Post on 01-May-2018
Material Testing
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
Static Material Testing
StrengthDeformationFractureDesign requirement compliance
Tensile testCompression testHardness test
Evaluation of Material
Standardized Tests
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
Tensile Test
Ensures meaningful and reproducible resultsUniform cross section
Standard Test Sample (dog bone)
Tensile Test ProcedureDog bone is created to test specifications
Dog bone is secured in tester
Tensile Test ProcedureA tension force (F) is applied to the dog bone until failure occursSimultaneously the applied tension force (F) and dog bone elongation ∆L are recorded
A plot is created from the stored load elongation data
F
∆L
Tensile Test Data
F
∆LTest 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?
Tensile Test DataLoad-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
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
lostre adss =area
Fσ = A
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
Tensile Test DataManipulating Elongation Results
To eliminate test results based on sample size, calculate sample strainStrain - the amount of stretch per unit length
Elongation under load divided by the original Length (L0)
amount of stretchstrain = original length
Tensile Test DataCalculate the strain in the dog bone with an elongation of 0.0625in.
0.0625in.ε = 1.000in.
= 0.0625
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)
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
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 LimitUppermost stress of elastic behavior
Elastic and proportional limit are almost identical, with the elastic limit being slightly higher
Tensile Test – Stress-Strain Curve
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
Yield PointWhen 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
Offset Yield StrengthDefines the stress required to produce a tolerable amount of permanent strain
Common value is 0.2%
Tensile Test – Stress-Strain Curve
Plastic DeformationUnrecoverable elongation beyond the elastic limit
When the load is removed, only the elastic deformation will be recovered
Tensile Test – Stress-Strain Curve
Tensile Test – Strength PropertiesStress Strain Curve
Plastic deformation represents failure
Part dimensions will now be outside of allowable tolerances
DeformationTest 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 StrengthLoad 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
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
Compare the material properties of these three metal samples
Tensile Test – Samples
BrittlenessMaterial failure with little or no ductility
Lack of ductility, not lack of strength
Tensile Test – Stress-Strain Curve
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
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
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
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
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