materials for automobiles ceen0903 summer 2011 classes of materials metals made of metallic...
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Materials for Automobiles
CEEN0903 Summer 2011
Classes of MaterialsMetals
Made of metallic elements: Sn, Cu, Fe
CeramicsCompounds between metallic and nonmetallic
elements: Sodium Carbonate (Na2CO3), Quartz
Includes both clay and glass
PolymersMost are organic compounds: PVC, Isoprene Includes plastic and rubber
Frame: MetallicWindows: Ceramic
Tires: Polymeric
Stress and StrainThree types of loads
Tensile
Compressive
Shear
Stress and StrainTension Tests
Specimen with a round cross-section is deformed until fracture
Record force versus elongation Elongation is dependent on cross-sectional area
Engineering Stress:
F is the load applied perpendicular to the cross section in units of newtons (N) or pounds force (lbf)
A0 is the original cross-sectional area of the specimen
Stress units are megapascals (MPa), 1 MPa=106 N/m2
ExampleHigh Heeled Shoes
One shoe has a heal with a diameter of 2cmOne shoe has a heal with a diameter of 6cmCalculate the stress on a women’s heal for each
shoeGiven:
Woman weighs 120lb (54 kg)g=9.8 m/s2
Assume the weight is equally distributed between the two feet, and ignore the presence of the toe part of the shoe
Stress and StrainEngineering Strain:
l0 is the original length of the specimen
li is the instantaneous length of the specimen
Δl is the change in lengthStrain is unitless but sometimes expressed as a
percentage, in which case it is multiplied by 100
Tension Test
Elastic DeformationHooke’s Law: σ=Eε
True for most metals in tension at low levels of stress and strain
E is the modulus of elasticity or Young’s modulusStiffness of a materialMetals: 45 GPa<E<407 GPaCeramics: 70 GPa<E<500 GPaPolymers: 0.007 GPa<E<4 GPa
Deformation in which stress and strain are proportional is called elastic deformation Deformation can be recovered upon removing the force
ExampleA piece of copper originally 305 mm (12 in.) long
is pulled in tension with a stress of 276 Mpa (40,000 psi). If the deformation is entirely elastic, what will be the resulting elongation?
Equations:
σ=Eε
Plastic DeformationBeyond a certain strain, stress and strain are no
longer proportional and deformation cannot be recoveredHooke’s law is no longer valid
Stress level at which deformation becomes plastic is called the yield strength, σy
Yield strength is a measure of resistance to plastic deformation and ranges from 35 MPa to 1400 MPa for metals
Stress-Strain Behavior
Toughness: ability of a material to absorb energy up to fracture
DuctilityDuctility measures the degree of plastic
deformation at fractureA material with little or no plastic deformation
prior to fracture is termed brittleA material with a large amount of deformation
prior to fracture is ductile
Ductile materials undergo necking prior to fractureNecking occurs after the maximum stress is
reached and all further deformation only occurs at the neck
Material Properties
Ceramics Polymers Metals
Elastic Modulus
70-500 GPa 0.007-4 GPa 45-407 GPa
Yield Strength
High Low Varies
Toughness Small Small Large
Ductility Brittle Ductile Ductile
Car Part Windows Seats, TiresFrame, Body, Engine
Metallic Car PartsFrame and Body
Soft/Formable
Light weight
Ex: Steel (Fe, C) look up type
Considerations: Quick acceleration Fuel economy Stability Crash-worthiness
Current research: Al and Mg instead of steel
Engine and Axels
Hard
Strong at high temperatures
Corrosion resistant
Ex: Cast Iron (Fe)
Axel- cyclic loading (different from constant loading)
Considerations: Vibration dampening both to
prevent fracture and for passenger comfort
Car WindowsSide Windows
Tempering Glass heated Cooled rapidly to room
temperature
Properties Compressive surface
stress on outside Tensile stress inside Higher fracture strength
Windshield
Tempered glass sandwich Polymer layer in the
middle Cracks run sideways and
not through the middle
EU versus US standards
Car TiresRubber
Vulcanization:Polymer chains cross-linked by heating the
material up and adding sulfur
Car Tires
Steel
Tempered Glass
Rubber
Tempered Glass
Sandwich
Cast Iron
Key Equations
Questions?
σ=Eε
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