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Transcript of Chapter 7: 7 - 1 features of fracture in metallic materials 4. Microstructural features of fracture...
Slide 1
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 1
Chapter 7:
Mechanical Properties:
Part Two
Chapter 7: Mechanical Properties: Part Two
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Slide 2
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 2
Learning Objectives
1. Fracture mechanics
2. The importance of fracture mechanics
3. Microstructural features of fracture in metallic materials
4. Microstructural features of fracture in ceramics, glasses, and composites
5. Weibull statistics for failure strength analysis
6. Fatigue
7. Results of the fatigue test
Chapter 7: Mechanical Properties: Part Two
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Slide 3
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 3
Learning Objectives
8. Application of fatigue testing
9. Creep, stress rupture, and stress corrosion
10. Evaluation of creep behavior
11. Use of creep data
Chapter 7: Mechanical Properties: Part Two
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Slide 4
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 4
Fracture Mechanics
Fracture mechanics: Discipline concerned with the behavior of materials containing cracks or other small flaws.
Fracture toughness: Measures the ability of a material containing a flaw to withstand an applied load.
The stress applied to the material is intensified at the flaw, which acts as a stress raiser. For a simple case, the stress intensity factor K is given by:
wheref geometry factor for the specimen and flaw the applied stress
a flaw size
Chapter 7: Mechanical Properties: Part Two
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Slide 5
© 2011 Cengage Learning Engineering. All Rights Reserved. 7 - 5
Figure 7.1 - Fracture Mechanics
Chapter 7: Mechanical Properties: Part Two
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Slide 6
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 6
The Importance of Fracture Mechanics
Selection of a material
If the flaw size a and the magnitude of the stresses are known, we can select a material of fracture toughness Kc or KIc to prevent the flaw from growing.
Design of a component
If the maximum size of any flaw and the material is known (and therefore its Kc or KIc has already been selected), the maximum stress that can be supported by the component can be calculated.
Design of a manufacturing or testing method
If the material has been selected, the applied stress is known, and the size of the component is fixed, the maximum size of a tolerable flaw can be calculated.
Chapter 7: Mechanical Properties: Part Two
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Slide 7
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 7
Figure 7.3
Chapter 7: Mechanical Properties: Part Two
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Slide 8
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 8
The Importance of Fracture Mechanics
Brittle fracture Any crack or imperfection limits the ability of a ceramic to
withstand a tensile stress. This is because a crack (sometimes called a Griffith flaw) concentrates and magnifies the applied stress.
wherea length of a surface crackr crack radius
wherea length of a surface crack surface energy
Chapter 7: Mechanical Properties: Part Two
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Slide 9
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 9
Figure 7.5
Chapter 7: Mechanical Properties: Part Two
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Slide 10
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 10
Chapter 7: Mechanical Properties: Part Two
Figure 7.10
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Slide 11
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 11
Chapter 7: Mechanical Properties: Part Two
Figure 7.11
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Slide 12
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 12
Figure 7.12
Chapter 7: Mechanical Properties: Part Two
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Slide 13
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 13
Weibull Statistics for Failure StrengthAnalysis
Chapter 7: Mechanical Properties: Part Two
Fatigue Lowering of strength or failure of a material due to repetitive stress that may be above or below the yield strength
Probability that the material will not fail under a applied stress
Probability of failure F(Vo) = 1 – P(Vo)
Weibull modulus (m) Measure of the variability of the strength of the material
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Slide 14
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 14
Figure 7.16
Chapter 7: Mechanical Properties: Part Two
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Slide 15
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 15
Figure 7.17
Chapter 7: Mechanical Properties: Part Two
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Slide 16
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 16
Figure 7.18 - Geometry for the Rotating Cantilever Beam Specimen Setup
Chapter 7: Mechanical Properties: Part Two
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Slide 17
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 17
Figure 7.19 - The Stress-Number of Cycles to Failure (S-N) Curves for a Tool Steel and an
Aluminum Alloy
Chapter 7: Mechanical Properties: Part Two
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Slide 18
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 18
Application of Fatigue Testing
Chapter 7: Mechanical Properties: Part Two
Mean stress
Stress amplitude
Goodman relationship
fs desired fatigue strength for zero mean stress
UTS tensile strength of the material
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Slide 19
© 2011 Cengage Learning Engineering. All Rights Reserved. 7 - 19
Figure 7.21
Chapter 7: Mechanical Properties: Part Two
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Slide 20
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 20
Application of Fatigue Testing
Chapter 7: Mechanical Properties: Part Two
Stress intensity factor
Number of cycles required for fracture to occur:
ai initial flaw sizeac flaw size required for fracture
Effects of temperature As the material’s temperature increases, both fatigue life and
endurance limit decrease.
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Slide 21
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 21
Creep, Stress Rupture, and StressCorrosion
Creep: Time dependent permanent deformation under a constant load or constant stress and at high temperatures.
Stress corrosion
Phenomenon in which materials react with corrosive chemicals in the environment, leading to the formation of cracks and lowering of strength.
Tempering produces an overall compressive stress on the surface of glass.
Chapter 7: Mechanical Properties: Part Two
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Slide 22
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 22
Figure 7.24
Chapter 7: Mechanical Properties: Part Two
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Slide 23
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 23
Figure 7.25
Chapter 7: Mechanical Properties: Part Two
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Slide 24
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 24
Figure 7.26
Chapter 7: Mechanical Properties: Part Two
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Slide 25
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 25
Evaluation of Creep Behavior
Chapter 7: Mechanical Properties: Part Two
Creep rate = strain
time
Combined influence of applied stress and temperature on the creep rate and rupture time (tr) follows an Arrhenius relationship
where
R gas constant
T temperature in kelvin
C, K, n, and m constants for the material
Qc activation energy for creep
Qr activation energy for rupture
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Slide 26
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 26
Figure 7.27 - Results From a Series of Creep Tests
Chapter 7: Mechanical Properties: Part Two
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Slide 27
© 2011 Cengage Learning Engineering. All Rights Reserved.7 - 27
Key Terms
Fracture mechanics
Fracture toughness
Griffith flaw
Transgranular
Microvoids
Intergranular
Chevron pattern
Delamination
Weibull distribution
Weibull modulus (m)
Beach or clamshell marks
Striations
Rotating cantilever beam test
Wöhler curve (S-N curve)
Endurance limit
Fatigue life
Fatigue strength
Shot peening
Tempering
Creep
Creep test
Creep rate
Rupture time
Stress-rupture curve
Chapter 7: Mechanical Properties: Part Two
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