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C h a p t e r 8
Failure
Dr. Mohammad Abuhaiba, PE

2. 8.6 Fracture Toughness Testing
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8.6 Fracture Toughness Testing
Impact test is often used to evaluate brittleness of a material under conditions of high strain rates (103 s-1).
Izod test is often used for plastic materials.
From amount of swing of pendulum, energy dissipated in breaking the specimen can be obtained.
This energy is the impact toughness of material.
Units of Charpy: J and Izod: J/m.
Impact Toughness: ability of a material to withstand an impact blow.
Fracture Toughness: ability of a material containing flaws to withstand applied load.
Dr. Mohammad Abuhaiba, PE

3. 8.6 Fracture Toughness Testing
1/18/2019 6:28 AM
8.6 Fracture Toughness Testing
Dr. Mohammad Abuhaiba, PE

4. 8.6 Fracture Toughness Testing
1/18/2019 6:28 AM
8.6 Fracture Toughness Testing
Dr. Mohammad Abuhaiba, PE

5. 1/18/2019 6:28 AM
8.6 Fracture Toughness Testing Ductile to Brittle Transition Temperature
DBTT: temperature at which a material changes from ductile to brittle fracture.
BCC metals have transition temperatures, but most FCC metals do not.
FCC metals have high absorbed energies, with the energy decreasing gradually as the temperature decreases.
Notch sensitivity: absorbed energies are much lower in notched specimens if the material is notch sensitive.
Dr. Mohammad Abuhaiba, PE

6. 1/18/2019 6:28 AM
8.6 Fracture Toughness Testing Ductile to Brittle Transition Temperature
Dr. Mohammad Abuhaiba, PE

7. 1/18/2019 6:28 AM
8.6 Fracture Toughness Testing Ductile to Brittle Transition Temperature
Relationship to stress-strain diagram:
Energy required to break a material during impact testing is not always related to tensile toughness.
Materials that have high impact resistance are generally those that have high strength and high ductility, and hence, high toughness.
Metals that show excellent tensile toughness may show a brittle behavior under high strain rates.
Dr. Mohammad Abuhaiba, PE

8. 1/18/2019 6:28 AM
8.6 Fracture Toughness Testing Ductile to Brittle Transition Temperature
Dr. Mohammad Abuhaiba, PE

9. 1/18/2019 6:28 AM
8.7 Cyclic Stresses
Dr. Mohammad Abuhaiba, PE

10. 8.8 THE S–N CURVE Rotating Cantilever Beam Test
Test is carried out at various stress amplitudes (Sa).
Stress amplitude is defined as max stress.
Number of cycles (N) it takes to cause total failure of specimen or part is recorded.
These curves are based on complete reversal of stress.
Dr. Mohammad Abuhaiba, PE

11. 8.8 THE S–N CURVE Rotating Cantilever Beam Test
Endurance limit or fatigue limit: Max stress to which material can be subjected without fatigue failure, regardless of # of cycles.
Fatigue life: tells us how long a component survives at a particular stress.
Dr. Mohammad Abuhaiba, PE

12. 8.8 THE S–N CURVE Rotating Cantilever Beam Test
Fatigue strength: max stress for which fatigue will not occur within a particular number of cycles.
For carbon steels, endurance limit is usually UTS.
Most materials are notch sensitive.
Shot peening
Dr. Mohammad Abuhaiba, PE

13. 8.9 Crack Initiation and Propagation
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8.9 Crack Initiation and Propagation
Fatigue failures typically occur in 3 stages:
Crack initiation at surface.
Crack propagation as load continues to cycle.
Sudden fracture.
In polymers, as the material is subjected to repetitive stresses, considerable heating can occur near crack tips and the inter-relationships between fatigue and creep affect the overall behavior.
Dr. Mohammad Abuhaiba, PE

14. 8.9 Crack Initiation and Propagation
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8.9 Crack Initiation and Propagation
Dr. Mohammad Abuhaiba, PE

15. 8.9 Crack Initiation and Propagation
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8.9 Crack Initiation and Propagation
Dr. Mohammad Abuhaiba, PE

16. 8.9 Crack Initiation and Propagation
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8.9 Crack Initiation and Propagation
Dr. Mohammad Abuhaiba, PE

17. 8.12 Generalized Creep Behavior
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8.12 Generalized Creep Behavior
Permanent elongation of a component under a static load maintained for a period of time at a high temperature.
Essentially, in creep the material begins to flow slowly.
Diffusion, DL glide or climb, or grain boundary sliding can contribute to the creep of metallic materials.
When a material does actually creep and then ultimately break the fracture is defined as stress rupture.
Normally, ductile stress-rupture fractures include necking & presence of many cracks that did not have the opportunity to produce final fracture.
Dr. Mohammad Abuhaiba, PE

18. 8.12 Generalized Creep Behavior
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8.12 Generalized Creep Behavior
Grains near the fracture surface tend to be elongated.
Ductile stress rupture failures generally occur at high creep rates and relatively low exposure temperatures and have short rupture times.
Brittle stress rupture failures show little necking and occur more often at smaller creep rates and high temperatures. Equiaxed grains are observed near the fracture surface.
Brittle fracture typically occurs by formation of voids at intersection of 3 grain boundaries and precipitation of additional voids along grain boundaries by diffusion processes.
Dr. Mohammad Abuhaiba, PE

19. 8.12 Generalized Creep Behavior
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8.12 Generalized Creep Behavior
Dr. Mohammad Abuhaiba, PE

20. 8.12 Generalized Creep Behavior
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8.12 Generalized Creep Behavior
Creep Rates and Rupture times
In 1st stage of creep of metals, many DLs climb away from obstacles, slip, and contribute to deformation.
Eventually, rate at which DLs climb away from obstacles equals rate at which DLs are blocked by other imperfections. This leads to 2nd stage. The slope of the steady portion is the creep rate.
Eventually, during 3rd stage, necking begins, & stress increases, & specimen deforms at an accelerated rate until failure occurs.
Rupture time: time required for failure to occur.
Either a higher temperature or a higher stress reduces the rupture time and increases the creep rate.
Dr. Mohammad Abuhaiba, PE

21. 8.13 Stress and Temperature Effects
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8.13 Stress and Temperature Effects
Dr. Mohammad Abuhaiba, PE

22. 12, 18, 27, 28 Home Work Assignment Due Wednesdays 28/12/2011
1/18/2019 6:28 AM
Home Work Assignment
12, 18, 27, 28
Due Wednesdays 28/12/2011
Dr. Mohammad Abuhaiba, PE