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longer proportional and deformation cannot be recoveredHooke’s law is no longer valid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