2012 time depandant mechanical properties, viscosity, viscoelasticity

7/27/2019 2012 Time Depandant Mechanical Properties, Viscosity, Viscoelasticity

1/24

TIMEDEPENDENT MECHANICALPROPERTIES


2/24

LECTURE OUTLINE:

Definitions and Significance of:

time dependent mechanical properties:

Viscosity

ViscoelasticityStress relaxation

Creep

Fatigue


3/24

Stress-strain curves

3

Used to compare materials mechanicalproperties whether in compression,tension or shear, especially whenstrain

is independent of the length of time theload is applied


4/24

Strain-time curves are sometimes used when straindepends on the time the load is maintained

(e.g. polymers, soft tissues)

4


5/24

PROPERTIES THAT AREAFFECTED BY TIME

Viscosity

Viscoelasticity

Creep

Fatigue


6/24

6

The study of flow or deformation of

materials is measured by:

Elasticity , Viscoelasticity, Creep : Solids

Viscosity = shear stress/shear strain: Liquids


7/24

VISCOSITY The resistance liquids to flow

Viscosity is high when the

forces binding molecules

together are high Shearing of a liquidBetween 2 plates


8/24

Fluid Level

Fluid Flow

Measuring of Viscosity

A specified volume of the liquid is put in a container withan orifice of specific dimension in its bottom.

The time required for the liquid to run out of the orifice is

measured.

The viscosity of the liquid is computed from standard table


9/24

VISCOELASTICITY

Viscoelasticity is the property of materials that exhibit both

viscousand elastic characteristics when undergoing

deformation.

Viscous materials, like honey, resist shear flowand strain

linearly with time when a stress is applied.

Elastic materials strain immediately when stretched and just

as quickly return to their original state once the stress is

removed.
http://www.answers.com/topic/viscosityhttp://www.answers.com/topic/elasticity-physicshttp://www.answers.com/topic/shear-flowhttp://www.answers.com/topic/stretch-ratiohttp://www.answers.com/topic/stretch-ratiohttp://www.answers.com/topic/shear-flowhttp://www.answers.com/topic/elasticity-physicshttp://www.answers.com/topic/viscosity


10/24

STRAIN-TIME CURVE

Unlike purely elastic substances, a vicoelastic material

l has an elastic component and a viscous component.The viscosity of a viscoelastic substance gives the

substance a strain rate dependent on time.


11/24

Viscoelasticity

Viscoelastic materials are materials for which

the relationship between the stress and strain

depends on time.

The stiffness depends on the rate of applicationof load.

The mechanical energy is dissipated by

conversion to heat in deformation of the material.

Viscoelastic effect occurs at small or large

stress


12/24

VISCOELASTICITY

Clinical Significance:

Some synthetic and biologic materials have

properties between that of a solid and a liquid,

which makes them susceptible to distortion

Synthetic materials: Polymers

Biologic materials: human soft tissues


13/24

STRESS RELAXATION

Gradual reduction in the stress when the

material is held at constant strain

(elongation)


14/24

CREEP

Is the flow of solids

Permanent deformation under loads less

than elastic limit due to constant stress

maintained for a long period


15/24

CREEP Is used to rate the resistance of a material to

plastic deformation under sustained load.

Creep resistance should be considered for

any part or structure that is subjected to

sustained load in service.


16/24

SIGNIFICANCE OF CREEP

Creep of blood vessels is important since it

have a steady state internal pressure that

gives rise to circumferential stress.

Creep occurs in polyethylene socket of total

hip replacement. It gives indentation of thesocket by ball joint.


17/24

FATIGUE Cyclic (Dynamic) Fatigue Failure:

is Failure of material by fracture due to repeated

application of load below its proportional limit over time.

(Cyclic stress+ Small stress + Long period of time)


18/24

FATIGUE

Same as creep in that the stress is below the

elastic limit but continuous

Fatigue is a function of the stress level and thenumber of cycles

S

tress

Time

Repeated cyclic loading


19/24

MECHANISM OF FAILURE

Cyclic stress initiate micro-cracks at the

centre of stress concentration within the

material or on the surface resulting in

growth and propagation of the cracks,

leading to failure.


20/24

20

Testing of materials Fatigue by 2 ways:

1. Fatigue life: application of stress cycles at a certain amount

and frequency and observe number of cycles needed to cause

failure.

2. Fatigue limit: select a number of cycles (e.g. 107) and

determine the value of the cyclic stress which is required to

cause fracture within this number of cycles .


21/24

SIGNIFICANCE OF FATIGUE:

Implants flexed many times during service ,

to avoid fatigue:

- during fabrication avoid notching and cracks

- design and sharp corners of the should be avoided.


22/24

REFERENCES

Biomaterials Science. An introduction to materials in

Medicine. Ratner B,D, Hoffman A, Schoen F, Lemons

J, E . First Edition,.1996

Biomaterials :An introduction Park j, Lakes R.S. Third

Edition, 2007


23/24

HOME WORK

List the time dependant mechanical properties

What is the significance of viscoelasticity?

Define fatigue. Explain the fatigue mechanism.


24/24

2012 time depandant mechanical properties, viscosity, viscoelasticity

Documents