Chapter 3 elasticity and strength of materials
Sponsored Links
This presentation is the property of its rightful owner.
1 / 26

Chapter 3 Elasticity and Strength of Materials PowerPoint PPT Presentation


  • 163 Views
  • Uploaded on
  • Presentation posted in: General

Chapter 3 Elasticity and Strength of Materials. References 1-Physics in biology and Medicine 3 rd e, Paul Davidovits 2- web sites 3- College Physics, 7 th e, Serway. Classification of matter. Matter is normally classified as being in one of three states:

Download Presentation

Chapter 3 Elasticity and Strength of Materials

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Chapter 3 Elasticity and Strength of Materials

References

1-Physics in biology and Medicine 3rd e, Paul Davidovits

2- web sites

3- College Physics, 7th e, Serway


Classification of matter

  • Matter is normally classified as being in one of three states:

  • A solid has a definite volume and shape.

  • Aliquid has a definite volume but no definite shape.

  • A gas it has neither definite volume nor definite shape. 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 a plasma.


Structure of matter

  • All matter consists of some distribution of atoms or molecules.

  • In a solid: The atoms, held together by forces that are mainly electrical, are located at specific positions with respect to one another and vibrate about those positions.

  • At low temperatures

  • The vibrating motion is slight and the atoms can be considered essentially 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.


Structure of matter

  • We can picture applied external forces as compressing these tiny internal springs.

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

  • An understanding of the fundamental properties of these different states of matter is important in all the sciences, in engineering, and in medicine.

  • Forces put stresses on solids, and stresses can strain, deform, and break those solids, whether they are steel beams or bones.


Solid Classification

  • Solids can be classified as either:

  • crystalline: NaCl,

  • or amorphous: Glass


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 after the force is removed.


Longitudinal Stretch and Compression

  • Stress, S

  • Longitudinal Strain, St

  • Hook`s Law


A Spring

energy E stored in the spring is given by


Fatigue

  • Fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic 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 wear is produced when the wear particles are detached by cyclic crack growth of microcracks on the surface. These microcracks are either superficial cracks or subsurface cracks.


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


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


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 6×10−3 sec and the

  • change in momentum is 2 kg m/sec, the average force that acted during the collision is


Fracture Due to a fall: Impulsive Force Considerations

  • The magnitude of the force that causes the damage is computed

  • the duration of the collision Dt is 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.


Example

  • When a person falls from a height h, his/her velocity on impact with the ground, neglecting air friction

  • W=mg

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

  • Measuring time is a problem

  • Vertical fall Dt=10-2 sec

  • bends his/her knees or falls on a soft surface


  • Table 3.1, the force per unit area that may cause a bone fracture is 109 dyn/cm2

  • person falls flat on his/her heels, the area of impact may be about 2 cm2.

    Body of mass of 70 kg, Dt = 10−2 sec


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.

30 cm

v

Decelerating force, F


For A =1000 cm2

  • At an impact velocity of 70 km/h

  • F= 4.45×106 dyn

  • Stress= 4.45×103 dyn/cm2 < The estimated strength of body tissue.

  • At a 105-km

  • F= 1010 dyn

  • Stress= 107 dyn/cm2. probably injure the passenger


Whiplash Injury

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

  • the body is accelerated in the

    forward direction by the back

    of the seat,

    the unsupported neck is then suddenly yanked back at full speed.


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 force remains below the magnitude for serious injury even at the terminal falling velocity of 62.5 m/sec (140 mph).


Nonomaterial

  • Nanotechnology

  • is the production of functional materials and structures in the range of 0.1 to 100 nanometers

  • one hydrogen atom is 0.1 to 0.2 nm and of a small bacterium about 1,000 nm

  • Nanotechnologies are predicted to revolutionize:

  • (a) the control over materials properties at ultrafine scales; and

  • (b) the sensitivity of tools and devices applied in various scientific and technological fields.

physical or chemical methods


Nanomaterials

  • 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 physical properties regardless of its size,

  • At the nanoscale this is often not the case. Size-dependent properties are observed such as quantum confinement in semiconductor particles, and superparamagnetism in magnetic materials, etc..


Example

  • For example,

  • the bending of bulk copper (wire, ribbon, etc.) occurs with movement of copper atoms/clusters at about the 50 nm scale.

  • Copper nanoparticles smaller than 50 nm are considered super hard materials that do not exhibit the same malleability and ductility as bulk copper.


Some recent publication in dentistry material science


  • Figure 1. Schematics and transmission electron microscopic images of composites studied. A. Composite with nanometric particles (× 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. μm: micrometer


Assignment

  • Solve the following problems

  • 1, 3, 5


  • Login