study of time dependence

7/27/2019 Study of Time Dependence

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Presented By:

Swati Sharma

TIME DEPENDENT PROPERTIES

OF FIBRES


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Time dependent properties study of different

properties of the fibres with respect to the time.

Time dependent study is important because it

provides means of predicting behavior in real

situation.

INTRODUCTION

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Creep: long times, from 1 minute to 1 month.

Stress relaxation: medium to long times, from 1 second

to 1 hour.

Stressstrain curves, including impact methods: short to

medium times, from 1/100 second to 10 minutes.

Dynamic testing: short times, from 0.1 millisecond to 1

second.

METHODS USED AT DIFFERENT TIME

SCALES ARE AS FOLLOW

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Creep is time dependent deformation under constant load

for a long time.

The total extension under applied load may therefore bedivided into three parts:

Immediate elastic deformation , which is instantaneous and

recoverable (a-b and d-e).

Primary creep, which is recoverable in time (e-f) . The secodary creep, which is non-recoverable(g-h) .

CREEP

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CREEP UNDER CONSTANT LOAD AND

RECOVERY UNDER ZERO LOAD

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PRIMARY AND SECONDARY CREEP

OF VISCOSE RAYON AT 60% R.H

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The creep of viscose rayon at various loads shows a

certain regularity when the elongation divided by the

stress was plotted against the time

The creep curves obtained at various loads and can be

fitted to a single master curve by vertical and lateral

shift and by multiplying the elongation scale by an

appropriate factor

GENERALIZED CREEP CURVES

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GENERALISED CREEP CURVES FOR

VISCOSE RAYON AT 6% R.H .

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Humidity:

Creep increases with the humidity. The change is small

below 40% r.h. but considerable at higher humidity.

Temperature:

Creep of fibre increases as the temperature increased

INFLUENCE OF VARIOUS FACTORS ON

CREEP

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Progressive loss of stress

(load) over time under

constant strain(deformation)

is known as stress

relaxation.

Both linear elastic and

viscous fluid behaviors are

present.

The material does notcompletely recover. There

is both elasticity and

plasticity

STRESS RELAXATION

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Stressstrain tests take some time, and consequentlythere is an opportunity for creep to occur.

The slower the test, the more time there is available,

and thus the greater the extension at a given load

Because of creep, the slower curves are nearer to thestrain axis than the faster curves. There are two reasonsfor this:

1. Near the end of the test, there has beenmore time for creep

2. above the yield point the rate of creep isgreater

STRESSSTRAIN CURVES

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Stressstrain curves at various rates of

extension viscose rayon

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DIFFERENCE BETWEEN CRE AND CRL

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The stressstrain curves obtained for a fibre thus

depend on the time taken in the test and on the way in

which the time is distributed

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Technique used to s tudy and characterize materials

One way is to use an impact test. In this method, amoving large mass is engaged with one end of thespecimen, while the other end is held fixed andconnected to a load-measuring device.

The dynamic tests explained here are of two types

(1) cyclic loading

(2) tests in such a short time that the propagation of thestress wave means that the s tress cannot be regarded asconstant along the specimen

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DYNAMIC TESTING


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Let at ant time t ,stress is = f and strain = e at time t

A sinusoidal strain is applied and the stress in the

material is measured.

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where em is the strain amplitude, and is the angular frequency in

radians/second

where fm is the stress amplitude and is the angular phase

difference
http://en.wikipedia.org/wiki/Sinusoidalhttp://en.wikipedia.org/wiki/Sinusoidal


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Dynamic structures and

steady-state response:

(a) imposed sinusoidal

strain variation;

(b) stress variation for

linear viscoelastic material;

(c) Stressvariation for a non-linear

material, showing also a

longer-term trend


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The subject is complicated by the number of ways inwhich the dynamic behavior can be represented

1) modulus from ratio of amplitudes = fm/em

2) phase angle or loss angle =

3) in-phase modulus (usually termed dynamic modulus)

4) It is a measure of how well a material can get rid ofenergy and is reported as the tangent of the phase anglealso known as diss ipation factor.

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Direct observation of stressstrain loop:

impose cyclic loading or extension and to record the

stressstrain loop

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METHODS OF DYNAMIC TESTING


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Free vibrations :

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Contd..


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Forced resonant vibrations:

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Direct observation Forced resonant vibrations:

Rheovibron tester


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Velocity of sound: continuous transmission


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THANK

YOU

study of time dependence

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