mechanical properties of metals
DESCRIPTION
Mechanical Properties of MetalsTRANSCRIPT
Mechanical Properties of Metals
0.0100.0080.0060.0040.0020.0000
100
200
300
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500
CONTINUED
Stre
ss (
MPa
)
Strain
Mechanical Properties• Stiffness - Elastic Modulus or Young’s Modulus (MPa)• Strength - Yield, Ultimate, Fracture, Proof, Offset Yield.
Measured as stress (MPa)• Ductility - Measure of ability to deform plastically without
fracture - Elongation, Area Reduction, Fracture Strain - (no units or mm/mm)
• Toughness, Resilience - Measure of ability to absorb energy (J/m3).
• Hardness - Resistance to indentation/abrasion (Various scales, e.g.; Rockwell, Brinell, Vickers.)
Stress and Strain• In a simplistic sense, stress may be thought
of as Load/Area.
• Similarly, strain is the deformation of the component/original length.
• A stress may be direct, shear, or torsional - leading to corresponding deformations.
• Stress cannot be measured directly, but deformation can be.
Direct Stress Examples
Load, P
P
Area Ao
Lo
L/2
L/2
Direct Stress - Tension
Load, P
P
Area Ao
Lo
L/2
L/2
Direct Stress - Compression
S P
Ao
e L
Lo
Engineering Stress
Engineering Strain
Tension Test
Typical Universal Testing Machine
ExtensometerMeasures L
Measures P
Modern Materials Testing System
HydraulicWedge Grips
SpecimenExtensometer
ASTM Tension Test Specimen
0.505" Dia
2” Gauge Length
Ao=0.20 in2
Lo
Raw Data ObtainedL
oad,
P (
kN)
Elongation, L (mm)
Uniform DeformationTotal Elongation
ElasticDeformation
X
Maximum Load, Pmax
Load, Pf
Engineering Stress-Strain CurveElongation
0.2% offset yield stress
Proportional Limit
E
E
(Ultimate)
Engineering Strain, e = L/Lo)
Eng
inee
ring
Str
ess,
S=
P/A
o
Sy
Su
Duke’s Quick Tip!
• Express Load in Newtons (N) and Area in mm2
to get Stress in MPa.
• Mechanical properties of metals are almost always given in MPa or ksi.
• Imperial units: Load in kips (1000 lbf) & Area as in2 gives Stress in ksi (kips/in2)
• 1000 psi = 1 ksi = 6.89 MPa
N
mm2 MPa
Hooke’s Law Elastic Deformation
• Elastic deformation is not permanent; it means that when the load is removed, the part returns to its original shape and dimensions.
• For most metals, the elastic region is linear. For some materials, including metals such as cast iron, polymers, and concrete, the elastic region is non-linear.
• If the behavior is linear elastic, or nearly linear-elastic, Hooke’s Law may be applied:
• Where E is the modulus of elasticity (MPa)
S Ee
Modulus of Elasticity - Stiffness
0.0100.0080.0060.0040.0020.0000
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CONTINUED
Str
ess
(M
Pa)
Strain
E S
e
(300 0)MPa
(0.015 0.0)2x105 MPa
Atomic Origin of Stiffness
Strongly Bonded
Weakly Bonded
Net
Inte
rato
mic
Forc
e
Interatomic Distance
E dF
dr
ro
Shear Stress and Strain
Shear Stress, Shear Strain,
shear stress, = Shear Load / Areashear strain, = angle of deformation (radians)shear modulus, G = /(elastic region)
G
She
ar S
tres
s
Shear Strain
Elastic Properties of Materials
• Poisson’s ratio: When a metal is strained in one direction, there are corresponding strains in all other directions.
• For a uniaxial tension strain, the lateral strains are constrictive.
• Conversely, for a uniaxial compressive strain, the lateral strains are expansive.
• i.e.; the lateral strains are opposite in sign to the axial strain.
• The ratio of lateral to axial strains is known as Poisson’s ratio, .
Poisson’s Ratio,
ex
ez
ey
ez
For most metals, 0.25 < < 0.35in the elastic range
Furthermore:
E 2G(1 )
Plastic DeformationS
tres
s
Strain0.002 0.002 0.002
Sy
Sy
Sy
Elastic Plastic
Most Metals - Al, Cu Clad Al-Alloys Low carbon Steel
Elastic Plastic
Elastic Plastic
Microstructural Origins of Plasticity
• Slip, Climb and Slide of atoms in the crystal structure.• Slip and Climb occur at Dislocations and Slide occurs
at Grain Boundaries.
Elastic and Plastic StrainS
tres
s
StrainPlastic
Elasticeeep
P
Total Strain
(e,S) e ee ep
ee S
Eep e ee
The 0.2% offset yield stressis the stress that gives a plastic(permanent) strain of 0.002.
Elastic Recovery
Strain
Str
ess
Loading
Unloading
Loading
Unloading
Reloading
elastic strainStrain
Ductility - EL% & AR%
• Elongation
• Area Reduction
EL% L f Lo
Lo
x 100
AR% Ao A f
Ao
x 100
Lo
Ao
Lf
Af
Ductile Vs Brittle Materials• Only Ductile materials will exhibit necking.
• Ductile if EL%>8% (approximately)
• Brittle if EL% < 5% (approximately)
X
XXA
B C
XD Brittle Ductile
A & B C & D
Eng
inee
ring
Str
ess
Engineering Strain
Toughness & Resilience
• Toughness: A measure of the ability of a material to absorb energy without fracture. (J/m3 or N.mm/mm3= MPa)
• Resilience: A measure of the ability of a material to absorb energy without plastic or permanent deformation.
(J/m3 or N.mm/mm3= MPa)
• Note: Both are determined as energy/unit volume
Toughness, Ut
Engineering Strain, e = L/Lo)
Eng
inee
ring
Str
ess,
S=
P/A
o
X
Ut S deo
e f
(Sy Su )
2
EL%
100
SuSy
X
Resilience, Ur
Engineering Strain, e = L/Lo)
Eng
inee
ring
Str
ess,
S=
P/A
o
Ur S deo
e y
Sy ey
2
Sy
2
2E
SuSy
E
ey
Typical Mechanical Properties
Material Yield Stress(MPa)
UltimateStress (MPa)
DuctilityEL%
Elastic Modulus(MPa)
Poisson’sRatio
1040 Steel 350 520 30 207000 0.301080 Steel 380 615 25 207000 0.302024 Al Alloy 100 200 18 72000 0.33316 Stainless Steel 210 550 60 195000 0.3070/30 Brass 75 300 70 110000 0.356-4 Ti Alloy 942 1000 14 107000 0.36AZ80 Mg Alloy 285 340 11 45000 0.29
Metals in annealed (soft) condition