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 results

Uniform 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 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

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?

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

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

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 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

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 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

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 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 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

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

Brittleness

Material 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