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

2. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

3. 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:
Tensile - tend to stretch the material
Compressive - tend to squeeze material
Shear – tend to slide material
Stress‑strain curve - basic relationship that describes mechanical properties for materials
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

4. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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

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

7. 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.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

8. Engineering Stress Defined as force divided by original area:
where e = engineering stress (tegasan kejuruteraan)
N/mm2),
F = applied force (N), and
Ao = original area of test specimen (mm2)
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

9. 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);
Lo = original gage length (mm)
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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

11. 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.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

12. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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

14. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

15. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

16. 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) =
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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

18. EXAMPLE
A tensile test uses a test specimen that has a gauge length of 50 mm and an area = 200mm2. 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.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

19. 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
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20. True Strain
Provides a more realistic assessment of "instantaneous" elongation per unit length
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21. True Stress-Strain Curve
Figure 2.6 ‑ True stress‑strain curve for the previous engineering stress‑strain plot in Figure 3.3.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

22. 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.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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

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

25. 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)
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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

27. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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

29. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

30. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

31. Bending Test
Figure 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.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

32. Bend-test Methods
Figure 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).
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

33. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

34. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

35. Brinell Hardness Test
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 Hardness testing methods: (a) Brinell
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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

37. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

38. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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

40. Table 2.1: Common Rockwell Hardness Scales
Rockwell Scale
Hardness Symbol
Indenter
Load (kg)
Typical Material Tested A
HRA
Cone 60
Carbides, ceramic B
HRB
1.6 mm ball
100
Nonferrous metals C
HRC
150
Ferrous metals, tool steels
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

41. 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)
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

42. 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.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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

45. 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
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

46. 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.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

47. 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.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

48. 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.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

49. Figure 2.14: Typical creep curve
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50. 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.
©2007 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 3/e

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

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