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Strong stuff!
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What: The stuff we build ships, subs
and planes out of!
Why: 1) So we can determine the best
material to build with.
2) So we can build an efficient
(weight and $$) and strong structure.
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USS Cole:
Tough Stuff
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Classifying Loads on Materials (5.1)
Normal Load - Force applied perpendicular to the
supporting material. Compression or buckling?
Normal Tension
Load
Normal
Compression Load
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Classifying Loads on Materials (5.1)
Shear Load - Material supporting the load is parallel
to the load. Similar to torsion/twisting.
Shearing
Load
Bending – Load is applied out of the plane. This is the
most common failure mode on a ship. Buckling may lead
to bending.
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Stress and Strain (5.2)
Tensile, Compressive and Shear “Stress” - Load
per unit area
Force(lb)
Stress( psi) 
Area(in 2 )
Note: This equation does not apply to bending.
Where:
load (F) is in pounds
the cross-sectional area (A) is in inches2
stress is in pounds per square inch (psi).
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Stress and Strain (5.2)
Example: What is the compressive stress
in the three “2 by 4’s” that are
supporting a 50 gallon water drum?
Force=50 gal x 8 lb/gal=400#
Area= 3 x (2” x 4”) = 24 in2
400lb

 16 psi
2
24in
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Stress and Strain (5.2)
If a material is loaded, some elongation occurs.
This is found by:
e  L  L0
Where:
L is the loaded length in feet
L0 is the original length in feet.
Strain compares the elongation of a material to
its original length (it is often measured in %):
e

L0
Ex: The strain for a 10 foot piece of
steel compressing 2.4” is 2.4/120=2%
Strain in Materials
• Steel will strain 2-30% before
fracturing
• Aluminum is similar
• Fiberglass is about 1%
• Plastics may go 300%
• Wood is about 15%
• Ceramics are usually less than 0.2%
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Stress-Strain Diagram (5.3)
• The values of stress versus strain for a given
material can be easily plotted. Relate the
stress/strain curve to the load/elongation curve.
• Evaluation of the diagram allows us to study the
behavior of the material and assess its
properties.
• Each material has a different stress-strain
diagram.
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Stress-Strain Diagram (5.3)
T.S.
Slope = E
3
Strain Hardening
5
Fracture
sy
2
S
t
r
e
s
s
Note:
1. Original unstressed condition.
2. Yield Strength
3. Tensile Strength
5. Fracture Strength
Plastic Region
Elastic Region
1
4
Strain
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Stress-Strain Diagram (5.3)
1-2: Elastic Region - If the
material is unloaded prior
to point 2, the material will
return to its original
condition. The stress and
strain are related by the
Modulus of Elasticity (E).
This relationship is given
by:
stress 
E ( psi ) 
strain


T.S.
Slope = E
3
Strain Hardening
s
5
Fracture
sy
2
S
t
r
e
s
s
Plastic Region
Elastic Region
1
4
e Strain
E for steel is 30,000,000 psi, alum is 10,000,000, wood is around 1E6
If E is constant, then the material is “linear elastic”
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Stress-Strain Diagram (5.3)
Beyond point 2 is the
Plastic Region
because if a material
is unloaded in this
region it will not
return to its original
condition. There will
be permanent
deformation.
T.S.
Slope = E
3
Strain Hardening
s
5
Fracture
sy
2
S
t
r
e
s
s
Plastic Region
Elastic Region
1
4
e Strain
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Stress-Strain Diagram (5.3)
If the material was
unloaded at point 3, its
final condition would be
at point 4. We know this
because the behavior of
the material would be in
accordance to the
Modulus of Elasiticity.
The distance between
points 1 and 4 indicate
the amount of permanent
deformation. On a ship
this is called the hungry
horse look.
T.S.
Slope = E
3
Strain Hardening
s
5
Fracture
sy
2
S
t
r
e
s
s
Plastic Region
Elastic Region
1
4
e Strain
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Stress-Strain Diagram (5.3)
Reloading the
material from point 4,
the curve will follow
back to point 3. Note
that the material now
has a higher yield
strength. Raising
the yield strength by
permanently
straining the material
is called Strain
Hardening.
T.S.
Slope = E
3
Strain Hardening
s
5
Fracture
sy
2
S
t
r
e
s
s
Plastic Region
Elastic Region
1
4
e Strain
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Stress-Strain Diagram (5.3)
The Tensile Strength
is the largest value
of stress on the
diagram and is
shown at point 3. It
is the maximum
stress which the
material can support.
T.S.
Slope = E
3
Strain Hardening
s
5
Fracture
sy
2
S
t
r
e
s
s
Plastic Region
Elastic Region
1
4
e Strain
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Stress-Strain Diagram (5.3)
If stretched beyond
point 3, the stress
decreases as
necking occurs.
Fracture will occur at
point 5.
T.S.
Slope = E
3
Strain Hardening
s
5
Fracture
sy
2
S
t
r
e
s
s
Plastic Region
Elastic Region
1
4
e Strain
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Material Properties (5.4)
• “Material properties” describe the
characteristics and performance of a
material.
• For example, these are what we compare
when approving a material for repairs.
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Material Properties (5.4)
• Strength (=the maximum allowable Stress) - is the ability
to resist failure. Strength is quantified from points
on the stress-strain curve:
• yield stress (y)(=max point of elastic range)
• ultimate tensile strength (UTS)(=max point on curve)
– Carbon steels and metal alloys have higher strength
than pure metals.
– Ceramics: high strength (but brittle).
• y ranges from 500 - 300,000 psi
• UTS ranges from 700 - 450,000 psi
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Material Properties (5.4)
• Hardness - A measure of the materials ability to
resist plastic deformation (i.e. indentation,
abrasion, and wear).
• Hardness is quantified by Rockwell, Brinnell, or
other hardness tests.
– Indenter pressed into specimen and depth of
penetration is measured and compared.
• Hardness and tensile strength are closely
related in metals.
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Material Properties (5.4)
• Ductility - Ability to plastically deform before
failure.
• Measure by percent elongation (i.e. “Stretch”)
of tensile test specimens.
• Examples include: some aluminum, low
carbon steel.
• “Ductile” is the opposite of “brittle”.
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Material Properties (5.4)
• Brittleness - A measure of a material’s inability
to deform before failure.
• Examples include glass, most composites,
high carbon steels, and many ceramic
materials.
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Material Properties (5.4)
Comparison of Brittle versus Ductile Material on a
Stress-Strain Diagram
Brittle
s
Ductile
S
t
r
e
s
s
Brittle
e Strain
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Material Properties (5.4)
• Toughness - A measure of a material’s ability to
absorb energy.
• Toughness is the area under the stress-strain
curve. Units of toughness are in-lb/in3 (i.e.
energy per unit volume).
• Can also be relatively measured by a “Charpy
V-notch test”.
• What needs to be tough, versus strong?
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Material Properties (5.4)
• Charpy V-Notch Test:
– Single material: different specimens at
different temperatures.
– Charpy tests evaluate the impact toughness
of a material as a function of temperature.
– See Figure 5.6 for a picture of the test
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Material Properties (5.4)
CT
h o
a u
r g
p h
y n
e
s
s
(in-lb)
Ductile Behavior
Heating most materials
will make them more
ductile! The opposite is
true too!
Brittle Behavior
Transition
Temperature (often near 0 degrees C)
Temperature (F) T
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Material Properties (5.4)
• Low temps; material is brittle so little energy
needed to fracture.
• At high temperatures, more ductile so greater
energy to fracture.
• The Transition Temperature is the boundary
between brittle and ductile behavior.
Important parameter in metal selection!
– Use alloys only in temps well above transition
range or catastrophies like Liberty Ships in WWII
may happen again!
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Material Properties (5.4)
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Material Properties (5.4)
FATIGUE
s
S
t
r
e
s
s
Steel
Endurance Limit
Aluminum
(psi)
Number of Loading Cycles, N
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Non-Destructive Testing (5.5)
• Inspections for material defects to ensure
quality control in acquisition and after
installation.
• Three Most Common Tests for External
(Surface) Defects are:
– Visual
– Dye Penetrant
– Magnetic Particle
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Non-Destructive Testing (5.5)
• Visual Testing (VT)
– Done during routine maintenance. “First
line of defense.”
– Quick, easy, and cheap.
– Examines surface of a material only.
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Non-Destructive Testing (5.5)
Dye Penetrant Testing (PT)
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Non-Destructive Testing (5.5)
• Magnetic Particle Testing (MT)
– Ferromagnetic materials only.
– Align the filings with defects.
– Detects surface and shallow
subsurface flaws and weld
defects.
– Power source is required.
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Non-Destructive Testing (5.5)
• Three Most Common Test for
Internal (Subsurface) Defects
are:
– Radiographic (RT)
– Ultrasonic (UT)
– Eddy Current Tests
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Non-Destructive Testing (5.5)
• Radiographic Testing (RT)
– Expose photographic film to x-ray sources.
– Detects internal flaws of thin or thick
sections and provides a permanent record.
– Requires trained technicians and presents
radiation hazards during testing.
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Non-Destructive Testing (5.5)
• Radiographic Testing (RT)
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Non-Destructive Testing (5.5)
• Ultrasonic Testing (UT)
– Sends sound waves through a material.
– Good for all metals and nonmetallic items.
– Detects deep flaws in tubing, rods, and
joints.
– Equipment is portable but requires a
trained technician.
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Non-Destructive Testing (5.5)
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Non-Destructive Testing (5.5)
• Eddy Current Testing
– Involves magnetic field variations.
– Detects seams and cracks in tubing.
– Good only for limited penetration depth on
very conductive materials.
– Being replaced by ultrasonic testing.
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Non-Destructive Testing (5.5)
• Additional Non Destructive Test which you are
likely to encounter:
 Hydrostatic Test.
– A system is isolated and pressurized.
– Inspected for leaks or the ability to hold
pressure.