material testing. reproducible evaluation of material properties material response to varying...
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Material Testing
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Material TestingReproducible evaluation of material properties
Material response to varying loading conditions, including magnitude, cycling, and mode
Dynamic Testing
Material response to constant loadingStatic Testing
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Static Material Testing
StrengthDeformationFractureDesign requirement compliance
Tensile testCompression testHardness test
Evaluation of Material
Standardized Tests
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Tensile TestUniaxial
A straight line axial force is applied to a test sample(typically in the y axis)
DestructiveForce is applied until sample fails
Image courtesy of NSW Department of Education and Training
Hounsfield Tensometer
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Tensile Test
Ensures meaningful and reproducible results
Uniform cross section
Standard Test Sample (dog bone)
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Tensile Test ProcedureDog bone is created to test specifications
Dog bone is secured in tester
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Tensile Test ProcedureA 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
A plot is created from the stored load elongation data
F
d
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Tensile Test Data
F
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?
A
B
NO – Why?
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Tensile Test Data
Load-elongation results are dependent upon sample size
How can test data be manipulated to represent a material and not an individual test sample?
Larger sample indicates larger load-elongation
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Tensile Test DataTo eliminate test results based on sample size, calculate sample stress
Divide load (F) by the original test sample cross-sectional area (A0)
Stress is load per unit area
lo
stread
ss =area
Fσ = A
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Tensile Test DataCalculate the stress in the dog bone with a 430 lb applied force.
2area = r2area = (0.0625 in.)
2area = 0.0123in.
Fσ = A
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Tensile Test DataManipulating 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)
amount of stretchstrain =
original length
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Tensile Test DataCalculate the strain in the dog bone with an elongation of 0.0625in.
0.0625in.ε =
1.000in. = 0.0625
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Tensile Test – Stress-Strain Curve
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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
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Modulus of Elasticity (E) The proportional constant (ratio of stress and strain)
A measure of stiffness – The ability of a material to resist stretching when loaded
An inherent property of a given material
σ stressE = =
ε strain
Tensile Test – Stress-Strain Curve
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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
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Elastic LimitUppermost stress of elastic behavior
Elastic and proportional limit are almost identical, with the elastic limit being slightly higher
Tensile Test – Stress-Strain Curve
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ResilienceThe 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 bh2
Tensile Test – Stress-Strain Curve
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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 well-defined yield point
Tensile Test – Stress-Strain Curve
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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
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Plastic DeformationUnrecoverable elongation beyond the elastic limit
When the load is removed, only the elastic deformation will be recovered
Tensile Test – Stress-Strain Curve
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Tensile Test – Strength PropertiesStress Strain Curve
Plastic deformation represents failure
Part dimensions will now be outside of allowable tolerances
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DeformationTest sample elongation
Cross-sectional area decreases
Load bearing ability increases – Why?
The material is getting stronger – How?
Tensile Test – Stress-Strain Curve
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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
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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
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Tensile Test – Stress-Strain Curve
FailureIf continued force is applied, necking will continue until fracture occursDuctilityAmount of plasticity before fractureThe greater the ductility, the more a material can be deformed
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Compare the material properties of these three metal samples
Tensile Test – Samples
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Brittleness
Material failure with little or no ductility
Lack of ductility, not lack of strength
Tensile Test – Stress-Strain Curve
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ToughnessWork per unit volume required to fracture a material
Total area under the stress-strain curve from test initiation to fracture (both strength and ductility)
Tensile Test – Stress-Strain Curve
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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
Compression Test
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Resistance to permanent deformation
Resistance to scratching, wear, cutting or drilling, and elastic rebound
Brinell Hardness TestA tungsten carbide ball is held with a 500lb force for 15 sec into the material
The resulting crater is measured and compared
Hardness Testing
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Rockwell TestA small diamond-tipped cone is forced into the test sample by a predetermined load
Depth of penetration is measured and compared
Hardness Testing
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Image ResourcesNSW Department of Education and Training (2011). Retrieved from
http://lrrpublic.cli.det.nsw.edu.au/lrrSecure/Sites/Web/tensile_testing/index.htm?Signature=%287e02281c-318a-461b-a8ed-3394db0c4fe6%29