MECHANICAL PROPERTIES OF MATERIALS

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MECHANICAL PROPERTIES OF MATERIALS

• Thursday, 3 March 2005

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MECHANICAL PROPERTIES AND TESTING OF MATERIALS

• The Tensile Test
• Compression
• The Bend Test
• The Hardness Test
• The Impact Test
• The Fatigue Test
• The Creep Test

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Structure-Property-Processing Relationship
Figure 1The three-part relation- ship between
structure, Properties, and processing Method.
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Properties of A Material

• Mechanical Properties
• How a material responds
• To an applied force, include strength and
  ductility.
• To a sudden, intense blow (impact)
• To a continually cycled through an alternating
  force (fatigue)
• To a high temperatures (creep)
• To an abrasive conditions (wear)
• Also determine the ease with which a material can
  be deformed into a useful shape.
• Physical Properties electrical, magnetic,
  optical, thermal, elastic, and chemical behaviour
  depend on both structure and processing of a
  material.

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The Tensile Test

• Measures the resistance of a material to a static
  or slowly applied force.
• Strength
• Ductility
• Toughness
• Elastic Modulus
• Strain Hardening
• Tension-test specimen
• Solid and round (ASTM lo50mm and ?12.5mm)
  standard, flat-sheet or tubular.

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Use of the Stress-Strain Diagram

• Tensile strength the stress that corresponds to
  the maximum load in a tensile test.
• Ductility the ability of material to be
  permanently deformed without breaking when a
  force is applied.
• Modulus of elasticity Youngs modulus, or the
  slope of the stress-strain curve.
• Toughness a qualitative measure of the impact
  properties of a material. A material that resists
  failure by impact is said to be tough.
• Strain hardening

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Stress-Strain Curves
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Mechanical Properties of Various Materials at
Room Temperature
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The Conversions from Load-Gage Length to
Stress-Strain

• Engineering stress ? F/Ao
• Engineering strain ? (l-lo)/lo
• Where
• Ao the original cross sectional area of the
  specimen before the test begins
• lo the original distance between the gage marks
• l the distance between the gage marks after
  force F is applied

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Table 1 Tensile Test Data of a 0.505 in. diameter
Al-alloy test bar
Measured Measured Calculated Calculated
Load (lb) Gage Length (in) Stress (psi) Strain (in./in.)
0 2.000 0 0
1000 2.001 5000 0.005
3000 2.003 15000 0.0015
5000 2.005 25000 0.0025
7000 2.007 35000 0.0035
7500 2.030 37500 0.0150
7900 2.080 39500 0.0400
8000 (max.load) 2.120 40000 0.0600
7950 2.160 39700 0.0800
7600 (fracture) 2.205 38000 0.1025
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Examples 1

• Convert the load-gage length data in Table 1 to
  engineering stress and strain and plot a
  stress-strain curve and calculate
• The tensile strength
• The elongation
• The engineering stress at fracture
• An aluminium rod is to widhstand an applied force
  of 45000 pounds. To assure a sufficient factor of
  safety, the max. allowable stress on the rod is
  limited to 25000 psi. The rod must be at least
  150 in. long but must deform elastically no more
  than 0.25 in. when the force is applied. Design
  an appropriate rod.

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Properties Obtained from the Tensile Test

• Yield Strength
• Tensile Strength
• Elastic Properties
• Ductility
• True Stress and True Strain
• Effect of Temperature
• Effect of Deformation Rate

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Yield Strength

• The strength at which plastic deformation becomes
  noticable.
• The stress required for dislocation to slip (in
  metals).
• The stress that divides the elastic and plastic
  behaviour of the material.
• When designing a part that will not plastically
  deform
• Select a material that has a high yield strength
• Make the component large so that the applied
  force produces a stress that is below the yield
  strength.

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• OFFSET YIELD STRENGTH in some materials, the
  stress at which the material changes from elastic
  to plastic behaviour is not easily detected.
• The stress-strain curve for certain low-carbon
  steels displays a double yield point.

Figure 2 (a) Determining the 0.2 offset yield
strength in gray cast iron and (b) Upper and
lower yield point behaviour in low carbon steel.
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Tensile Strength (UTS)

• The stress obtained at the highest applied force,
  which is the maximum stress on the engineering
  stress strain curve.
• If the specimen is loaded beyond its UTS, it
  begins to neck (locally deformed region). As the
  test progresses, the engineering stress drops
  further and the specimen finally fractures at the
  necked region. The engineering stress at fracture
  is known as breaking or fracture stress.

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Elastic Properties

• The ratio of stress to strain in the elastic
  region modulus of elasticity or Youngs modulus
  (after T. Young, 1773-1829), E.
• The modulus is a measure of stiffness of the
  material.
• Hookes law (after R. Hooke, 1635-1703)
• Modulus of elasticity, E ?/?
• Modulus of Resilience (Er)the area contained
  under the elastic portion of a stress-strain
  curve, is the elastic energy that a material
  absorbs during loading and subsequently releases
  when the load is removed.
• Er yield strength/(2)(strain at yielding)
• Poissons ratio, ? (after S. D. Poisson,
  1781-1840)
• ? - ?lateral/ ? longitudinal

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Ductility

• Measures the amount of deformation that a
  material can withstand without breaking.
• elongation (lf-lo)/lox100
• reduction in area (Ao-Af)/Aox100
• Example 2
• The Al alloy in example 1 has a final gage length
  after failure of 2.195 in. and final diameter of
  0.398 in. at the fractured surface. Calculate the
  ductility of this alloy.

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True Stress and True Strain

• True Stress is the ratio of the load F to the
  actual (instantaneous) cross-sectional area A of
  the specimen.
• True stress, ? F/A
• True Strain, ? ln (l/lo)

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Strain Hardening

• We can represent the true stress-true strain
  curve by the equation
• ? K ?n
• Where
• K the strength coefficient.
• n the strain hardening (work hardening)
  exponent.
• Plotting the corrected curve in Fig. 2.5c on a
  log-log graph approximately a straight line
  (Fig. 2.5d). The slope of the curve is equal to
  exponent n. Thus, the higher the slope, the
  greater the strain hardening capacity of the
  material the stronger and harder it becomes as
  it is strained.

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Effect of Temperature
Figure 3 The effect of temperature (a) on the
stress-strain curve (b) on the tensile Properties
of an Al alloy.
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Effect of Deformation Rate

• Deformation rate is defined as the speed at which
  a tension test is being carried out, in units of,
  say m/s or ft/min.
• The strain rate is a function of the specimen
  length.
• Increasing the strain rate increases the strength
  of the material (strain-rate hardening).

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Compression

• Forging, rolling, extrusion are performed with
  the workpiece subjected to compressive forces.
• Compression test compressing a solid cylindrical
  specimen between two flat dies (platens). The
  friction between the specimen and the platens,
  the specimens cylindrical surface bulges
  (barreling).
• True stress-true strain curves for the tensile
  and compression tests for ductile material
  coincide. But this comparability does not hold
  true for brittle materials.

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Disk Test

• For brittle materials such as ceramic and
  glasses. The disk is subjected to compression
  between two hardened flat platens. When the
  material is loaded, tensile stress develop
  perpendicular to the vertical centerline along
  the disk, fracture begins, and the disk splits in
  half vertically. The tensile stress,? is uniform
  along the centerline and can be calculated as
• ? 2P/?dt
• P load at fracture
• d diameter of the disk
• t thickness of the disk.

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The Bend Test

• In brittle materials, failure occurs at the
  maximum load, where the tensile strength and
  breaking strength are the same. How about with
  ductile materials ?
• In many brittle materials- flaws at the surface ?
  Bend test
• By applying the load at 3 points and causing
  bending, a tensile force acts on the material
  opposite the midpoint. Fracture begins at this
  location.

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• Flexural strength or modulus of rupture describes
  the materials strength.
• Flexural strength
• 3FL/(2wh2)
• Flexural modulus is the modulus elasticity in
  bending and is calculated in the elastic region.
• Flexural modulus L3F/4wh3?

Figure 4 (a) The bend test often used for
Measuring the strength of brittle materials, and
(b) The deflection ?? obtained by bending.
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Hardness Test

• Measures the resistance to penetration of the
  surface of a material by a hard object.
• Brinell, Rockwell, Vickers, Knoop.
• Tensile strength (psi) 500 HB
• Hardness correlates well with wear resistance.

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Impact Test
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Impact Test

• To evaluate the brittleness of a material under a
  sudden, intense blow, in which a strain rate is
  extremely rapid.
• Charpy and Izod test.
• Izod for nonmetallic materials. The specimen may
  be either notched or unnotched.
• Knowing the initial and final elevations of the
  pendulum, we can calculate the difference in
  potential energy (impact energy absorbed by the
  specimen during failure).
• Toughness ??

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Properties Obtained from the Impact Test

• Transition temperature
• Results of a series of impact tests performed at
  various temperature.
• Notch sensitivity
• Caused by poor machining, fabrication or design
  concentrate stresses and reduce the toughness of
  the material. Compare the absorbed energy between
  notched and unnotched specimen.
• Relationship to the stress-strain diagram.

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The area contained within the true stress- True
strain curve is related to the impact Energy.
Although material B has a lower Yield strength,
it absorbs a greater energy Than material A.
Charpy V-notch properties for BCC Carbon steel
and a FCC ss. FCC crystal Structure typically
leads to higher Absorbed energies and no
transition Temp.
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The Fatigue Test

• A component often subjected to the repeated
  application of a stress below the yield strength
  of the material.
• Cyclical stress rotation, bending, or vibration.
• Although the stress is low, the material may fail
  after a large number of applications of the
  stress.

The rotating cantilever beam Fatigue test.
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3 Stages of Fatigue

• Tiny cracks initiates at the surface.
• The crack gradually propagates as the load
  continues to cycle.
• Sudden fracture of the material occurs when the
  remaining cross section of the material is too
  small to support the applied load.

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Results of the Fatigue Test

• Endurance limit the stress below which there is
  a probability that failure by fatigue will never
  occur design criterion.
• Fatigue life how long a component survives at a
  particular stress
• Fatigue strength max stress for which fatigue
  will not occur within a particular number of
  cycles, such as 500000 cycles (for Al and
  polymers which have no endurance limit).
• In some materials, endurance limit ?? half
  tensile strength.
• Endurance ratio endurance limit/tensile strength
  ?? 0.5

The stress-number of cycles to Failure (S-N)
curves for a tool Steel and an Al alloy.
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The Creep Test

• CREEP Plastic deformation at high temperatures
  stress high temperature.
• As soon as the stress is applied, the specimen
  stretches elastically a small amount depending on
  the applied stress and the modulus elasticity ??o
  of the material at high temperature.

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