Mechanical Properties Chapter 4

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Mechanical PropertiesChapter 4
Professor Joe Greene CSU, CHICO
MFGT 041
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Chapter 4 Objectives

• Objectives
• Mechanical properties in solids (types of forces,
  elastic behavior and definitions)
• Mechanical properties of liquids_ viscous flow
  (viscous behavior and definitions)
• Viscoelastic materials (viscoelastic behavior and
  definitions, time dependent)
• Plastic stress-strain behavior (plastic behavior
  and definitions, interpretation and mechanical
  model of plastic behavior)
• Creep and Toughness
• Reinforcements and Fillers

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Viscoelastic Materials

• Polymers are Viscoelastic materials that exhibit
• liquid (viscous) or
• solid (elastic) properties
• Depending upon the time scale of the event
• Short time (fast) event will act like a solid
• Long time (slow) event will act like a liquid.
• Depending upon the temperature of the event
• Example, Silly Putty
• Roll into a ball and drop it to the ground and it
  BOUNCES like a solid
• Place it on a table and leave overnight and it
  will FLOW and flatten out into a puddle like a
  liquid.
• Heat up the silly putty and the drop it and it
  will STICK to the ground like a liquid.
• Chill silly putty to bellow room temperature and
  leave rolled up on a table and it will STAY
  rolled up at that cold temperature

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Fundamentals of Mechanical Properties

• Mechanical Properties
• Deal directly with behavior of materials under
  applied forces.
• Properties are described by applied stress and
  resulting strain, or applied strain and resulting
  stress.
• Example 100 lb force applies to end of a rod
  results in a stress applied to the end of the rod
  causing it to stretch or elongate, which is
  measured as strain.
• Strength ability of material to resist
  application of load without rupture.
• Ultimate strength- maximum force per cross
  section area.
• Yield strength- force at yield point per cross
  section area.
• Other strengths include rupture strength,
  proportional strength, etc.
• Stiffness resistance of material to deform under
  load while in elastic state.
• Stiffness is usually measured by the Modulus of
  Elasticity (Stress/strain)
• Steel is stiff (tough to bend). Some beds are
  stiff, some are soft (compliant)

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Fundamentals of Mechanical Properties

• Mechanical Properties
• Hardness resistance of materials to surface
  indentation or abrasion.
• Example, steel is harder than wood because it is
  tougher to scratch.
• Elasticity ability of material to deform without
  permanent set.
• Rubber band stretches several times and returns
  to original shape.
• Plasticity ability of material to deform outside
  the elastic range and yet not rupture,
• Bubble gum is blown up and plastically deforms.
  When the air is removed it deflates but does not
  return to original shape.
• The gum has gone beyond its elastic limit when it
  stretches, set it remains plastic, below the
  breaking strength of the material.
• Energy capacity ability of material to absorb
  energy.
• Resilience is used for capacity in the elastic
  range.
• Toughness refers to energy required to rupture
  material

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Mechanical Test Considerations

• Principle factors are in three main areas
• manner in which the load is applied
• condition of material specimen at time of test
• surrounding conditions (environment) during
  testing
• Tests classification- load application
• kind of stress induced. Single load or Multiple
  loads
• rate at which stress is developed static versus
  dynamic
• number of cycles of load application single
  versus fatigue
• Primary types of loading

compression
tension
torsion
flexure
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Standardized Testing Conditions

• Moisture
• 100F, 100 R.H.
• 1 Day, 7 Days, 14 Days
• Temperature
• Room Temperature Most common
• Elevated Temperature Rocket engines
• Low Temperature Automotive impact
• Salt spray for corrosion
• Rocker Arms on cars subject to immersion in NaCl
  solution for 1 Day and 7 Days at Room Temperature
  and 140 F.
• Acid or Caustic environments
• Tensile tests on samples after immersion in
  acid/alkaline baths.

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Stress

• Stress Intensity of the internally distributed
  forces or component of forces that resist a
  change in the form of a body.
• Tension, Compression, Shear, Torsion, Flexure
• Stress calculated by force per unit area. Applied
  force divided by the cross sectional area of the
  specimen.
• Stress units
• Pascals Pa Newtons/m2
• Pounds per square inch Psi Note 1MPa 1
  x106 Pa 145 psi
• Example
• Wire 12 in long is tied vertically. The wire has
  a diameter of 0.100 in and supports 100 lbs.
  What is the stress that is developed?
• Stress F/A F/?r2 100/(3.1415927 0.052 )
  12,739 psi 87.86 MPa

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Stress

• Example
• Tensile Bar is 10in x 1in x 0.1in is mounted
  vertically in test machine. The bar supports 100
  lbs. What is the stress that is developed? What
  is the Load?
• Stress F/A F/(widththickness)
  100lbs/(1in.1in ) 1,000 psi 1000 psi/145psi
  6.897 Mpa
• Load 100 lbs
• Block is 10 cm x 1 cm x 5 cm is mounted on its
  side in a test machine. The block is pulled with
  100 N on both sides. What is the stress that is
  developed? What is the Load?
• Stress F/A F/(widththickness) 100N/(.01m
  .10m ) 100,000 N/m2 100,000 Pa 0.1 MPa 0.1
  MPa 145psi/MPa 14.5 psi
• Load 100 N

100 lbs
1 cm
5cm
10cm
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Strain

• Strain Physical change in the dimensions of a
  specimen that results from applying a load to the
  test specimen.
• Strain calculated by the ratio of the change in
  length and the original length. (Deformation)
• Strain units (Dimensionless)
• When units are given they usually are in/in or
  mm/mm. (Change in dimension divided by original
  length)
• Elongation strain x 100

lF
l0
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Stress-Strain Diagrams

• Stress-strain diagrams is a plot of stress with
  the corresponding strain produced.
• Stress is the y-axis
• Strain is the x-axis

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Stiffness

• Stiffness is a measure of the materials ability
  to resist deformation under load as measured in
  stress.
• Stiffness is measures as the slope of the
  stress-strain curve
• Hookean solid (like a spring) linear slope
• steel
• aluminum
• iron
• copper
• All solids (Hookean and viscoelastic)
• metals
• plastics
• composites
• ceramics

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Modulus

• Modulus of Elasticity (E) or Youngs Modulus is
  the ratio of stress to corresponding strain
  (within specified limits).
• A measure of stiffness
• Stainless Steel E 28.5 million psi (196.5 GPa)
• Aluminum E 10 million psi
• Copper E 16 million psi
• Molybdenum E 50 million psi
• Nickel E 30 million psi
• Titanium E 15.5 million psi
• Tungsten E 59 million psi
• Carbon fiber E 40 million psi
• Glass E 10.4 million psi
• Composites E 1 to 3 million psi
• Plastics E 0.2 to 0.7 million psi

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Modulus Types

• Modulus Slope of the stress-strain curve
• Initial Modulus slope of the curve drawn at the
  origin.
• Tangent Modulus slope of the curve drawn at the
  tangent of the curve at some point.
• Secant Modulus Ratio of stress to strain at any
  point on curve in a stress-strain diagram. It is
  the slope of a line from the origin to any point
  on a stress-strain curve.

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Testing Procedure

• Tensile tests yield a tensile strain, yield
  strength, and a yield stress
• Tensile modulus or Youngs modulus or modulus of
  elasticity
• Slope of stress/strain
• Yield stress
• point where plastic
• deformation occurs
• Some materials do
• not have a distinct yield point
• so an offset method is used

Yield stress
1000 psi
Stress
Yield strength
Slopemodulus
0.002 in/in
Strain
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Expected Results

• Stress is measured load / original
  cross-sectional area.
• True stress is load / actual area.
• True stress is impractical to use since area is
  changing.
• Engineering stress or stress is most common.
• Strain is elongation / original length.
• Modulus of elasticity is stress / strain in the
  linear region
• Note the nominal stress (engineering) stress
  equals true stress, except where large plastic
  deformation occurs.
• Ductile materials can endure a large strain
  before rupture
• Brittle materials endure a small strain before
  rupture
• Toughness is the area under a stress strain curve

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Energy Capacity

• Energy Capacity ability of a material to absorb
  and store energy. Energy is work.
• Energy (force) x (distance)
• Energy capacity is the area under the
  stress-strain curve.
• Hysteresis energy that is lost after repeated
  loadings. The loading exceeds the elastic limit.

Stress
Strain
Elastic strain
Inelastic strain
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Creep Testing

• Creep
• Measures the effects of long-term application of
  loads that are below the elastic limit if the
  material being tested.
• Creep is the plastic deformation resulting from
  the application of a long-term load.
• Creep is affected by temperature
• Creep procedure
• Hold a specimen at a constant elevated
  temperature under a fixed applied stress and
  observe the strain produced.
• Test that extend beyond 10 of the life
  expectancy of the material in service are
  preferred.
• Mark the sample in two locations for a length
  dimension.
• Apply a load
• Measure the marks over a time period and record
  deformation.

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Creep Results

• Creep versus time

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Mechanical Properties in Liquids (Viscous Flow)

• Polymer Flow in Pressure Flow (Injection Molding)

FIGURE 2. (a) Simple shear flow. (b) Simple
extensional flow. (c) Shear flow in cavity
filling.(d) Extensional flow in cavity
filling. Ref C-MOLD Design Guide
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Viscous Flow

• Viscosity is a measure of the materials
  resistance to flow
• Water has low viscosity easy to flow
• Syrup has higher viscosity harder to flow
• Viscosity is a function of Shear Rate, Temp, and
  Pressure
• increase Shear Rate Viscosity Decreases
• Increase Temperature Viscosity Decreases

Ref C-MOLD Design Guide
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Newtonian and Non-Newtonian Flow

• Viscosity is a measure of the materials
  resistance to flow.
• Newtonian Material. Viscosity is constant
• Non-Newtonian Viscosity changes with shear rate,
  temperature, or pressure
• Polymers are non-Newtonian, usually shear thinning

Fig 4.4
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Viscosity Measurements

• Viscosity is a measure of the materials
  resistance to flow.
• Liquids (paints, oils, thermoset resins, liquid
  organics) Measured with rotating spindle in a cup
  of fluid, e.g., Brookfield Viscometer
• Resistance to flow is measured by torque.
• The spindle is rotated at several speeds.
• The fluid is heated to several temperatures.

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Viscosity Measurements

• Melts (plastic pellets, solid particles)
• Resistance to flow is measured by torque in
  cone-and-plate, e.g., Rheometrics viscometer
• The plates are heated and the toque is measured
• Resistance to flow is measured by flow through
  tube
• Capillary rheometer
• Melt Indexer

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Viscosity Testing

• Melt Flow Index

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Melt Index

• Melt index test measure the ease of flow for
  material
• Procedure (Figure 3.6)
• Heat cylinder to desired temperature (melt temp)
• Add plastic pellets to cylinder and pack with rod
• Add test weight or mass to end of rod (5kg)
• Wait for plastic extrudate to flow at constant
  rate
• Start stop watch (10 minute duration)
• Record amount of resin flowing on pan during time
  limit
• Repeat as necessary at different temperatures and
  weights

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Viscoelastic models

• Plastics exhibit viscoelastic behavior, to an
  applied stress
• Viscous liquid Continuously deform while shear
  stress is applied
• Elastic solid Deform while under stress and
  recover to original shape

Ref C-MOLD Design Guide
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Viscoelastic models

• Plastics exhibit viscoelastic behavior, to an
  applied stress
• Viscous liquid Simple dashpot
• Viscoelastic liquid Spring and Dashpot in series
  (Maxwell model)
• Viscoelastic solid Spring and Dashpot in
  Parallel (Voight model)
• Elastic solid Simple Spring
• Figure 4-6

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Viscoelastic models

• Time Dependence of Viscoelastic properties
• Viscous liquid Constant viscosity Newtonian
• Viscoelastic liquid Viscosity changes at
  different rates, e.g., higher shear rate reduces
  viscosity or Shear thinning plastics
• Viscoelastic solid Solid part has a memory to
  applied stress and needs time for the stress to
  reach zero after an applied load.
• Elastic solid Simple Spring Hooks Law on
  spring constant
• Figure 4-7