Material Testing

Material Testing Reproducible evaluation of material properties

Material response to varying loading

conditions, including magnitude, cycling,

and mode

Dynamic Testing

Material response to constant loading

Static Testing

Static Material Testing

Strength

Deformation

Fracture

Design requirement compliance

Tensile test

Compression test

Hardness test

Evaluation of Material

Standardized Tests

Tensile Test Uniaxial

A straight line axial force is

applied to a test sample

(typically in the y axis)

Destructive Force is applied until sample fails.

Image courtesy of NSW Department of Education and Training

Hounsfield Tensometer

Tensile Test

Ensures meaningful and reproducible results.

Uses a uniform cross section.

Standard Test Sample (dog bone)

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.

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 Data

To 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 Data

Calculate the stress in the dog bone with a

430 lb applied force.

2area = r

2area = (0.0625 in.)

2area = 0.0123in.

Fσ = A

2

430 lb

0.0123 in.

35,000 psi

Tensile Test Data

Manipulating Elongation Results

To eliminate test results based on sample

size, calculate sample strain.

Strain (e) – is the amount of stretch per unit

length.

Elongation (d) under load, divided by the

original length (L0)

amount of stretchstrain =

original length

Tensile Test Data Calculate the strain in the dog bone with an

elongation of 0.0625in.

0.0625in.ε =

1.000in. = 0.0625

amount of stretchstrain =

original length

Tensile Test – Stress-Strain Curve

Elastic Range

Initial response is linear.

Stress and strain are proportional

to one another.

Tensile Test – Stress-Strain Curve

Proportional

Limit

Proportional Limit

Stress at which material starts elongating

more than the proportion in force.

Tensile Test – Stress-Strain Curve

Proportional

Limit

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 Limit = Yield Point

Uppermost stress of elastic behavior .

Elastic limit and yield strength mean the

same thing.

Elastic limit 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 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 Deformation

Unrecoverable elongation beyond

the elastic limit.

When the load is removed, only the

elastic deformation will be recovered.

Tensile Test – Stress-Strain Curve

Yield

Point

Tensile Test – Strength Properties Stress Strain Curve

Plastic deformation represents failure.

Part dimensions will now be outside of

allowable tolerances

Plastic Deformation without necking

Elongation continues, some is permanent

Cross-section decreases along entire

sample.

Load can continue increasing.

Tensile Test – Stress-Strain Curve

Tensile Strength

Load bearing ability peaks.

Less force is now required to

continue elongating.

Weakest location begins to

decrease in area more than other

locations – Necking

Tensile Test – Stress-Strain Curve

Plastic Deformation with Necking

Sample can now be stretched with less

force.

Tensile Test – Stress-Strain Curve

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.

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

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).

Tensile Test – Stress-Strain Curve

Tensile Testing Examples

steel rebar tensile test

steel cylinder tensile test

welded steel

concrete cylinder

metal cable

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

Video examples

concrete test

composite test

soda can

concrete 2

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 500 lb

force for 15 sec into the material.

The resulting crater is measured and

compared.

Hardness Testing

Brinell testing video

Rockwell Test A small diamond-tipped cone is forced into

the test sample by a predetermined load

Depth of penetration is measured and

compared.

Hardness Testing

Rockwell testing video

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-a8ed-

3394db0c4fe6%29

Askeland, Donald R. (1994). The Science and Engineering of

Materials, 3rd ed. PWS Publishing: Boston.