Material Testing - bp025.k12.sd.us

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