down_1227ch 3 mechanical properties.ppt

8/10/2019 down_1227Ch 3 mechanical properties.ppt

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Chapter 3

Elasticity and Strength of Materials

References

1-Physics in biology and Medicine 3rde, Paul Davidovits

2- web sites

3- College Physics, 7the, Serway

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Classification of matter

Matter is normally classified as being in one ofthree states:

A solidhas a definite volume and shape.

Aliquidhas a definite volume but no definiteshape.

A gas it has neither definite volume nor definiteshape. Because gas can flow, however, it shares

many properties with liquids. Often this classification system is extended to

include a fourth state of matter, called aplasma.

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Structure of matter

All matter consists of some distribution of atoms or molecules.

In a solid: The atoms, held together by forces that are mainlyelectrical, are located at specific positions with respect to oneanother and vibrate about those positions.

At low temperatures

The vibrating motion is slight and the atoms can be consideredessentially fixed.

As energy is added to the material,

The amplitude of the vibrations increases.

A vibrating atom can be viewed as being bound

in its equilibrium position by springs attached to

neighboring atoms. A collection of such atoms

and imaginary springs is shown in Fig.1.

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Structure of matter

We can picture applied external forces ascompressing these tiny internal springs.

When the external forces are removed, the solidtends to return to its original shape and size.Consequently, a solid is said to have elasticity.

An understanding of the fundamental propertiesof these different states of matter is important inall the sciences, in engineering, and in medicine.

Forces put stresses on solids, and stresses canstrain, deform, and breakthose solids, whetherthey are steel beams or bones.

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Solid Classification

Solids can be classified as either:

crystalline: NaCl,

or amorphous: Glass

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Stress- Strain

Examine the effect of forces on a body 1-stretched,

compressed,

bent,

Twisted

Elasticityis the property of a body that tends

to return the body to its original shape afterthe force is removed.

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Longitudinal Stretch and

Compression Stress, S

Longitudinal Strain, St

Hook`s Law

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A Spring

energy E stored in the spring is given by

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Fatigue

Fatigueis the progressive and localized structural

damage that occurs when a material is subjected tocyclic loading.

Fatigue life, Nf, is the number of stress cycles of a

specified character that a specimen sustains before

failure of a specified nature occurs.

Surface fatigue: Surface fatigue is a process by which

the surface of a material is weakened by cyclic loading.

Fatigue wearis produced when the wear particles aredetached by cyclic crack growth of microcracks on the

surface. These microcracks are either superficial cracks

or subsurface cracks.October 17, 2014 10


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Bone Fracture: Energy Considerations

Knowledge of the maximum energy that parts

of the body can safely absorb allows us to

estimate the possibility of injury under various

circumstances.

Assume that the bone remains elastic until

fracture, the corresponding force is

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Example

A leg bone 90 cm and an average

area of about 6 cm2

Y=14

1010

dyn/cm2

This is the amount of energy in the impact of a

70-kg person jumping from a height of 56 cm(1.8 ft), given by the product mgh.

E= 70x10xH=384 J

H=384/700=0.56 m= 0.56 cm

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Impulsive Forces

In a sudden collision, a large force is exerted for

a short period of time on the colliding object.

For example, if the duration of

a collision is 6

103

sec and the change in momentum is 2 kg m/sec, the

average force that acted during the collision is

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Fracture Due to a fall: Impulsive

Force Considerations

The magnitude of the force that causes the damage

is computed

the duration of the collision Dtis difficult to

determine precisely

If the colliding objects are hard, very short~ few

milliseconds If the objects is soft and yields during the collision,

the duration of the collision is lengthened, and as a

result the impulsive force is reduced.October 17, 2014 14


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Example When a person falls from a height h, his/her

velocity on impact with the ground, neglectingair friction

W=mg

After the impact the body is at rest : mvf= 0

Measuring time is a problem

Vertical fall Dt=10-2sec

bends his/her knees or falls on a soft surfaceOctober 17, 2014 15


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Table 3.1, the force per unit area that may

cause a bone fracture is 109dyn/cm2

person falls flat on his/her heels, the area of

impact may be about 2 cm2.

Body of mass of 70 kg, Dt = 102

sec

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Airbags: Inflating Collision

Protection Devices

The impact force may also be calculated from

the distance the center of mass of the body

travels during the collision under the action of

the impulsive force.

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30 cm

Decelerating force, F

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For A =1000 cm2

At an impact velocity of 70 km/h

F= 4.45

106 dyn Stress= 4.45103dyn/cm2< The estimated

strength of body tissue.

At a 105-km F= 1010dyn

Stress= 107dyn/cm2. probably injure the

passengerOctober 17, 2014 18


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Whiplash Injury

the impact is sudden, as in a rear-end collision,

the body is accelerated in the

forward direction by the backof the seat,

the unsupported neck is then suddenly yanked

back at full speed.

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Falling from Great Height

Falling on a hard surface

Cause injury Energy=mgh=1/2 mv2

Falling on a soft surface Example:

decelerating impact force acts over a distance

of about 1 m, the average value of this forceremains below the magnitude for serious

injury even at the terminal falling velocity of

62.5 m/sec (140 mph).

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Nonomaterial Nanotechnology

is the production of functional materials andstructures in the range of 0.1 to 100 nanometers

one hydrogen atom is 0.1 to 0.2 nm and of asmall bacterium about 1,000 nm

Nanotechnologies are predicted to revolutionize:

(a) the control over materials properties at ultrafinescales; and

(b) the sensitivity of tools and devices applied in

various scientific and technological fields.October 17, 2014 21

physical or chemical methods


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Nanomaterials

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It studies materials with morphological

features on the nanoscale, and especially

those that have special properties stemming

from their nanoscale dimensions.

A bulk material should have constant physicalproperties regardless of its size,

At the nanoscale this is often not the case. Size-

dependent properties are observed such as quantum

confinementin semiconductorparticles, and

superparamagnetismin magneticmaterials, etc..
http://en.wikipedia.org/wiki/Nanoscalehttp://en.wikipedia.org/wiki/Quantum_confinementhttp://en.wikipedia.org/wiki/Quantum_confinementhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Superparamagnetismhttp://en.wikipedia.org/wiki/Magnetichttp://en.wikipedia.org/wiki/Magnetichttp://en.wikipedia.org/wiki/Superparamagnetismhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Quantum_confinementhttp://en.wikipedia.org/wiki/Quantum_confinementhttp://en.wikipedia.org/wiki/Nanoscale


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Some recent publication in


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Some recent publication in

dentistry material science

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Figure 1. Schematics and

transmission electron

microscopic images of

composites studied. A.

Composite with nanometricparticles ( 60,000

magnification). B. Composite

with nanocluster particles

(300,000 magnification). C.Composite with hybrid fillers

(300,000 magnification).

nm: Nanometers. APS:

Average particle size. m:

micrometerOctober 17, 2014 25


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Assignment

Solve the following problems

1, 3, 5

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down_1227ch 3 mechanical properties.ppt

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