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ENSC1004 > Mechanical failure of materials chap 4 > Flashcards

Mechanical failure of materials chap 4 Flashcards

1
Q

What are the main types of material failure?

A
  • Plastic yielding
  • fracture
  • fatigue
  • creep
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2
Q

What is material failure?

A

loss of load carrying capacity

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3
Q

What is failure by plastic yielding?

A
  • Yielding is defined as a form of material failure
  • Yielding = onset of plastic deformation
  • materials begins to deviate from a linear behavior between stress and strain
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4
Q

Safety factor in failure by plastic yielding

A
  • Engineers prefer to work with materials below plastic yielding
    to design against plastic yielding divide the yield strength by a safety factor and the the division will equal the acceptable work stress of the material
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5
Q

What is failure by fracture?

A
  • spontaneous breaking of interatomic bonds, fast,
  • happens in load bearing structures like bridges, trucks, pressure vessels, gas pipelines
  • when a tensile stress is applied
  • materials stores energy,
    when the energy stores equals the energy required to fracture and the limit of the bond strength is reached = material fractures
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6
Q

Crack propagation

A
  • when there is a defect in a material, this is an area of stress concentration
  • when tensile stress is applied the crack propagates to where the bonds are still intact
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7
Q

Examples of failure by fracture

A
  • Liberty ships (1946) = lack of understanding of brittle to ductile transitions
  • 1250/4700 brittle fractures
  • 230 of 1250 serious
  • 12 fractures in which they broke into two
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8
Q

Energy criterion of failure

A

E stored = E failure

  • defects reduce energy of fracture
  • Elastic region in stress and strain curve moved to end of curve = energy stored in the material that can be stored to do work to create fracture
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9
Q

Examples of energy criterion of failure

A

Examples:
Pin into fully inflated balloon
Ef (intact) > Es > Ef (defect)

Pin w. partially inflated
Estored > Ef (defect)
- balloon may not pop

Pin then blow up
Estored = Ef

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10
Q

Modes of fracture
crack formation
crack propagtion

A

Brittle fracture

  • catastrpphic
  • ceramics, high strength metals, high strength brasses
  • fracture with little or no plastic defo
  • low energy stored
  • Flat fracture surface morphology (cleavage fracture)

Ductile fracture

  • fails are yield strength
  • reveals itself
  • polymers, soft metals Au and Pb
  • local Plastic deformation before crack propagates
  • high energy stored
  • cup and cone surface morphology (dimple texture)
  • slope (45 degree)

Mixed fracture

  • carbon steels and engineering alloys
  • bit of necking followed by brittle failure
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11
Q

Steps for brittle failure

A
  • low applied force = uniform stress
  • high enough applied force = cracks formed = crack tips stress concentrations with stress higher than initial constant stress
  • crack propagates = failure
  • stress maximum = stress at failure
  • stress at fracture is a measure of a material’s ability to resist breakage of interatomic bonds
  • theoretical strength = Young’s modulus divided by 10

Summary: crack spreads rapidly even without more applied stress, deemed unstable

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12
Q

Steps for ductile failure

A
  • Start loading = Necking
  • At crack tips = stress concentrations limited at the yield strength by local plastic deformation in front of crack tip –> creation of microvoids
  • Microvoids expands and merge to advance crack tip

Summary: a lot of plastic deformation at crack site, propagates slowly so crack is deemed stable,

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13
Q

Brittle to Ductile transition

A

Material can change behaviour based on temp and sometimes these changes are irreversible
Eg steel = brittle at low temp
PVC = high temps = brittle

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14
Q

Ductile vs brittle failure

A

Ductile

  • via plastic deformation
  • yield strength of metals decreases as temp increases
  • because energy increases –> easier to break bonds/ dislocation slips easier–> yield strength decreases

Brittle

  • via cleavage of interatomic bonds
  • fracture strength required for cleavage insensitive of temps - once material is brittle not much change occurs after that
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15
Q

Ductile Brittle Transition temperature (DBTT)

A
  • Material with DBTT ductile at high temp and brittle at low temps
  • temp at which behaviour changes = DBTT
  • materials with low strength ( FCC= pure metals, Cu, Ni ) = yield strength is less than fracture strength = always fracture via ductile failure
  • materials with high strength (BCC = steel alloys, HCP = Zn alloy) = yield strength is greater than fracture strength = fail in brittle manner
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16
Q

Charpy test

A

Measures DBTT
Impact vs temp curve
mgH - mgh

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17
Q

Stress near singularities

A

tips of cracks in materials= stress risers
magnitude of stress-dependent on the geometry of crack and material
For an elliptical-shaped crack = stress max = initial uniform stress multiplied by 1 +2 times (crack size divided by the crack radius of curvature ) to a half
- larger crack = higher stress concentration
- smaller radius of curvature - sharper tip = higher stress

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18
Q

Stress intensity factor

A
  • stress alone not valid to predict fracture failure of materials
    K = (Y)(theta)Square root(pi * a)
19
Q

What is simple fracture?

A

separation of a body into two or more pieces in response to an imposed stress that is static

20
Q

Classification of type of fracture is based on…

A

Ability of material to experience plastic deformation

21
Q

What type of applied stresses result in fracture

A
  • compressive
  • tensile - what we focus on in this unit!!
  • shear
  • torsion
22
Q

Fracture toughness

A

Increase load K increases when K is at a critical value = Kc = fracture toughness

23
Q

Mechanical criterion for brittle fracture

A

K >= Kc

24
Q

Three modes of fracture propagation

A
  • Plain strain fracture or Normal tensile mode Kic
  • Sliding mode Kiic
  • Tearing mode Kiiic
25
Q

Comparing Kc

A

Metals - high
Ceramics - Mid
Polymers - Low

26
Q

Designing with Kc

A
  • designing against brittle fracture and yielding
  • K <= Kc
  • Maximum crack size tolerance
27
Q

Designing against fracture

A
  • determined by applied stress and pre-existing cracks captured in stress intensity factor (K)
  • K = (Y)(theta)Square root(pi * a)
  • Kic = fracture toughness = given value
  • Failure criterion K= Kic or (Y)(theta)Square root(pi * a) = Kic
28
Q

L15 What is fatigue

A
  • failure by a material after cyclic stresses
  • cycles of fluctuating stress
  • 90% of failures
  • striations
29
Q

Characteristics of fatigue

A
  • occurs at lower stresses than the tensile yield strength for static loads
  • occurs after a lengthy period of time of repeated stresses and strain cycling
  • brittle manner even in ductile materials
  • Catastrophic - sudden and without warning
30
Q

Low cycle fatigue failure

A

< 10^4 -10^5

  • high loads - elastic and plastic deformations
  • limited lifetime of important components
31
Q

High cycle fatigue failure

A

> (greater)10^4 - 10^5 cycles
low loads - elastic deformation
common - despite designing the material below yield strength continual unloading and loading can lead to failure

32
Q

How fatigue occurs

A
  1. Crack initiation
    - at surface
    - flaw/ scratch
    - defects on surface
  2. Crack growth
    - incremental growth of crack after every cycle
    - beach mark - mm - macroscopic - each band is a display of growth over multiple cycles
    - striations - microscopic - advance of a crack after a loading cycle
  3. Final failure
    - area is insufficient to hold the load
    - can be brittle or ductile
    - surface texture points to origin of crack
33
Q

What do beach marks look like

A
34
Q

What do striations look like

A
35
Q

Applied stress may be

A
  • Axial (tension/compression)
  • Flexural (bending)
  • Torsional (twisting)
    cyclic with fluctuating stress modes
36
Q

Reversed cycle

A
  • fixed frequency
  • amplitude is symmetrical about mean stress zero
  • Stress max = - stress min
    stress mean = 0
    range of stress = - 1
37
Q

Repeated cycle

A
  • sinusoidal time dependency
  • fixed frequency
  • max and min are asymmetric
38
Q

Random

A

non-sinusodial

- frequency and amplitude are random

39
Q

Cyclic stress parameters (Stress)

A

Mean stress = 1/2(stress max + stress min)
Range of stress = stress max - stress min
Stress amplitude = 1/2 range of stress
Stress ratio = stress min/ stress max

40
Q

SN curves

A
  • used to predict fatigue life
  • ## displays stress amplitude versus no of cycles to fatigue failure
41
Q

Materials with fatigue limit

A

Materials with fatigue limit = ferrous metals + Ti alloys - Stress level below which material will never fail by fatigue - above which stress increases and no of cycles to failure decreases
- for materials with fatigue limit = fatigue strength = stress at which material will survive 10^7 fatigue cycles

42
Q

Materials without fatigue limit

A
  • non- ferrous metals
  • Al, Cu, Mg
    Fatigue life (no of cycles to failure at a particular stress) increases and lowered stress amplitude
43
Q

Factors that affect fatigue failure

draw diagrams

A

a) Mean stress - as mean stress increases fatigue life decreases
Materials with lowest mean stress - highest possible stress amplitude
b) Stress raisers decrease fatigue life
- notches
- grooves
- holes
- shaper discontinuity = more severe stress concentration

44
Q

Improving fatigue life

A
  • Lower mean stress
  • reduce stress raisers
  • impose surface residual compressive stresses by shot peening
  • improve surface harness - rough, fine grinding, honing, polishing (most expensive)

ENSC1004 (7 decks)

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