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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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