Hardness_Eng

Laboratory 2:

1Mechanical Metallurgy

LaboLaborraattooryry 22

Hardness Testing

Objectives

• Students are required to understand the principles of hardness testing, i.e., Rockwell, Brinell and Vickers hardness tests.

• Students are able to explain variations in hardness properties of selected materials such as aluminium, steel, brass and welded metals and can explain factors that might affects their hardness properties.

• Students can select appropriate macro-micro hardness testing techniques for suitable materials-property analysis.

• Students are able to analyze the obtained hardness values in relevant to the nature of each material to be measured and use this information as a tool for selecting suitable materials for engineering applications.

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1 . L itera tu re Rev ie w

Hardness is one of the most basic mechanical properties of engineering materials. Hardness test is practical and provide a quick assessment and the result can be used as a good indicator for material selections. This is for example, the selection of materials suitable for metal- forming dies or cutting tools. Hardness test is also employed for quality assurance in parts which require high wear resistance such as gears.

The nomenclature of hardness comes in various terms depending on the techniques used for hardness testing and also depends on the hardness levels of various types of materials. A scratch hardness test is generally used for minerals, giving a wide range of hardness values in a Moh.s scale at minimum and maximum values of 1 and 10 respectively. For example, talcum provides the lowest value of 1 while diamond gives the highest of 10. The basic principle is that the harder material will leave a scratch on a softer material. Hardness values of metals generally fall in a range of 4-8 in Moh.s scale, which is not practical to differentiate hardness properties for engineering applications. Therefore, indentation hardness measurement is conveniently used for metallic materials. A deeper or wider indentation indicates a less resistance to plastic deformation of the material being tested, resulting in a lower hardness value.

The indentation techniques involve Brinell, Rockwell, Vickers and Knoop. Different types of indenters are applied for each type. The standard test methods according to the American Society Testing and Materials (ASTM) available are, for instance, ASTM E10-07a (Standard test method for Brinell hardness of metallic materials), ASTM E18-08

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(Standard test method for Rockwell hardness of metallic materials) and ASTM E92-41 (Standard test method for Vickers hardness of metallic materials) These hardness testing techniques are selected in relation to specimen dimensions, type of materials and the required hardness information. Their principles and testing methods are mentioned as follow.

1 .1 B rin ell Ha rdn e ss T es t

Brinell hardness test was invented by J.A. Brinell in 1900 using a steel ball indenter with a

10 mm diameter. The steel ball is pressed on a metal surface to provide an impression as demonstrated in figure 1. This impression should not be distorted and must not be too deep since this might cause too much of plastic deformation, leading to errors of the hardness values.

c) Impression on Brinell hardness test sample

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Different levels of material hardness result in impression of various diameters and depths. Therefore different loads are used for hardness testing of different materials as listed in table 1. Hard metals such as steels require a 3,000 kgf load while brass and aluminium involve the loads of 2,000 and 1,000 or 500 kgf respectively. For materials with very high hardness, a tungsten carbide ball is utilized to avoid the distortion of the ball.

c) Impression on Brinell hardness test sample

Figure 1: (a) Brinell indentation (b) measurement of impression diameter and c) Impression on Brinell hardness test sample [1].

In practice, pressing of the steel ball on to the metal surface is carried out for 30 second, followed by measuring two values of impression diameters normal to each other using a low magnification macroscope. An average value is used for the calculation according to equation 1

BHN = P

= P

; (1)(πD / 2)(D − D 2 − d 2 )

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πDt

where P is the applied load, kgD is the diameter of the steel ball,

mmd is the diameter of the indentation, mmt is the depth of impression, mm

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Note: This BHN values has a unit of kgf.mm-2 (1 kgf.mm-2 = 9.8 MPa) which cannot be compared to the average mean pressure on the impression.

Generally, the metal surface should be flat without oxide scales or debris because these will significantly affect the hardness values obtained. A good sampling size due to a large steel ball diameter is advantageous for materials with highly different microstructures or microstructural heterogeneity. Scratches or surface roughness have very small effects on the hardness values measured. However, there are some disadvantages of Brinell hardness test. These are errors arising from the operator themselves (from diameter measurement) and the limitation in measuring of too small samples.

Figure 2:Plastic deformation surrounded by elastic material underneath a Brinell indenter

If we considered the plastic zone beneath the Brinell indenter, this plastic region is surrounded by elastic material which obstructs the plastic flow. This condition is said to be plane strain compressive where plastic deformation is limited. If the metal is very rigid, the metal flow upwards surrounding the indenter is possible as illustrated in figure 1 a). However this situation is rarely seen because the

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metal displaced by the indenter is accounted for by the reduced volume of elastic material.

1 .2 Ro ckw ell Ha rd n ess T es t

Rockwell hardness test is commonly used among industrial practices because the Rockwell testing machine offers a quick and practical operation and can also minimize errors arising from the operator. The depth of an indentation determines the hardness values. There are two types of

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indenters, Brale and steel ball indenters. The former is a round-tip cone with an included angle of120o whereas the latter is a hardened steel ball with their sizes ranging from 1.6-12.7 mm. Thereforedifferent combinations of indenters and loads selected are suitable for hardness testing of various materials. This is for example; the R scale is employed for soft materials such as polymers while the A scale is suitable for hardness testing of hard materials such as tool materials according to table 1.

The testing procedure starts with indenting a flatly ground metal surface with a diamond or hardened steel ball with a minor load of 10 kgf to position the metal surface as shown in figure 3. . The depth of the impression caused by the minor load will be recorded as H1onto the machine before applying a major load level according to a standard as shown in table 2 and is recorded as H2. The difference of the depths (∆H= H1-H2) when applying the minor and the major loads indicates the hardness value of the material. If the depth difference is small, the deformation resistance of the metal is high, resulting in a high Rockwell hardness value. The hardness value will be displayed on a dial or a screen, having 100 divisions and each division represents a depth of 0.002 mm. Therefore the hardness value can be determined from a relationship as follows

HRX = M − ∆H

0.002; (2)

Where ∆H is H1-H2 and M is the maximum scale which equals 100 in general for testing with the diamond indenter (scale A, C and D). The M value equals 130 when testing with a steel ball for Rockwell scales B, E, M, and R.

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Figure 3: Rockwell hardness measurement showing positions to apply the minor and major loads.

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The Rockwell hardness units are in RA, RB and RC (or HRA, HRB, HRC), depending on material.s hardness. Tables 1 and 2 summarize loads and types of an indenter utilized for each scale. There are two types of indenters used, Brale indenter and steel ball indenters as mentioned previously. The applied major loads vary from 60, 100 and 150 kgf, also depending on the Rockwell hardness scale utilized. For instance, hardened steel is tested on a Rockwell scale C using a Brale indenter and at a major load of 150 kgf. On the Rockwell scale C, the obtained hardness values range from RC 20 F RC 70. Metals with lower hardness are tested on a Rockwell scale B using a 1.6 mm diameter steel ball at a 100 kgf major load, providing RB 0 F RB 100 hardness values. Rockwell scale A offers a wider range of hardness values which can be used to test materials ranging from annealed brass to cemented carbide. Due to high accuracy, the Rockwell hardness test is commonly conducted for measuring hardness of heat-treated steels. Furthermore, the smaller indenter (in comparison to that of Brinell hardness test) facilitates hardness measurement in small areas. However, this technique requires good surface preparation since the hardness values obtained is significantly affected by rough and scratched surfaces.

There are several considerations for Rockwell

hardness test- Require clean and well positioned indenter and anvil- The test sample should be clean, dry, smooth and oxide-free surface- The surface should be flat and perpendicular to the indenter

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- Low reading of hardness value might be expected in cylindrical surfaces- Specimen thickness should be 10 times higher than the depth of the indenter- The spacing between the indentations should be 3 to 5 times of the indentation diameter- Loading speed should be standardized.

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Table 1: Rockwell hardness scale for various mateirals

Scale IndenterMinor Load kgf Major Load kgf

A Diamond cone 10 50

B 1/16" steel ball 10 90

C Diamond cone 10 140

D Diamond cone 10 90

E 1/8" steel ball 10 90

F 1/16" steel ball 10 50

G 1/16" steel ball 10 140

H 1/8" steel ball 10 50

K 1/8" steel ball 10 140

L 1/4" steel ball 10 50

M 1/4" steel ball 10 90

P 1/4" steel ball 10 140

R 1/2" steel ball 10 50

S 1/2" steel ball 10 90

V 1/2" steel ball 10 140

Table 2: Applied loads and types of indenter used in Rockwell scale A,B and C hardness testing.

1 .3 V i ckers H a r dn ess T e st

Vickers hardness test requires a diamond pyramid indenter with an included angle of 136o. This technique is also called a diamond pyramid hardness test (DPH) according to the shape of the indenter. To carry on the test, the diamond indenter is pressed on

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to a prepared metal surface to cause a square-based pyramid indentation as illustrated in figure 4.

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c) Impression on Vickers hardness c) Impression on Vickers hardness test sample

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c) Impression on Vickers hardness test sample

Figure 4: Vickers hardness test (a) Vickers indentation, (b) measurement of impression diagonal.

The Vickers hardness value (VHN) can be calculated from the applied load divided by areas of indentation, at which the latter is derived from the diagonals of the pyramid as expressed inthe equation below

VHN =2P sin(θ / 2)

d 2=

1.854P

d 2 ;(2)

Where P

is the applied load, kgdθ

is is

the average length of the diagonals = (d1+d2)/2) , mmthe angle between the opposite faces

Generally, the applied load should be carefully selected to achieve a perfect square-based pyramid indentation for accurate hardness values, see figure 5 (a). The pincushion indentation as shown in figure 5 (b) normally observed in annealed metal results from sinking of metal surrounding the pyramid faces. The measured diagonals would be too long, thus, giving an under-estimated hardness value. In figure 5 (c), a barrel-shaped indentation usually achieved from cold-worked metals provides an indentation with metal pile-up at the pyramid faces. In such a case,

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the measured diagonals would be too small and lead to an over-estimated hardness value obtained.

Vickers hardness is widely used in experimental and research areas because the VHN scale practically offers a wide range of hardness values. For instance, the VHN values range from 5 to1,500 can be obtained from measuring materials from dead soft to full hard. This method is therefore more convenient and provides a wider range of the hardness values in comparison to those obtained

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from Rockwell and Brinell hardness tests. The applied loads vary from 1-120 kg, which depends on the materials being tested. However, Vickers hardness test is incommonly used for company daily checks. This is due to errors which might occur in the measurement of the diagonals and longer time required to finish the test.

Figure 5: Vickers hardness indentations a) perfect indentation, b) pincushion and c) barrel-shaped.

1 .4 Micro V ick ers ha rdn e ss t est

Micro Vickers hardness requires a micro-sized indenter (figure 6), which allows hardness measurement in very limited areas such as surfaces of fine wires, thin sheets and foils. Moreover hardness measurements at specific microstructural phases of materials, for instance, hardness measurment of ferrites and pearlites existing in steels is also possible. This is beneficial for identifying any hardness variation caused by metallurgical changes such as hardening, quenching, plating, welding, bonding processes, where the larger indenter used for macro Vickers hardness test limits its application in this case. The testing procedure of micro Vickers hardness is similar to that of macro Vickers hardness. However, the prepared surface should be well polished without any fine scratches in

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order to minimize errors which might occur when indenting on these scratches.

Another useful type of micro hardness test employs a Knoop indenter as shown in figure 6 (right) in order to accommodate limited testing areas such as on cross-sections of heat-treated surfaces. The Knoop hardness number (KHN) can be calculated from the applied load divided by theunrecovered projected area of the indention as follows

KHN = 14 .2 P

l 2 ;(3)


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Where

P is the applied load, kgl is the length of the long diagonals, mm

Figure 6: Micro hardness indentations a) Vickers diamond-pyramid indenter, b) Knoop diamond- pyramid indenter.

Furthermore, the strength of some metals can be determined from the plastic area under the stress-strain curve. This is of interest when the strength of the materials can not be measured directly from the standard tensile test. In this case, the yield strength at 0.2% offset can be determined from the Vickers hardness number as shown in the expression

σ = VHN

(0.1) n o

3

;(5)

where σo is the yield strength at 0.2% offset, kgf mm-2 (= 9.8 MPa)VHN

nis is

the Vickers hardness number, VHNthe work hardening exponent


Laboratory 2: In summary, hardness measurements for

example Brinell, Rockwell, Vickers and Knoop are considered to be fast and easy ways to acquire hardness values of materials. Suitable hardness measurements should be selected depending on the nature of the materials, dimensions, specimen locations to be measured, metallurgical microstructures or phases of interest, etc. Analysis of the

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hardness data leads to better understanding of materials and further development in advanced materials. The selection of proper materials to be used in desired applications can be therefore effectively made. Moreover, prediction of material strength is possible by interpreting the hardness values if the work hardening exponent is known.

!"# 7: 3456789:;<=>?:@=>?ABCADC83EFG@=HDIJKJMCกกC<@JOP?3?? Micro

Vicker/Knoop 3RS Rockwell scale C.

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!"# 8: 3456789:;<=>?:@=>?ABCADC83EFG3RSABCADC83EFG3<GEPG Carbon steel

3RS Alloy steel

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For convenience, the hardness values measured using different methods such as Brinell, Rockwell or Vickers testing can be converted using the hardness value conversion table as shown in table 3.

Table 3 Hardness value conversion table for Brinell, Rockwell 3RS Vickers hardness values.

Rockw

Superficial Rockwell

Brinel

Vickers

Tensile strength

Diamond

1/16" Ball

"N" Brale Penetrater

10 mm Ball,3000 kgf

150 kgfC

60 kgfA

100 kgfD

100 kgfB

15 kgLoad

30 kgLoad

45 kgLoad

Diam. Of Ball

Impress

Hardness

Equivalent 1000 lb. Sq.

I80

92

87

97

92

87

186579

92

86

92

87

178778

91

85

96

91

86

171077

91

84

91

85

163376

90

83

96

90

84

155675

90

83

89

83

147874

89

82

95

89

82

140073

89

81

88

81

132372

88

80

95

87

80

124571

87

80

87

79

116070

87

79

94

86

78

107669

86

78

94

85

77

100468

86

77

85

79

9426

785

76

93

84

75

8946

685

76

93

83

73

8546

584

75

92

82

72

2.25

745

8206

484

74

81

74

2.30

710

7896

383

73

92

80

70

2.30

710

7636

283

73

91

79

69

2.35

682

7466

182

72

91

79

68

2.35

682

7206

081

71

90

78

67

2.40

653

6975

981

70

90

77

66

2.45

627

674

3265

880

69

89

76

65

2.55

578

653

3155

780

69

89

75

63

2.55

578

633

3045

679

68

88

74

62

2.60

555

613

2945

579

67

88

73

61

2.60

555

595

2875

478

66

87

72

60

2.65

534

577

2795

377

65

87

71

59

2.70

514

560

2695

277

65

86

70

57

2.75

495

544

2615

176

64

86

69

56

2.75

495

528

2545

076

63

86

69

55

2.80

477

513

2454

975

62

85

68

54

2.85

461

498

2384

875

61

85

67

53

2.90

444

484

232

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47

74

61

84

66

51

2.90

444

471

225


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Rockw


Brinel

Vickers

Tensile strength

Diamond

1/16" Ball


10 mm Ball,3000 kgf

150 kgfC

60 kgfA

100 kgfD

100 kgfB

15 kgLoad

30 kgLoad

45 kgLoad

Diam. Of Ball

Impress

Hardness


I46

73

60

84

65

50

2.95

432

458

2194

573

59

83

64

49

3.00

415

446

2114

473

59

83

63

48

3.00

415

434

2064

372

58

82

62

47

3.05

401

423

2024

272

57

82

61

46

3.10

388

412

1984

171

56

81

60

44

3.10

388

402

1914

070

55

80

60

43

3.15

375

392

1853

970

55

80

59

42

3.20

363

382

1813

869

54

79

58

41

3.25

352

372

1763

769

53

109

79

57

40

3.30

341

363

1713

668

52

109

78

56

39

3.35

331

354

1683

568

52

108

78

55

37

3.35

331

345

1633

467

51

108

77

54

36

3.40

321

336

1593

367

50

107

77

53

38

3.45

311

327

1543

266

49

106

76

52

34

3.50

302

318

1503

166

48

106

76

51

33

3.55

293

310

1463

065

48

105

75

50

32

3.60

285

302

1422

965

47

104

75

50

30

3.65

277

294

1382

864

46

103

74

49

29

3.70

269

286

1342

764

45

103

73

48

28

3.75

262

279

1312

663

45

102

73

47

27

3.80

255

272

1262

563

44

101

72

46

26

3.80

255

266

1242

462

43

100

72

45

24

3.85

248

260

1222

362

42

99

71

44

23

3.90

241

254

1182

262

42

99

71

43

22

3.95

235

248

1162

161

41

98

70

42

21

4.00

229

243

1132

061

40

97

69

42

20

4.05

223

238

1111

895

4.10

217

230

1071

6*94

4.15

212

222

1021

4*92

4.25

203

213

981

2*90

4.35

192

204

921

0*89

4.40

187

195

908

*87

4.50

179

187

876

*85

4.60

170

180

834

*84

4.65

166

173

792

*82

4.80

156

166

770

*81

4.80

156

160

747

94

.901

491

56737

75

.001

431

50707

45

.101

371

43677

25

.201

311

37657

05

.301

261

3262

Laboratory 2:


Rockw


Brinel

Vickers

Tensile strength

Diamond

1/16" Ball


10 mm Ball,3000 kgf

150 kgfC

60 kgfA

100 kgfD

100 kgfB

15 kgLoad

30 kgLoad

45 kgLoad

Diam. Of Ball

Impress

Hardness


I68

5.40

121

127

606

55

.501

161

22585

.601

121

1756

In summary, hardness testing methods for example Brinell, Rockwell, Vickers and Knoops are practical in measuring mechanical properties of metals and other engineering materials. It is essential for engineers to select an appropriate hardness testing method for the desired applications or materials used. This is depending on size and shape of the test pieces, metallurgical phases and their locations to be analysed. The correct hardness values are beneficial for material selection and design together with material development for higher performance. Moreover, the hardness values can be used for estimating other related mechanical properties of the materials, for example, tensile strength or yield strength.


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2 . Ma ter i a l s an d e qu i p me n t

2.1 Test specimens

2.2 Brinell hardness machine

2.3 Rockwell hardness machine

2.4 Vickers hardness machine

2.5 Micro Vickers hardness machine

3 . Exp er i me n t a l p r o c e du re

3.1 Surfaces of aluminium, brass steel and weld samples must be flattened and ground using sand papers. Polishing of the metal surface is required for only Rockwell and Vickers hardness tests while Brinell hardness test requires only flat and ground surfaces.

3.2 Hardness measurement is carried out using Brinell, Rockwell and Vickers hardness testing techniques on the prepared surfaces at 10 positions on each sample.

3.3 Hardness profile testing is conducted across the weld sample at 10 positions and 1 mm intervals using a Vickers hardness testing machine.

3.4 Micro Vickers hardness testing is carried out using the polished samples.

3.5 Summarize the experimental results on the table provided and exhibit the results graphically. Compare and discuss the obtained results in order to relate hardness properties of the metals to their microstructure. Give conclusions.

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4 . Res u l ts

4.1 Brinell hardness values (BHN)

Position Aluminium

Mild steel Brass

Position 1

Position 2

Position 3

Position 4

Position 5

Position 6

Position 7

Position 8

Position 9

Position 10

Mean

Stdev

Table 2: Brinell hardness values of aluminium, mild steel, brass and weld

Figure 4: Graph showing Brinell hardness values of aluminium, mild steel and brass.

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4.2 Rockwell hardness values (HRA, HRB, HRC)

Position Aluminium

Mild steel Brass

Position 1

Position 2

Position 3

Position 4

Position 5

Position 6

Position 7

Position 8

Position 9

Position 10

Mean

Stdev

Table 3: Rockwell hardness values of aluminium, mild steel, brass and weld

Figure5: Graph showing Rockwell hardness values of aluminium, mild steel and brass.

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4.3 Vickers hardness values

Positi

Aluminiu

Mild steel

Bras

Weld

Position 1

Position 2

Position 3

Position 4

Position 5

Position 6

Position 7

Position 8

Position 9

Position 10

Mean

Stdev

Table 4: Vickers hardness values of aluminium, mild steel, brass and the

weld.

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Figure 6: Graph showing Vickers hardness values of aluminium, mild steel and brass.

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4.4 Micro Vickers hardness (VHN)

Position Aluminium

Mild steel Brass

Position 1

Position 2

Position 3

Position 4

Position 5

Position 6

Position 7

Position 8

Position 9

Position 10

Mean

Stdev

Table 5: Micro Vickers hardness values of aluminium, mild steel, brass and weld

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Figure 7: Graph showing micro Vickers hardness value of aluminium, mild steel and

brass.


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4.5 Hardness profile of welded sample in relevant to the weld microstructure

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5 . D i sc u ss i o n

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6 . C on c l u s i on s

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7 . Q u es t i on s

7.1 Which metal does provide the highest hardness values? Why?

7.2 Explain why the hardness values in the welded area are different from the hardness values obtained in the base metal.

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7.3 Compare Macro Vickers and micro Vickers hardness values obtained from the experimental results.

7.4 Explain the relationship between hardness and tensile strength values.

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8 . Refere n ces

8.1 Dieter, G.E., M ec han i c a l m e t a ll u r gy , 1988, SI metric edition, McGraw-Hill, ISBN 0-07-

100406-8.

8.2 Hashemi, S. Founda ti o n s of m a t e r i a l s sc i e n ce an d e n g i n eer i n g , 2006, 4th edition, McGraw- Hill, ISBN 007-125690-3.

Documents

Hardness_Eng