Failure mechanism: CREEP

i. Fracture
ii. Fatigue failure
iii. creep
Keval K. Patil, M.E.-DESIGN, (kevalpatil@gmail.com)
1

Keval K. Patil, M.E.-DESIGN, (kevalpatil@gmail.com)
Part 3
2

 Materials are often placed in service at
elevated temperatures and exposed to static
mechanical stresses (e.g., turbine rotors in jet
engines and steam generators that experience
centrifugal stresses, and high-pressure steam
lines). Deformation under such circumstances
is termed creep
 Creep is slow plastic deformation of metal
under constant stresses at constant
temperature for prolonged period
 Defined as the time-dependent and permanent
deformation of materials when subjected to a
constant load or stress, creep is normally an
undesirable phenomenon and is often the
limiting factor in the lifetime of a part.
Keval K. Patil, M.E.-DESIGN, (kevalpatil@gmail.com) 3

 When temperature of material increases, the mobility of atoms increases rapidly,
which changes mechanical properties of material.
 High temperature will also result in greater mobility of dislocation by the
mechanism of dislocation climb.
 The equilibrium concentration of vacancies increase and new deformation may form
at high .
 In some metals, the slip system changes or additional slip system are introduced
with increasing temperature.
 At high temperature, cold-worked metals will recrystallize and undergo grain
coarsening while age hardening alloys may overage by prolonged exposure at high
temperature and loose strength.
 Successful use of metals at high temperature involves a number of problems. But
modern technology demand better high temperature strength and oxidation
resistance.

 The specimen to be tested is placed in the electric furnace where it is heated
to a given temperature.
 The usual method of creep testing consist of subjecting the specimen at
constant tensile stress at constant temperature and measuring the extent of
deformation or strain with the time.
 The typical creep testing machine is shown in figure even though, it
appears to be simple, it requires considerable laboratory equipment, great
care and precision in performance.
 The time of each test may be matter of hour, weeks, months or even years.

 Creep is also determined in compression, shear and bending.
 The data is presented by plotting creep curve as deformation verses
time at constant temperature and stress.
 The test specimen may be circular, square or rectangular in cross-
section.
 Either a continuous record of deformation with time or sufficient
number of deformation readings with time should be taken over the
entire period of

 It is frequently important to be able to exploit creep or stress-rupture data into
regions where data are not available.
 Therefore, common methods of plotting creep data are based on plots which yield
reasonable straight line.
 Figure shows the common method of presenting the influence of stress on the
state or minimum creep rate.
 Note that a log-log plot is used, so that extrapolation of one log-cycle represents a
tenfold change.
 A change in slope of the line will sometimes occur.
 It has been shown that the value of the minimum creep rate depends on the length
of time the creep test has been carried out.
 It has been shown for long-time creep test(t> 10,000 hours) that the creep
strength based on ‘1%’ creep n true strain is essentially equal to creep
strength based minimum creep rate.

 A creep curve is a plot between the total creep or strain and the time for the entire
duration of test.
 Primary creep:
 The primary or transient creep is a decreasing creep rate because of the work
hardening process resulting from deformation

 Secondary creep:
 During the secondary or steady state creep(i.e. minimum creep rate), the
deformation continues at an approximately constant rate.
 During this process, a balance exist between the rate of work hardening and
rate of softening because of recovery or recrystallization.
 The steady state creep may be essentially viscous or plastic in character,
depending upon the stress level and temperature.
 Tertiary Creep:
 Creep rate increases with time until fracture occurs in this stage
 Tertiary creep can occurs due to necking of the specimen or grain boundaries
sliding at high temperature.

 Creep is a deformation process in which three main features to be
involved are:
 The normal movement of dislocation along slip planes.
 Process ‘dislocation climb’ which is responsible for rapid creep at
temperatures above 0.5 times of melting temp.
 Slipping at grain boundaries.
 The following mechanism are known to be responsible for creep in
crystalline materials.
 Dislocation climb.
 Vacancy diffusion.
 Grain boundary sliding.

 In the primary stages of creep, dislocation move quickly at first but
soon becomes pilled up at various barriers.
 At temperature in excess of 0.5 tm, thermal activation is sufficient to
promote a process known as ‘dislocation climb’.
 It is shown in figure this would bring into use new slip planes and so
reduce the rate of work hardening.

 In addition to plastic deformation by dislocation movement, deformation by
a form of slip at grain boundaries also occurs during the secondary stage of
creep. These movements possibly leads to the formation of ‘vacant sites’,
that is lattice position from which atoms are missing and this in turn makes
possible ‘ dislocation climb’.
 In the tertiary stage of creep micro-cracks are initiated at grain boundaries
due to the movement of dislocations. In some cases, there is migration of
vacant site, as a result necking and consequent rapid failure follows.

 the diffusion of vacancies control creep rate. In
this mechanism, grain boundary acts as a
source and sink for vacancies. The mechanism
depends on the migration of vacancies from one
side of a grain to another. Referring to figure a
grain ABCD is under stress ‘p’, the atoms
moved from faces ‘BC’ and ‘AD’, along the
path shown and the grain creeps in the
direction of stress
 Movement of atoms creating vacancies on face
‘AB’ & ‘DC’ and destroying them on the
other faces.

 the sliding of neighbouring grains with respect to the boundary that
separates them. Figure shows that, grain boundaries lose their
strength at lower temperature than the grains themselves. This effect
arises from non crystalline structure of the grain boundary

 Andrade’s work on analysis of classical creep curve is focused on
topic of creep. He considered that the constant stress creep curve
represents the superposition of two separate creep processes which
occur after the sudden strain which results from applying the load.
The first component of the creep curve is a ‘transient creep’ with a
creep rate decreasing with time. Added to this is a constant-rate
‘viscous-creep’ component figure shows Andrade's analysis of the
classical creep curve.
 Andrade found that the creep urve could be represented by the
following empirical equation.
 e = e0 [1+ t^(1/3)] e^(kt)
 Where, e= strain, t= time, & k = constant, e0 = instantaneous strain.
c

 Creep resistant material are required for structural and machine
components used at elevated temperatures. They should be capable of
withstanding these temperatures without undergoing creep beyond the
specified limit, which may cause dimensional changes beyond
permissible limit used in the design.
 The following are the requirements of a creep resistance material.
 It should have high melting point because, the creep becomes significant
above 0.4 tm (tm=melting point). If the melting point is high, material can be
used at higher temperature, e.g. iron, nickel, cobalt.

 It should have coarse grained structure. The grain boundary region becomes
quasi-viscous at creep temperature . Since in coarse grained materials grain
boundary area is less, so that less amount of quasi- viscous region is formed
with a less tendency to flow, reducing the creep deformation.
 It should be precipitation hardenable. It should have fine insoluble
precipitates at the operating temperature. If coherent precipitates are present,
maximum creep resistance is obtained e.g. in nickel base and iron-nickel-
base superalloy coherent precipitates of Ni3 (Al, Ti) is formed.
 Dispersion hardening improves creep resistance.
 It should have high oxidation resistance i.e. the oxide film should follow
either a logarithmic or a cubic law of growth.

Failure mechanism: CREEP

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