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MODULE 1 : ENGINEERING MECHANICS 6 hrs
Statically determinate and indeterminate systems.Stress- strain relations.
Variation of Stress and Strain.
.
Stress Concentration.
Creep and Viscoelasticity.
PP Benham, RJ Crawford, CG Armstrong, Mechanics of
Engineering Materials, 2nd
8/10/2011 Module 1
pages 573-597 of the above book
sections 21.1-14
Originated from: A/P SHU DongWei
6. Creep and Viscoelasticity
,
turbines, nuclear processes, etc
Creep = strain increases at constant stress.
Thermoplastics susceptible at room
.
Part 1: Metals
Part 2: Plastics
Creep of metals0.3Tm, and 0.5Tm
constant stress /load
constant temperature
Andrade 1910; test on lead at room temp
long duration:4 stages
initial elastic
primary
secondary (const
ra e
tertiary to fracture
Empirical creep strain: general
=f . f t . f T
f1() = A1 ; A2 sinh(/0) ; A3 exp(/0)
f2(t) = t1/3
+ t + t3
f3(T) = exp(H/(RT))
R universial gas constant
T absolute temperature
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Empirical Creep strain: Linear
= + n
Secondary creep strain
Eqn 21.6
0.5% strain at 100,000 hrsextrapolation
Test results for
different
temperatures
Temperature
Plotted from previous graph and then
extrapolate to 100,000
21.3 Creep-Rupture test
Used for selection of material for long term test
Determine the rupture strength
Applied varying stress sufficient to cause rupture, successive tests
at constant T
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21.5 Creep during beam bending
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Creep under multi-axial stresses
of the
&
are
in
Creep under multi-axial stresses
Creep under multi-axial stresses
E21.1
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5
4
23
4
503
Where th=time in hours
Sub the design values:
4.
2. xpxxx
p4=794.4x108
p=530N/m2
Time-dependent increase in strain at constant stress
21.7 stress relaxation: thermal(Time-dependent decrease in stress at constant strain)
tightening stress in
bolts:
Loosening of bolts
in tension
stress relaxation
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21.8 stress relaxation in a bolt
relaxation: variable stress/temp
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Time-hardening hyp
Creep rate = f(stress, t)
Strain hardening hyp
Strain rate = f(stress,p.strain)
Creep and fatigue Creep and fatigue, Sa-Sm diagram(0.26% carbon steel at 400degc 100
Points along
x-axis=> creep.
Points along
y-axis=>
Fati ue
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21.11:Plastics material: viscoelasticity
tf.viscoelasticity
tf ,
time-dependent
strain, i.e. relaxation.
When applied stress
is remove, strain
recovery over period
of time.Can happen at
ambient temp.
Creep and recovery: Plastic material
Creep curves/isometri/isochro Creep modulus
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Creep modulus/temperature
Effect of temp on the creep
rate of plastic materials.
T=(-30deg c) to (+40deg c)
Creep/stress levels
Designing for creep in
plastic: Most acceptable
approac pseu o-e as c
design method.
Use of published
experimental creep data for
the material
Fig 21.18: Creep curves for
acetal at 20deg C
Creep/stress levelsfrom previous curves
A solid circular acetal rod,
0.15m long, clamped at one
end and the other free end is
25N. Determine a suitable
diameter for the rod for a
limiting strain of 2% in 1 year.What would be the max.
deflection at this time.
1 year Isochronous curve:
allowable stress of 17.1MN/m2 is
obtained at 2% strain limit.
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