Metallurgy & Material science, UNIT-2, Sreenidhi, SNIST,K.srinivasulureddy, ductile & brittle fracture, creep,fatigue

1. TYPES OF FRACTURE

2. Brittle vs. Ductile Fracture
• Ductile materials - extensive plastic deformation and energy
absorption (“toughness”) before fracture
• Brittle materials - little plastic deformation and low energy
absorption before fracture.
Fracture: Refers to the failure of a material under load by
breaking into two or more pieces. Fracture can occur under
all service conditions.
➢ Materials subjected to alternating or cyclic loading (as in
machines) fail due to fatigue(fatigue fracture)
➢ Materials used at high temperatures can fail due to creep
fracture

3. 3
Fracture mechanisms
• Ductile fracture
– Occurs with plastic deformation
• Brittle fracture
– Little or no plastic deformation
– Catastrophic
Ductile Brittle

4. Ductile, Creep
Brittle
Cracks choose the path of least resistance
Brittle fractures display either transgranular or intergranular fracture

8. A. Very ductile, soft metals (e.g. Pb(lead), Au(Aurum-Gold) at room
temperature, other metals, polymers, glasses at high temperature.
B. Moderately ductile fracture, typical for ductile metals
C. Brittle fracture, cold metals, ceramics, ice

10. (a) Necking (b) Formation of microvoids (c) Coalescence of microvoids
to form a crack (d) Crack propagation by shear deformation
(e) Fracture

11. ➢ Ductile fracture is the rupture of a material after a considerable amount
of plastic deformation.
➢ Material begin to neck beyond the ultimate tensile strength
➢ The neck refers to the reduced cross-section of the specimen near the
middle.

12. ➢ Cracks are found to nucleate at brittle particles, either the natural kind
found in multiphase materials, e.g., cementite in steel, or foreign
inclusions, e.g., oxide inclusions in copper.
➢ When a brittle particle is present, it is difficult to maintain compatibility
in the neck region between the continuously deforming matrix and the
nondeforming particle. This results in the formation of very tiny voids
near the matrix-particle interface.
➢ If fracture initiates at pores in the neck region, then the voids are
already present. The voids grow with increasing deformation and
ultimately reach sizes of the order of a mm. At this stage, the material
may tear apart.
➢ This model of ductile fracture is supported by the finding that materials
of ultra high purity, which do not have inclusions or pores, do rupture
in a fully ductile manner, that is after thinning down to a point or an
edge.

13. The basic steps are: void formation, void coalescence (also known as crack
formation), crack propagation, and failure, often resulting in a cup-and-cone
shaped failure surface.

14. • Evolution to failure:
• Resulting
fracture
surfaces
(steel)
particles
serve as void
nucleation
sites.
Fracture surface of tire cord wire
loaded in tension.
Moderately Ductile Failure
necking
s
void
nucleation
void growth
and linkage
shearing
at surface
fracture

15. Rough surface

16. In ductile fracture, extensive plastic deformation takes place before fracture.
The terms rupture or ductile rupture describe the ultimate failure of tough
ductile materials loaded in tension. Rather than cracking, the material "pulls
apart," generally leaving a rough surface. In this case there is slow
propagation and an absorption of a large amount energy before fracture.
Many ductile metals, especially materials with high purity, can sustain very
large deformation of 50–100% or more strain before fracture under favorable
loading condition and environmental condition. The strain at which the
fracture happens is controlled by the purity of the materials.
At room temperature, pure iron can undergo deformation up to 100% strain
before breaking, while cast iron or high-carbon steels can barely sustain 3%
of strain.
Because ductile rupture involves a high degree of plastic deformation, the
fracture behavior of a propagating crack as modeled above changes
fundamentally. Some of the energy from stress concentrations at the crack
tips is dissipated by plastic deformation before the crack actually propagates.

19. Brittle fracture in CI

22. Ductile vs Brittle Failure
Very
Ductile
Moderately
Ductile
Brittle
Fracture
behavior:
Large Moderate Small
• Ductile
fracture is usually
desirable!
Ductile:
warning before
fracture
Brittle:
No
warning

23. 23
• Ductile failure:
--one piece
--large deformation
Example: Failure of a Pipe
• Brittle failure:
--many pieces
--small deformation

24. Ductile vs. Brittle Failure
cup-and-cone fracture brittle fracture

25. Since brittle fracture is most unpredictable, usually a notch will be
introduced to simulate the conditions.
Notch-impact testing

26. 26
Impact Testing
final height initial height
(Charpy)

28. Two standard tests, the Charpy and Izod, measure the impact energy
(the energy required to fracture a test piece under an impact load),
also called the notch toughness.

29. DBTT

30. Fracture toughness is a property which describes the ability of a material containing a
crack to resist fracture, and is one of the most important properties of any material for
many design applications.
Fracture toughness is a quantitative way of expressing a material's resistance
to brittle fracture when a crack is present. If a material has much fracture toughness it will
probably undergo ductile fracture. Brittle fracture is very characteristic of materials with less
fracture toughness.
An impact toughness versus temperature graph for a
steel is shown in the image. It can be seen that at
low temperatures the material is more brittle and
impact toughness is low.
At high temperatures the material is more ductile
and impact toughness is higher.
The transition temperature is the boundary between
brittle and ductile behavior and this temperature is
often an extremely important consideration in the
selection of a material.

31. Greater the impact energy, the more ductile the material.
copper remains ductile at all temperatures in the range covered by this experiment
FCC metals remain ductile down to very low temperatures (Cu, Ni)

32. ➢It can be seen that copper remains ductile at all temperatures in the range covered by
this experiment.
➢Nylon can be seen to be going through its glass transition at above 50 °C, and hence
shows a brittle to ductile transition.
➢In contrast, acrylic remains brittle, as seen from the consistently low impact energies.
➢Zinc becomes more ductile at temperatures above room temperature.
➢Mild steel shows a ductile-brittle transition at around -60 °C.
➢Results for aluminium show that it become slightly less ductile as the temperature is
increased, and all the values for impact energy lie between the ductility of copper and
the brittleness of acrylic. Flow is always easier than cleavage in aluminium so there is
never a ductile-brittle transition observed. The decrease of overall ductility with
temperature seen in aluminium arises from dynamic recovery.

33. 33
• Pre-WWII: The Titanic • WWII: Liberty ships
• Problem: Used a type of steel with a DBTT ~ Room temp.
Design Strategy:
Stay Above The DBTT!

34. 34
Engineering Fracture Design
• Avoid sharp corners!
s
r ,
fillet
radius
w
h
o
smax

35. Ship-cyclic loading from waves.
FATIGUE

36. Computer chip-cyclic
thermal loading.
Hip implant-cyclic
loading from walking.
Fatigue Failure
Fatigue is the weakening of a material caused by repeatedly applied loads. It is
the progressive and localized structural damage that occurs when a material is
subjected to cyclic loading. The nominal maximum stress values that cause such
damage may be much less than the strength of the material typically quoted as
the ultimate tensile stress limit, or the yield stress limit.
Fatigue occurs when a material is subjected to repeated loading and unloading.

37. Crankshaft after fatigue failure

40. Aircraft Landing Gear

42. Fatigue
Fatigue is caused by repeated application of stress to the metal.
It is the failure of a material by fracture when subjected to a cyclic
stress.
Fatigue is distinguished by three main features.
i) Loss of strength
ii) Loss of ductility
iii) Increased uncertainty in strength and service life
This type of failure occurs in Metals and Polymers, While Ceramics do not undergo
this type of failure!!

43. Fatigue is an important form of behavior in all materials including
metals, plastics, rubber and concrete.
All rotating machine parts are subjected to alternating stresses.
Example: aircraft wings are subjected to repeated loads, oil and gas
pipes are often subjected to static loads but the dynamic effect of
temperature variation will cause fatigue.
Because of the difficulty of recognizing fatigue conditions, fatigue
failure comprises a large percentage of the failures occurring in
engineering.
To avoid: stress concentrations, rough surfaces and tensile residual
stresses, fatigue specimens must be carefully prepared.

45. Fatigue – Stress patterns
• Three different fluctuating stress-
time modes are possible:
(i) Reversed stress cycle,
(ii) Repeated stress cycle,
(iii) Random stress cycle.(Irregular
stress)

46. Stress cycles that can cause fatigue failure are characterized by following parameters:

47. Mean Stress affects Fatigue Life
Note, Mean σm = 0, for
Reverse stress cycling.
Mean σm = +ve
quantity for Repeat
stress cycling
Mean (σm) has an
effect on fatigue life.
Increasing the mean
stress level leads to a
decrease in fatigue life.

49. Crack Initiation and Propagation
The process of fatigue failure is characterized by three distinct steps:
(1) Crack initiation:
Cracks always nucleate on the surface of a component at some point
of stress concentration.
Crack nucleation sites:
surface scratches,
sharp fillets,
keyways,
threads,
dents, and the like…
(2) Crack Propagation:
Crack advances incrementally (during this step) with each stress cycle
(3) Failure:
Failure occurs very rapidly once the advancing crack has reached a
critical size.

50. Fatigue Testing – The S-N Curve
• Fatigue testing is carried out using standard machines, to
simulate the real service conditions:
(i) Stress level,
(ii) Time-frequency,
(iii) Stress pattern.

51. Rotating Beam test
specimen, post test with crack

52. Specimen: ASTM E466 standards
Reverse bending type fatigue testing machine

55. Stress (MPa)
No.of cycles to
failure
30 9500000
60 4000000
90 2000000
120 850000
140 600000
30 9400000
60 4000000
90 2100000
120 840000
140 595000

56. Fatigue Testing – The S-N
Curve
• Fatigue Limit (Endurance Limit):
For some steels and Ti alloys,
the S-N curve becomes
horizontal at higher ‘N’ values
for a limiting stress, below
which fatigue failure does not
occur. For steels, fatigue limit
range between 35% and 60% of
the tensile strength.
• Fatigue strength: Non-ferrous
alloys (Al, Cu, Mg) do not have
Fatigue Limit. Fatigue strength is
defined as the stress level at
which failure will occur for
some specified number of
cycles (e.g, 107 cycles).

57. Fatigue Testing – The S-N
Curve
• Fatigue Life: Number of
cycles to cause failure at a
specified stress level, as
taken from the S-N plot.

58. The S-N Curve
A very useful way to visual the failure for a specific material is with the S-N
curve.
The “S-N” means stress verse cycles to failure, which when plotted using
the stress amplitude on the vertical axis and the number of cycle to failure
on the horizontal axis.
An important characteristic to this plot as seen is the “fatigue limit”.

61. ➢ The point at which the curve flatters out is termed as fatigue limit and is
well below the normal yield stress.
➢ The significance of the fatigue limit is that if the material is loaded below
this stress, then it will not fail, regardless of the number of times it is
loaded.
➢ Materials such as aluminium, copper and magnesium do not show a
fatigue limit; therefore they will fail at any stress and number of cycles.
➢ Other important terms are fatigue strength and fatigue life.
➢ The fatigue strength can be defined as the stress that produces failure in a
given number of cycles usually 107.
➢ The fatigue life can be defined as the number of cycles required for a
material to fail at a certain stress.

62. Fracture surface of a
rotating steel shaft that
experienced fatigue
failure of metals.
Benchmark ridges are
visible to the unaided
eye as shown in the
photograph.
Striations: Each bench
mark will contain many
striations which can be
seen using SEM.
Crack Initiation and Propagation

63. TEM showing
fatigue
STRIATIONS in
Al.
Note presence
of striations and
benchmark
ridges confirm
that the failure
is fatigue.

64. Fatigue failure of metal
surface.
A crack formed at the top
edge.
The smooth region also
near the top corresponds
to the area over which
the crack propagated
slowly.
Dull and fibrous fracture
surface reveals fracture
was rapid.

66. Failure can be recognized by the appearance of fracture.
For a typical fracture ,Two distinct zones can be distinguished – a smooth
zone near the fatigue crack itself which, has been smoothened by the
continual rubbing together of the cracked surfaces, and a rough crystalline-
looking zone which is the final fracture.
Occasionally fatigue cracks show rough concentric rings which correspond to
successive positions of the crack.

68. Fatigue life improves by incorporating rounded fillet into a rotating
shaft at the point where there is a change in diameter.
(a) No fillet; bad design. (b) Fillet provided; good design.

69. Factors affecting fatigue properties
Surface finish:
Scratches dents identification marks can act as stress raisers and so reduce
the fatigue properties.
Electro-plating produces tensile residual stresses and have a detrimental
effect on the fatigue properties.
Temperature:
As a consequence of oxidation or corrosion of the metal surface increasing,
increase in temperature can lead to a reduction in fatigue properties.

70. Residual stresses:
Residual stresses are produced by fabrication and finishing processes.
Residual stresses on the surface of the material will improve the fatigue
properties.
Heat treatment:
Hardening and heat treatments reduce the surface compressive stresses;
as a result the fatigue properties of the materials are getting affected.
Stress concentrations:
These are caused by sudden changes in cross section holes or sharp
corners can more easily lead to fatigue failure. Even a small hole lowers
fatigue-limit by 30%.
Factors affecting fatigue properties

71. Fatigue fracture results from the presence of fatigue cracks, usually initiated
by cyclic stresses, at surface imperfections such as machine marking and slip
steps.
The initial stress concentration associated with these cracks are too low to
cause brittle fracture they may be sufficient to cause slow growth of the
cracks into the interior.
Eventually the cracks may become sufficiently deep so that the stress
concentration exceeds the fracture strength and sudden failure occurs.
The extent of the crack propagation process depends upon the brittleness of
the material under test.
In brittle materials the crack grows to a critical size from which it propagates
right through the structures in a fast manner, whereas with ductile materials
the crack keeps growing until the remaining area cannot support the load
and an almost ductile fracture suddenly occurs.
Fatigue Failure

72. To secure satisfactory fatigue life
▪ Modification of the design to avoid stress concentration eliminating sharp
recesses and severe stress raisers.
▪ Precise control of the surface finish by avoiding damage to surface by
rough machining, punching, stamping, shearing etc.
▪ Control of corrosion and erosion or chemical attack in service and to
prevent of surface decarburization during processing of heat treatment.
▪ Surface treatment of the metal.
The endurance limit for the material will be approximately 45 to
50% of the UTS if the surface of the test specimen is smooth and
polished.

73. 1. Small scratches, tool
marks on surface will
limit the fatigue life.
2. Improving the surface-
finish by polishing will
enhance fatigue life.
3. Residual compressive
stresses are introduced
into ductile metals
mechanically by
localized plastic
deformation, with in
the outer surface region
by Shot peening.
Shot Peening

74. Case hardening improves fatigue life
Case hardening with
carbon or nitrogen
increases the hardness
of the surface for steels.
The increased hardness
of the case improves
the wear resistance and
fatigue life.

75. PHYSICAL METALLURGY AND MATERIALS ENGINEERING
Hall-Petch equation:
Yield stress is inversely
proportional to the
square root of grain
diameter (grain size).
Where,
σy, is yield stress;
σ0 and ky are material
dependent constants;
d, is grain diameter
(grain size).
𝝈 𝒚 = 𝝈 𝟎 +
𝒌 𝒚
𝒅

76. Equicohesive Temperature
1. Strength of the GB = Strength of Grain at the ECT
2. Below ECT fine grain sized material is stronger due to large number of
grain boundaries are present.
3. Above ECT large grain sized material is stronger due to less tendency for
grain boundary sliding.

77. FAILURE DUE TO TORSION

78. Until the advent of variable speed drives (VSDs), torsional fatigue
failures were rare: Equipment designers could anticipate operating
speeds and excitation frequencies and engi-neer around them.
The purpose of a VSD is to allow operation at a wide range of
speeds. That, unfortunately, has led to many motor and driven-shaft
failures due to torsional-fatigue factors.
While the most common torsional fatigue cracks start at the sharp
corner (stress concentration) at the bottom of the keyway when
couplings are poorly fitted, another common appearance is the
diagonal shaft crack

83. PREVENTING TORSIONAL FATIGUE FAILURES
Torsional fatigue failure prevention:
Correct machining, assembly and installation to eliminate:
➢Incorrect fit/finish of a shaft and bore.
➢Small radius in a keyseat.
➢Excess clearance between the key and keyseat.
➢Misalignment.
These are the easiest to fix. Frequently, the effort stops here and the failures continue.
However, with more investigation, torsional fatigue failures can be eliminated.