# bc.inter.edufacultadeherreraMaterials_20Science by ewghwehws

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```									       ELASTIC DEFORMATION
1. Initial       2. Small load    3. Unload

bonds
stretch

initial

F

Elastic means reversible!
PLASTIC DEFORMATION (METALS)
1. Initial   2. Small load              3. Unload

F

Plastic means permanent!   linear               linear
elastic              elastic

plastic
ENGINEERING STRESS
• Tensile stress, s:               • Shear stress, t:

Ft
s
Ao
original area
N/m2 or lb/in2
ENGINEERING STRAIN
• Tensile strain:
/2
l l f  lo
    
lo     lo                            Lo
wo
/2
• Shear strain:
L/2        L/2
/2

 = tan           Strain is always
dimensionless.
/2 - 

/2             /2
LINEAR ELASTIC PROPERTIES
• Modulus of Elasticity, E:
(also known as Young's modulus)
• Hooke's Law:
s=E
• Poisson's ratio, n:
x  y
n  
z  z
Units:
E: [GPa] or [psi]
n: dimensionless
STRESS-STRAIN TESTING
• Typical tensile specimen   • Typical tensile
test machine
TENSILE STRENGTH, TS
• Maximum possible engineering stress in tension.

• Metals: occurs when noticeable necking starts.
• Ceramics: occurs when crack propagation starts.
• Polymers: occurs when polymer backbones are
aligned and about to break.
PLASTIC (PERMANENT)
DEFORMATION
(at lower temperatures, T < Tmelt/3)
• Simple tension test:
YIELD STRENGTH, sy
• Stress at which noticeable plastic deformation has
occurred.
when p = 0.002
tensile stress, s
sy

engineering strain, 
p = 0.002
DUCTILITY and %EL
L f  Lo
• Plastic tensile strain at failure: %EL           x100
Lo

Ao  A f
• Another ductility measure:     %AR           x100
Ao
RESILIENCE, Ur
• Resilience is the capacity of a material to absorb
energy when it is deformed elastically and then, upon

1        sy
2

U r  s y y 
2         2E
TOUGHNESS
• Energy to break a unit volume of material
• Approximate by the area under the stress-strain
curve.
Engineering     smaller toughness (ceramics)
tensile                           larger toughness
stress, s                         (metals, PMCs)

smaller toughness-
unreinforced
polymers

Engineering tensile strain, 

```
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