Material Testing - bp025.k12.sd.us

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Material Testing
Material Testing
Reproducible evaluation of material properties
Static Testing
Material response to constant loading
Dynamic Testing
Material response to varying loading
conditions, including magnitude, cycling,
and mode
Static Material Testing
Evaluation of Material
Strength
Deformation
Fracture
Design requirement compliance
Standardized Tests
Tensile test
Compression test
Hardness test
Tensile Test
Uniaxial
A straight line axial force is
applied to a test sample
(typically in the y axis)
Hounsfield Tensometer
Image courtesy of NSW Department of Education and Training
Destructive
Force is applied until sample fails
Tensile Test
Standard Test Sample (dog bone)
Ensures meaningful and reproducible results
Uniform cross section
Tensile Test Procedure
Dog bone is created to test specifications
Dog bone is secured in tester
Tensile Test Procedure
A tension force (F) is applied to the dog bone
until failure occurs
Simultaneously the applied tension force (F)
and dog bone elongation (d) are recorded
F
d
A plot is created from the stored load
elongation data
Tensile Test Data
A
F
B
d
Test sample A and B are 230 red brass. Test
sample A has a diameter of 0.125 in. Test
sample B has a diameter of 0.375 in.
If both samples are tested to failure, will the
applied tension force and elongation be the
same for both tests? NO – Why?
Tensile Test Data
Load-elongation results are dependent upon
sample size
Larger sample indicates larger load-elongation
How can test data be manipulated to
represent a material and not an individual
test sample?
Tensile Test Data
To eliminate test results based on sample
size, calculate sample stress
Stress is load per unit area
Divide load (F) by the original test sample
cross-sectional area (A0)
stre ss =
lo ad
area
σ =
F
A
Tensile Test Data
Calculate the stress in the dog bone with a
430 lb applied force.
σ =
F
A
area =  r
2
area =  (0.0625 in.)
area = 0.0123in.
2
2
Tensile Test Data
Manipulating Elongation Results
To eliminate test results based on sample
size, calculate sample strain
Strain (e) - the amount of stretch per unit
length
 Elongation (d) under load divided by the
original Length (L0)
strain =
am ount of stretch
original length
Tensile Test Data
Calculate the strain in the dog bone with an
elongation of 0.0625in.
ε =
0.0625in.
1.000in.
= 0 .0 6 2 5
Tensile Test – Stress-Strain Curve
Tensile Test – Stress-Strain Curve
Initial response is linear
Stress and strain are proportional to one
another
Elastic Range
Proportional Limit (The stress at which
proportionality ceases)
Tensile Test – Stress-Strain Curve
Modulus of Elasticity (E)
The proportional constant (ratio of stress
and strain)
E =
σ
ε
=
stress
strain
A measure of stiffness – The ability of a
material to resist stretching when loaded
An inherent property of a given material
Tensile Test – Stress-Strain Curve
If the load is removed, the
test sample will return to
its original length
The response is elastic or
recoverable
Exaggerated stretch to
illustrate principle
Tensile Test – Stress-Strain Curve
Elastic Limit
Uppermost stress of elastic behavior
Elastic and proportional limit are almost
identical, with the elastic limit being slightly
higher
Tensile Test – Stress-Strain Curve
Resilience
The amount of energy per unit volume that a
material can absorb while in the elastic range
Area under the stress-strain curve
Why would this be important to designers?
Hint: car bumper
1 bh
2
Tensile Test – Stress-Strain Curve
Yield Point
When the elastic limit is exceeded
A very small increase in stress produces
a much greater strain
Most materials do not have a welldefined yield point
Tensile Test – Stress-Strain Curve
Offset Yield Strength
Defines the stress required to
produce a tolerable amount of
permanent strain
Common value is 0.2%
Tensile Test – Stress-Strain Curve
Plastic Deformation
Unrecoverable elongation beyond
the elastic limit
When the load is removed, only the
elastic deformation will be recovered
Tensile Test – Strength Properties
Stress Strain Curve
Plastic deformation represents failure
Part dimensions will now be outside of
allowable tolerances
Tensile Test – Stress-Strain Curve
Deformation
Test sample elongation
Cross-sectional area decreases
Load bearing ability increases – Why?
The material is getting stronger – How?
Tensile Test – Stress-Strain Curve
Weakest point is stretched and becomes
stronger
New weakest point is stretched and becomes
stronger, and so on
This keeps occurring until the decrease in
area overcomes the increase in strength
Tensile Test – Stress-Strain Curve
Tensile Strength
Load bearing ability peaks
Force required to continue
straining the test sample
decreases
Weakest location at the peak
continues to decrease in area –
Necking
Tensile Test – Stress-Strain Curve
Failure
If continued force is applied,
necking will continue until fracture
occurs
Ductility
Amount of plasticity before fracture
The greater the ductility, the more a
material can be deformed
Tensile Test – Samples
Compare the material properties of these
three metal samples
Tensile Test – Stress-Strain Curve
Brittleness
Material failure with little or no ductility
Lack of ductility, not lack of strength
Tensile Test – Stress-Strain Curve
Toughness
Work per unit volume required to fracture a material
Total area under the stress-strain curve from test
initiation to fracture (both strength and ductility)
Compression Test
Stress and strain relationships are similar to tension
tests – elastic and plastic behavior
Test samples must have large cross-sectional area
to resist bending and buckling
Material strengthens by stretching laterally and
increasing its cross-sectional area
Hardness Testing
Resistance to permanent deformation
Resistance to scratching, wear, cutting or
drilling, and elastic rebound
Brinell Hardness Test
A tungsten carbide ball is held with a 500lb
force for 15 sec into the material
The resulting crater is measured and
compared
Hardness Testing
Rockwell Test
A small diamond-tipped cone is forced into
the test sample by a predetermined load
Depth of penetration is measured and
compared
Image Resources
NSW Department of Education and Training (2011). Retrieved from
http://lrrpublic.cli.det.nsw.edu.au/lrrSecure/Sites/Web/tensile_testin
g/index.htm?Signature=%287e02281c-318a-461b-a8ed3394db0c4fe6%29

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