1. Material Testing

2. Material Testing Static Testing Dynamic 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

3. Static Material Testing
Evaluation of Material
Strength Deformation Fracture Design requirement compliance
Standardized Tests
Tensile test Compression test Hardness test

4. Tensile Test Uniaxial Destructive
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

5. Tensile Test Standard Test Sample (dog bone)
Ensures meaningful and reproducible results
Uniform cross section

6. Tensile Test Procedure
Dog bone is created to test specifications
Dog bone is secured in tester

7. 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

8. Tensile Test Data F d A B
Test sample A and B are 230 red brass. Test sample A has a diameter of in. Test sample B has a diameter of in.
If both samples are tested to failure, will the applied tension force and elongation be the same for both tests?
NO – Why?

9. 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?

10. 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)

11. Tensile Test Data
Calculate the stress in the dog bone with a 430 lb applied force.

12. 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)

13. Tensile Test Data
Calculate the strain in the dog bone with an elongation of in.

14. Tensile Test – Stress-Strain Curve

15. Tensile Test – Stress-Strain Curve
Proportional Limit
Elastic Range
Initial response is linear
Stress and strain are proportional to one another

16. Tensile Test – Stress-Strain Curve
Proportional Limit
Proportional Limit
Stress at which material starts elongating more than the proportional in force

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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%

23. Tensile Test – Stress-Strain Curve
Yield Point
Plastic Deformation Unrecoverable elongation beyond the elastic limit
When the load is removed, only the elastic deformation will be recovered

24. Tensile Test – Strength Properties
Stress Strain Curve
Plastic deformation represents failure
Part dimensions will now be outside of allowable tolerances

25. Tensile Test – Stress-Strain Curve
Plastic Deformation without necking
Elongation continues, some is permanent
Cross-section decreases along entire sample
Load can continue increasing
The bonds between atoms in the material

26. 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

27. Tensile Test – Stress-Strain Curve
Plastic Deformation with Necking
Sample can now be stretched with less force.

28. 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

29. Tensile Test – Samples
Compare the material properties of these three metal samples

30. Tensile Test – Stress-Strain Curve
Brittleness
Material failure with little or no ductility
Lack of ductility, not lack of strength

31. 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)

32. 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

33. 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

34. 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

35. Resources
NSW Department of Education and Training (2011). Retrieved from Askeland, Donald R. (1994). The Science and Engineering of Materials, 3rd ed. PWS Publishing: Boston.