2012 time depandant mechanical properties, viscosity, viscoelasticity
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TIMEDEPENDENT MECHANICALPROPERTIES
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LECTURE OUTLINE:
Definitions and Significance of:
time dependent mechanical properties:
Viscosity
ViscoelasticityStress relaxation
Creep
Fatigue
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Stress-strain curves
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Used to compare materials mechanicalproperties whether in compression,tension or shear, especially whenstrain
is independent of the length of time theload is applied
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Strain-time curves are sometimes used when straindepends on the time the load is maintained
(e.g. polymers, soft tissues)
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PROPERTIES THAT AREAFFECTED BY TIME
Viscosity
Viscoelasticity
Creep
Fatigue
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The study of flow or deformation of
materials is measured by:
Elasticity , Viscoelasticity, Creep : Solids
Viscosity = shear stress/shear strain: Liquids
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VISCOSITY The resistance liquids to flow
Viscosity is high when the
forces binding molecules
together are high Shearing of a liquidBetween 2 plates
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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
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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 -
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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.
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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
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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
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STRESS RELAXATION
Gradual reduction in the stress when the
material is held at constant strain
(elongation)
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CREEP
Is the flow of solids
Permanent deformation under loads less
than elastic limit due to constant stress
maintained for a long period
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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.
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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.
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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)
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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
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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.
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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 .
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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.
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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
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HOME WORK
List the time dependant mechanical properties
What is the significance of viscoelasticity?
Define fatigue. Explain the fatigue mechanism.
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