Visco Elasticity 1

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VISCO ELASTICITY

1. INTRODUCTION

2.VARIOUS TESTSa. CREEPb. STRESS RUPTUREc. STRESS RELAXATION

3. DESIGN AGAINST CREEP


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1. INTRODUCTION:

The time dependent material behavior is often referredto as viscoelasticity .

TWO TYPES: 1.Linear 2.nonlinear viscoelasticity:


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In general, for stress , the creep compliance D(t)can be given as the ratio of strain to stress at a certain time.

D(t) = e(t) /

This property is often characterized as linear viscoelasticity .

The strain range in which a specimen is linear viscoelasticcan be determined by a creep test.

A transition from linear to nonlinear viscoelasticityis shown in Figure 9.


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2.VISCOELASTICITY: DETERMINATION TESTS

1. CREEP TEST :-Constant stress- strain variation-

large test period (2000-10,000h)

2. STRESS RUPTURE TEST:-High range constant loads

- Short term(


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1. CREEP

It is a time - dependent deformation under a tensile load.

Generally occurs at high temperature (thermal creep),but can also happen at room temperature in certain materials(e.g. lead or glass), albeit much slower.

As a result, the material undergoes a time dependentincrease in length, which could CAUSE failure while in service.


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TYPICAL CREEP TEST


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The creep curve


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A typical creep curve shows three distinct stages withdifferent creep rates. After an initial rapid elongation o,

the creep rate decrease with time until reaching thesteady state.

1) Primary creep provides- decreasing creep rate.

2) Secondary creep -gives the representing constantcreep rate.

3) Tertiary creep yields a rapid creep rate till failure.


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EFFECT OF TEMPERATURE AND STRESS ON CREEP RATE:

T1


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Recovery of Strain after Unloading

After unloading, some of the strain is recovered, but an appreciableplastic strain has become permanent.


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DESIGN AGAINST CREEP CREEP RESISTANCE FACTORS:

Strategy I: Employ materials of high melting temperatures. High melting point alloys normally has high creep resistance.

Ex- metallic materials- stainless steels,

Refractory metals- tungsten, Molybdenim, tantalum..

Super alloys- Cobalt alloys, Niclel alloys, Iron alloys..alloyong elements(Tn, Mo, Ta)


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Strategy II: Obstruct the motion of dislocations. Creep life could be extended by obstructing the motion of dislocations. This could be done in a number of ways:

-Solid solution additions :to eliminate vacancies (major effect)- Second Phase Particles: The effect of second phaseparticles can pin the dislocations and in so doing reduce therate of creep. (major effect)


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Strategy III: Obstruct diffusional flow.

Diffusional flow can be retarded by: Increase the grain size- Use of single crystal material


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Case Study I: Turbine Blade

The efficiency of gas turbines increases as the operation temperatureincreases. Today engines operate at temperatures as high as 1400 oC. Thetemperature of the turbine blades reach 850oC.

The alloy from which the blades are made mustmeet very tough requirements:

-Resistance to creep-Resistance to high-temperature oxidation.-High strength and toughness-Thermal fatigue resistance

-Thermal stability-Low density

Modern blades are made out of very sophisticated Ni -superalloys whichare prepared by investment casting.


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Effect of solid solution and secondphase particles


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Effect of grain size


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2.STRESS RUPTURE TESTS

Determines the time necessary for material toresult in failure under a overload.

Useful in materials selection wheredimensional tolerances are acceptable, butrupture cannot be tolerated.

Generally performed at elevated temperatures.

Smooth, notched, flat specimens or samples of any combination can be tested


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Creep vs. Stress Rupture Test


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3. Stress relaxation ( constant strain)

Stress relaxation is defined as a gradual decrease instress with time under a constant deformation or strain


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Stress relaxation test can be used for some practicalapplications.

For example, low stress relaxation is desired for threadedbottle closures.

The stress data obtained from stress relaxation test can beused to calculate transient modulus for plastics design bysimply dividing the stress at a particular time by the appliedstrain.


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CONCLUSION

Understanding of Viscoelasticity and varioustests

creep resistance factors, grain size, solidsolutions.. have been analysed

Cerep analysis applied to turbine blade toincrease the life.

Visco Elasticity 1

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