Chapter 7: 7 - 1 features of fracture in metallic materials 4. Microstructural features of fracture in ceramics, ... Figure 7.1 - Fracture Mechanics Chapter 7:

$Page 1: Chapter 7: 7 - 1 features of fracture in metallic materials 4. Microstructural features of fracture in ceramics, ... Figure 7.1 - Fracture Mechanics Chapter 7:$
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


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


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Slide 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


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Slide 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


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Slide 5

© 2011 Cengage Learning Engineering. All Rights Reserved. 7 - 5

Figure 7.1 - Fracture Mechanics


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Slide 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.


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Slide 7


Figure 7.3


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Slide 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


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Slide 9


Figure 7.5


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Slide 10



Figure 7.10

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Slide 11



Figure 7.11

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Slide 12


Figure 7.12


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Slide 13


Weibull Statistics for Failure StrengthAnalysis


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


Figure 7.16


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Slide 15


Figure 7.17


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Slide 16


Figure 7.18 - Geometry for the Rotating Cantilever Beam Specimen Setup


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Slide 17


Figure 7.19 - The Stress-Number of Cycles to Failure (S-N) Curves for a Tool Steel and an

Aluminum Alloy


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Slide 18


Application of Fatigue Testing


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


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Slide 20


Application of Fatigue Testing


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


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.


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Slide 22


Figure 7.24


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Slide 23


Figure 7.25


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Slide 24


Figure 7.26


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Slide 25


Evaluation of Creep Behavior


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


Figure 7.27 - Results From a Series of Creep Tests


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Slide 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


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Documents

Chapter 7: 7 - 1 features of fracture in metallic materials 4. Microstructural features of fracture in ceramics, ... Figure 7.1 - Fracture Mechanics Chapter 7: