MTM

1. Chapter 8: Failure
Aloha
Airlines
flight
243
Fractured
Oil
Tanker
Intentional Incision in plastic

2. Concept Check 8.1
Cite two situations in which the possibility of failure is part of
the design of a component or product.
Answer: Several situations in which the possibility of failure is
part of the design of a component or product are as follows:
1.The pull tab on the top of aluminum beverage cans
2.Aluminum utility/light poles that reside along freeways—a
minimum of damage occurs to a vehicle when it collides with
the pole.
3.In some machinery components, a shear pin is used to
connect a gear or pulley to a shaft—the pin is designed shear
off before damage is done to either the shaft or gear in an
overload situation

3. Topics to be covered
Simple fracture
– Ductile
– Brittle
Fundamentals of fracture mechanics
Fracture toughness testing
Ductile-to-brittle transition
Fatigue (when cyclic stresses are imposed)
Creep (time-dependent deformation, normally at elevated temperatures)

4. Chapter 8: Failure
Cavities, Cracks, Voids, Notches, Surface Scratches, Corners

8. Tougher absorbing
energy

9. Very
ductile
Very
Brittle
Cleavage
planes

10. a. Initial necking
b. Cavity formation
c. Coalescence of cavities to form crack
d. Crack propagation (slip)
e. Final shear fracture at 45° relative to tensile direction
Mechanism of ductile Fracture

11. Tensile loading Shear loading
Ductile Fracture

13. Brittle Fracture: SEM is used to study surface
V – Shaped chevrons
Fan-like markings
Ceramics will have shiny surface

14. Ductile Brittle
deformation extensive little
Crack propagation slow, needs stress fast
type of materials most metals (not too cold) ceramics, ice, cold metals
warning permanent elongation none
strain energy higher lower
fractured surface rough smoother
necking yes no

17. Trans-granular - faceted Inter- granular ( weakened boundaries )

18. Fracture Mechanics:
Quantification of relationship between material properties,
stress level, crack propagating flaws, propagating mechanism
Fracture strength for most materials is lower than theoretical one

19. Stress
Raisers
Much smaller forces are required for fracture. Why

20. Amplification will depend on crack orientation & Geometry
Cracks, Voids, sharp corners, notches, scratches amplify stresses

21. Radius of curvature
Stress concentration factor

22. Stress required for crack propagation in brittle materials
All brittle materials contain cracks & flaws of various sizes,
geometries, orientation. When critical stress is reached,
crack starts propagating.
Stress-raiser-free whiskers can be made

23. Example problem 8.1:
Determine the maximum flaw size at which fracture occurs
Material = Large Glass sheet
Tensile Stress = 40 MPa
Specific surface Energy = 0.3 J/ m2
Modulus of Elasticity = 69 GPa
a = 8.2 µm

24. Fracture Toughness:
Critical stress (σc ) related to length of crack (a)
: definition
Kc depends on length of crack Vs thickness of material that cracks

25. Y = 1.0 Y = 1.1
Crack in a plate
of infinite
width
Crack ( notch ) in a plate
of semi-infinite width
Fracture
toughness and
value of Y
( plane strain condition )

26. Plane strain condition:
1.No deformation in
Z direction
2. Thickness B much
larger than notch
depth a

27. No strain component
normal to front and back
face.
Under Plane Strain condition i.e
Rather
than
Plane strain fracture toughness

28. Design Using Fracture Mechanics
KIC depends on Temperature, loading rate, microstructure,
etc.

29. Fracture Mechanics and
→ Possibility of Fracture

30. Note: σy , TS, and %EL are all functions of loading rate

31. 1) Impact energy determination as function of T
2 ) Range of temperature over which ductile-to-brittle
transition takes place
Note: Both KIC & Impact tests determine the fracture
properties of materials

33. Specimen for both Izod and Charpy ( mostly in US )

34. Most Important Objective of CVN (Charpy V-Notch) testing:
Finding ductile-to-brittle transition Temperature
For steel
Ignore this graph
brittle
ductile

35. Most Important Objective of CVN (Charpy V-Notch) testing:
Finding ductile-to-brittle transition Temperature
Shiny and cleavage = Brittle Fibrous/dull = Ductile

36. Ductile materials require
more energy to fracture
Ductile materials become
brittle as temperature is lowered

37. Low strength (remain ductile, large E)
(remain brittle, small E )
low strength steel

38. Increasing carbon contents in steel
Note: Grain size also affects DBTT

39. Fatigue
( fracture when stress-strain fluctuates in time)
Stress may be Axial, Flexural or Torsional
1. occurs usually at lower applied stress
2. responsible for ~ 90 % damage
3. is brittle-like
4. called fatigue because each cycle incrementally adds
to the damage (to the strain, crack growth)
5. slow moving crack which rapidly picks up speed

40. Laboratory Fatigue Tester
Laboratory Bicycle Handle Tester
(Various loads at once)
Shoe Fatigue Tester Airplane Fatigue Tester

41. TS or σy

42. mean
Range
amplitude

46. σa stress ( S ) is applied, usually 2 / 3 of static tensile strength
Can be Tensile-compression also

48. https://www.youtube.com/watch?v=LhUclxBUV_E
Demonstration of Fatigue Testing

49. Close to 0.5 of TS

50. cycles fixed ( what stress value, strength )
Stress fixed ( how many cycles, life )
Fig. Materials which do not show fatigue limit.

51. or fatigue limit
for many
steels 35 –
60%
Of TS
ferrous
nonferrous

56. Nucleation

58. Factors Affecting Fatigue:
1.Mean Stress
2.Surface Effects
3.Design Factors
4.Surface Treatments
5.Environmental Factors (temperature and Corrosion)

59. ( no. of cycles to fracture at certain stress )

60. How do glaciers move?
What limits the life of turbine blades in a jet engine?
Why do Tungsten Filament in Bulb fuse/melt?
These all are due to a phenomenon called
Creep!

61. Primary Creep:
 Starts at a rapid rate and slows with time.
Slowing indicate strain hardening
Secondary Creep:
It has a relatively uniform rate.
Tertiary Creep:
It has an accelerated creep rate and terminates
when the material breaks or ruptures. It is associated with
both necking and formation of grain boundary voids.

62. Creep ( Deformation )
( time dependent permanent deformation which occurs when materials are
exposed to static loads at high temperatures ( 0.4Tm )
for long times
Stress ( usually axial ) is kept constant
2. strain hardening
3. strain hardening – recovery competing
( grain
boundary
separation,
cracks, flaws )
1. Elastic
4.

64. Effect of Stress
Effect of Temperature

68. Empirical relationships have been developed in which
the steady-state creep rate as a function of stress and
temperature:
When the influence of temperature is included the
relation becomes:

70. The need often arises for engineering creep data that are
impractical to collect from normal laboratory tests.
This is especially true for prolonged exposures (on the
order of years).
One solution to this problem involves performing creep
and/or creep rupture tests at temperatures in excess of
those required, for shorter time periods, and at a
comparable stress level, and then making a suitable
extrapolation to the in-service condition. A commonly
used extrapolation procedure employs the Larson–Miller
parameter, m, defined as:

73. Factors Affecting Creep:
Temperature
Stress
Melting Point
Elastic Modulus
Grain Size
Greater the melting point, higher the elastic modulus and larger
the grain size of a material, the higher will be its creep life.
Stainless Steel and Super Alloys are especially resilient to creep.
Directional solidification and solid-solution alloying enhances the
creep life of a material.