2mech_propb

7/28/2019 2mech_propb

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Resistance to penetration

or localized plastic deformation

Hardness

Commonly measured by:

Rockwell or Brinell test

Hardness

IndenterRockwell test

Rockwell test

Hardness

Indenters: conical diamond or hardened steel balls

(1/16, 1/8, 1/4 or inch dia)

Depth

Brale

Depth

Ball

F F

Hardness number: from the difference in depth

of penetration resulting from the application of

an initial minor load (10 kg) followed by a larger

major load (60, 100 or 150 kg)

Hardness

Rockwell test

Depth

Brale

Depth

Ball

F F


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Indenter: hardened steel or tungsten carbide ball

10 mm dia

Load: 500 to 3000 kg in increments of 500 kg

Hardness

Diameter

F

Brinell test

Hardness is related to load and diameter of indentation

The Brinell hardness number is designated as HB

Hardness

Diameter

F

Brinell test

Differences between Rockwell and Brinell tests

Hardness

Indenters: a conical diamond in Rockwell

for hard materials

Hardness related to

depth of penetration In Rockwell

width of indentation in Brinell

Brinell test uses much higher loads than Rockwell

Hardness

Knoop and Vickers microhardness tests

Lower loads and smaller indenter size

Indenter (both tests):a very small pyramidal diamond

Load: between 1 and 1000 gmuch lower than Rockwell and Brinell tests


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Hardness

They measure the hardness of small specimens

Knoop is used for brittle materials like ceramics

Knoop and Vickers hardness numbers: HK and HV

Knoop and Vickers microhardness tests

Tensile Strength versus Hardness

Both indicate resistance to plastic deformation

Relation between Tensile Strength (TS) and

Brinell Hardness (HB) for steels: TS (psi) = 500 HB

0

50

100

150

200

250

300

0 100 200 300 400 500 600

Brinell Hardness

350650Fractures before yieldingE

2107200.14850700D

3105000.15550415C

1501050.40120100B

2102650.23340310A

Elastic

Modulus

1000 MPa

Fracture

Strength

MPa

Strain at

Fracture

Tensile

Strength

MPa

Yield

Strength

MPa

Alloy

Which of these materials:

(a) is the hardest?

(b) will experience the greatest % reduction in area?

(c) is the strongest?

(d) is the stiffest?

(e) has the highest modulus of resilience?

Explain why.

(a) Which of these materials is the hardest? Why?

-- Alloy D is the hardest

because it has the highest yield strength


2107200.14850700D

3105000.15550415C

1501050.40120100B

2102650.23340310A

Elastic

Modulus

1000 MPa

Fracture

Strength

MPa

Strain at

Fracture

Tensile

Strength

MPa

Yield

Strength

MPa

Alloy


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(b) Which of these materials will experience the greatest

% reduction in area? Why?

- Alloy B. It has the highest strain at fracture, and,

therefore is most ductile.


2107200.14850700D

3105000.15550415C

1501050.40120100B

2102650.23340310A

Elastic

Modulus1000 MPa

Fracture

StrengthMPa

Strain at

Fracture

Tensile

StrengthMPa

Yield

StrengthMPa

Alloy

(c) Which of these materials is the strongest? Why?

- Alloy D is the strongest as it has the highest yield strength.


2107200.14850700D

3105000.15550415C

1501050.40120100B

2102650.23340310A

Elastic

Modulus1000 MPa

Fracture

StrengthMPa

Strain at

Fracture

Tensile

StrengthMPa

Yield

StrengthMPa

Alloy

(d) Which of these materials is the stiffest? Why?

- Alloy E is the stiffest as it has the highest elastic modulus.


2107200.14850700D

3105000.15550415C

1501050.40120100B

2102650.23340310A

Elastic

Modulus

1000 MPa

Fracture

Strength

MPa

Strain at

Fracture

Tensile

Strength

MPa

Yield

Strength

MPa

Alloy

(e) Which of these materials has the highest modulus ofresilience? Why?

- Modulus of resilience = 0.5y2/E

Alloy D has the highest value.


2107200.14850700D

3105000.15550415C

1501050.40120100B

2102650.23340310A

Elastic

Modulus

1000 MPa

Fracture

Strength

MPa

Strain at

Fracture

Tensile

Strength

MPa

Yield

Strength

MPa

Alloy


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Stress calculations are approximate

Safety Factor

Design allowances must be made to protect against

unanticipated failure

Need to establish a safe stress or a working stress

Working stress = Yield strength/Factor of safety

w = y/N

Safety Factor

Factor of safety: 1.2 to 4.0 (normal range)

Based on consequences of failure, economics,

previous experience and the accuracy of theproperty data

Impact Test

What if a material is subjected to a sudden,

intense blow?

It would behave in a much more brittle mannerthan observed in the tensile test

This behavior is tested by impact test (Charpy test)

Impact Test


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Impact Test Impact Test

hf

ho

Impact Test

Impact enegry needed for fracture

is measured from h (i.e. ho-hf)

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