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ENGMP211 > Fracture > Flashcards

Fracture Flashcards

1
Q

What is Fracture?

A

The separation or fragmentation of a solid body into 2 or more parts under the action of stress

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2
Q

What are the main fracture modes?

A

Ductile and Brittle fracture.

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3
Q

What is Fatigue?

A

failure due to repeated stress/strain

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4
Q

What is Creep?

A

time dependent strain under static load (often at high temperature)

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5
Q

What is propagation?

A

To increase the extent of a structural flaw.

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6
Q

What are the macroscopic characteristics of ductile fracture?

A

Significant plastic deformation, energy absorption and slow “stable” crack growth.

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7
Q

What are the macroscopic characteristics of brittle fracture?

A

Very little plastic deformation, unstable propagation, and failure at stresses lower than the yield stress.

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8
Q

What are Factors that affect facture modes?

A

Temperature, Strain rate and stress state.

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9
Q

Explain the shear fracture process of a ductile material.

A

The is a formation of micro voids at the centre of the specimen then the is propagation of a crack by shear deformation at 45 degrees.

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10
Q

What is the difference between transgranular and intergranular?

A

Transgranular is the fracture through grain while intergranular is the fracture along grain boundaries.

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11
Q

What is the general form of fracture in ceramics?

A

Most ceramics exhibit brittle transgranular fracture.

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12
Q

What is the general form of fracture in polymers?

A

Polymers exhibit a very wide range of fracture behaviour.

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13
Q

What are the stress forms that can cause fatigue failure?

A

Axial (compression/tension), Flexural (bending), Torsion (twisting/rotation)

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14
Q

What is the difference between a material with a fatigue limit and one with no fatigue limit?

A

For material with a fatigue limit below a certain stress, no failure will occur but for materials with no fatigue limit the material will eventually fail at any repeated stress level.

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15
Q

What are the main regions of a fracture surface?

A

Initiation site (a point where the stress is concentrated, Fatigue zone (area resulting from the slow crack propagation), and Rupture zone (area of final failure).

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16
Q

How can we protect against fatigue?

A

Intelligent design, reduce the mean stress, surface finishing, case hardening.

17
Q

How do we define cracks is materials?

A

Crack are defined as elliptical with a tip radius given by pt = b^2/a

18
Q

What is the change is elastic energy?

A

A loaded material with store potential energy as elastic strain, the formation of a crack will unload certain volumes of material and lower elastic strain. Given by U.

19
Q

What is S (increase in surface energy)?

A

Is the energy needed to transform a uncracked system into a higher energy cracked system.

20
Q

How is the change in energy in the system calculated?

A

Change in W = U + S

21
Q

What is the difference between stable crack growth and unstable crack growth?

A

Stable crack growth requires extra external input to progress while in unstable crack growth the crack will grow spontaneously and often rapidly.

22
Q

What is yp?

A

The energy of plastic work per unit of surface area created in the crack.

23
Q

What is Gc and why do we use it over yp?

A

Gc is the critical strain energy release rate and we use it because it is easier to determine.

24
Q

What are the limitation of Gc?

A

Gc is found experimentally and is based on geometry so does not give a generalised result for a material which could be applied during design.

25
Q

What is KC?

A

Is the critical fracture toughness, the measure of resistance to unstable crack propagation.

26
Q

What is Mode I?

A

Is the tensile or opening mode and is considered the worst case.

27
Q

How is KC related to Gc?

A

K_C=√(G_c E’)

28
Q

What is KIC?

A

The plane strain fracture toughness, which is the resistance to brittle fracture.

29
Q

How is the fast fracture of pressure vessels avoided?

A

Ensure the wall thickness (t) is less than the critical crack size (a or 2a depending on crack type).
This is so that there will be leakage and a pressure loss in the vessel before failure.

30
Q

What are the stages of fatigue after initiation?

A

1) Slow crack growth
2) Crack changing directions cause striation and benchmarks
3) Striations give a way to tear dimples
4) Fracture

31
Q

What is the relationship between the microscopic and macroscopic modes of fracture?

A

Transgranuler and intergranular are related to brittle fracture and microvoid coalesce is related to ductile fracture.

ENGMP211 (9 decks)

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