHRB1.6 mm ball100Nonferrous metals CHRCCone150Ferrous metals, tool steels

41 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 2.53 VICKERS HARDNESS TEST This test, also developed in the early 1920s, uses a pyramid- shaped indenter made of diamond. It is based on the principle that impressions made by this indenter are geometrically similar regardless of load. where F = applied load (kg) D = diagonal of the impression made by the indenter (mm)

42 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e 2.54 KNOOP HARDNESS TEST  Developed by F. Knoop in 1939 uses a diamond indenter in the shape of an elongated pyramid, with applied loads ranging generally from 25 g to 5 kg.  Because of the light loads that are applied, it is a microhardness test.  Therefore, it is suitable for very small or very thin specimens and for brittle materials such as carbides, ceramics, and glass.

43 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Knoop Hardness Test where F = load (kg) D = long diagonal of the indenter (mm)

44 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Hardness-testing Methods and Formulas

45 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Dynamic Properties Dynamic loadings:  Sudden impact or loads  Repeated cycles of loading and unloading  Changes in the mode of loading, such as tension to compression

46 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e FATIGUE  In many engineering applications, products or components are subjected to various dynamic stress/ load.  Cyclic stresses may be caused by fluctuating mechanical load, such as on gear teeth, or by thermal stresses, such as on a cool die coming into repeated contact with hot workpiece.  Under these conditions, the part fails at a stress level below than at which failure would occur under static loading.  This phenomenon is known as fatigue failure.

47 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e CREEP Creep is the permanent elongation of a component under a static load maintained for a period of time. It is a phenomenon of metals and of certain nonmetallic materials, such as thermoplastics and rubber, and it can occur at any temperature.

48 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Creep Tests  The creep test typically consists of subjecting a specimen to a constant tensile load at a certain temperature and of measuring the changes in length at various time increments.  A typical creep curve usually consists of primary, secondary, and tertiary stages.  The specimen eventually fails by necking and fracture, called rupture or creep rupture.  The creep rate increases with temperature and the applied load.

49 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Figure 2.14: Typical creep curve

50 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e IMPACT  A typical impact test consists of placing a notched specimen in an impact tester and breaking it with a swinging pendulum.  In the Charpy test, the specimen is supported at both ends.  In the Izod test, it is supported at one end like a cantilever beam.  Materials that have high impact resistance are generally those that have high strength and high ductility, and hence high toughness.

51 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Impact Test Specimens Figure 2.15 Impact test specimens: (a) Charpy; (b) Izod.

52 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e Material Failures Figure 2.16 Schematic illustrations of types of failures in materials: (a) necking and fracture of ductile materials; (b) buckling of ductile materials under a compressive load; (c) fracture of brittle materials in compression; (d) cracking on the barreled surface of ductile materials in compression

53 ©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e


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