SRM UNIVERSITY, RAMAPURAM CAMPUS
DEPARTMENT OF MECHANICAL ENGINEERING
UNIT 2
FRACTURE BEHAVIOUR
Part - A (Two marks)
1.
Define fracture and list the types.
Fracture
 It is the end result of plastic deformation process.
 Separation of a body under stress into two or more parts.
 Caused either by physical or chemical forces.
 Results in creation of new surfaces.
Types of fracture
2.
3.
4.
5.
 Ductile fracture
 Brittle fracture
 Creep fracture
 Fatigue fracture
Explain Shearing fracture.
 Occurs as a result of extensive slip on active slip plane.
 Promoted by shear stress.
 Single crystal of h.c.p metals may slip on successive basal planes until finally the
crystal separates by shear.
Differentiate Brittle fracture and ductile fracture.
Ductile fracture
S.No.
Brittle fracture
1
Characterized by rapid rate of crack
propagation with minimal energy
absorption
Characterized by appreciable plastic
deformation prior to and during
propagation of crack
2
Occurs by separation normal to tensile
stress
Preceded by localized deformation by
necking
3
Observed in BCC and HCP metals
Observed in FCC metals
Define Ductile to Brittle transition temperature.
 When BCC metals subjected to impact loads, comparatively at low temperatures, a
transition occurs from ductile fracture (requiring higher energy) to brittle fracture
(requires low energy). It is called as Ductile to Brittle temperature.
 Depends on Rate of strain, crystal structure, grain size and stress distribution, etc.
Define Stress intensity factor.
 It is defines as the local stresses near the crack that depends on the product of
nominal stress and the square root of half of the flaw length.
k =  (c)
 If the value of k is same for two flaws of different geometry, stress filed around each
flaw is identical.
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SRM UNIVERSITY, RAMAPURAM CAMPUS
DEPARTMENT OF MECHANICAL ENGINEERING
6.
Define the term Fracture toughness.
 It is defined as a property that measures the resistance of a material to brittle
fracture when a crack is present.
kc = y  (c)
y – Dimensionless parameter depends on crack, specimen size, geometry and
load
y = 1 for thorough thickness of crack
y = 0.6 for semicircular crack
y = 0.8 for semi elliptical crack
y = 1.12 for Edge crack
y = 0.86 for Cone crack
7.
Define creep strength.
 Property of material by virtue of which it deforms continuously under a steady load
for prolonged time period.
 Viscous flow in metals involving applied stress, time and temperature.
8.
List out the different mechanisms takes place in creep fracture.
 Dislocation glide
 Dislocation climb
 Creep due to grain boundaries / Coble creep
 ro-Hering creep
9.
Differentiate dislocation glide and dislocation climb.
S.No.
Dislocation climb
Dislocation glide
1
Takes place at low temperature and
higher stress
Takes place at high temperature and
low stress
2
Rate of creep increased by thermal
activation
Rate of creep depends on rate of
escape of dislocation past the
obstacles
3
Transient creep takes place
Viscous creep takes place
10. What is the use of Deformation mechanism map?
 It indicates the dominant deformation mechanism takes place for different Stress –
Temperature combination.
 The boundary represents the combination of stress-temperature at which strain rate
of two mechanisms is equal.
 Allows selection of two or three variables (stress, strain rate or temperature), for
applications.
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SRM UNIVERSITY, RAMAPURAM CAMPUS
DEPARTMENT OF MECHANICAL ENGINEERING
11. Differentiate transient creep and viscous creep.
S.No.
Viscous creep
Transient creep
1
Takes place at low temperature and
higher stress
Takes place at high temperature and
low stress
2
Occurs when operated at stresses in
plastic range
Occurs when operated above flow
temperature
3
Takes place highly in Crystalline
materials
Takes place highly in Amorphous
materials
4
Low temperature creep
High temperature creep
12. Define fatigue limit.
 It’s also called as Endurance limit
 A definite stress below which failure by fatigue does not takes place, no matter how
many times the stress repeated or reversed.
 When series of stress is plotted against the number of cycles to failure at each
stresses, it becomes perfectly horizontal at this limit.
13. What are the different types of Fatigue loading?
 Reversed load
 Alternating load
 Fluctuating load
 Repeated load
 Irregular load
14. What are the different mechanisms takes place during fatigue fracture?
 Crack initiation
 Slip-band crack growth
 Crack growth on planes of high tensile stress
 Ultimate ductile fracture
15. What are the variables affecting the fatigue life?
 Surface condition of material
 Temperature
 Range of stress / Mean stress
 Frequency of stress cycle
 Environment
 Metallurgical factor
16. State Cumulative damage theory.
 The theory states that each series of stress cycles account for a certain fraction of
the total damage and when these fractions addup to unity, failure occurs.
 The fraction of total damage done by one series of cycles at a particular stress level
is given by the ratio of number of cycles (n) actually endured at that level to the
fatigue life (N) at that level.
Cycle ratio, C = ni / Ni
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SRM UNIVERSITY, RAMAPURAM CAMPUS
DEPARTMENT OF MECHANICAL ENGINEERING
17. What is meant by Plastic blunting process?
 Occurs at stage-II crack propagation.
 As a result of repeated tensile and compressive load on the crack tip, the slip
direction reverses at the crack tip. Due to plastic shear the crack grows and crack tip
becomes sharp and blunt simultaneously. This phenomenon is called as Plastic
Blunting.
18. Define Notch sensitivity.
 Reduction of nominal strength, static or impact, by the presence of stress
concentration.
 Expressed as ratio of the notched to the un-notched strength.
 Makes a ductile material to undergo brittle fracture.
19. What are the sources of failure?
 Improper material selection
 Improper processing
 Inadequate design
 Misuse of components
20. Define Equi-cohesive temperature.
 When the temperature is increased the role of grain boundary is reversed from one
of resisting deformation to one of aiding deformation. This temperature of inversion is
called as Equi-cohesive temperature.
 It is the temperature at which strength of grains and grain boundaries are equal.
21. State Paris law.
 Paris – Erdogan law relates the stress intensity factor range to subcritical crack
growth under fatigue stress regime.
𝑑𝑎
= 𝑐 𝑘 𝑚
𝑑𝑁
Where,
a
N
k
c&m
– Crack length
– Number of cycles
– Stress intensity factor – kmax - kmin
– Material constant
Material Technology
1.Mr. D.K.Mohan Kumar.,(Course Coordinator) Dept.of.Mech.Engg.SRM University, Chennai.
2.Mr.Easwar., Dept.of.Mech.Engg.SRM University, Chennai
3..Mr.Akbhar Aasiq Husain.., Dept.of.Mech.Engg.SRM University, Chennai
HOD/MECH
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SRM UNIVERSITY, RAMAPURAM CAMPUS
DEPARTMENT OF MECHANICAL ENGINEERING
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