1. Mechanical Properties and Their Measurement
METL 1301
Introduction to Metallurgy
Lecture3
Mechanical Properties and Their Measurement

2. Yield Strength
Yield strength represents the stress necessary to cause significant plastic deformation.
It is important to realize-that the yield strength does not represent the stress necessary just to begin plastic deformation.

3. Yield Strength
The necessity for a component or structure to operate at stresses below the yield strength can be illustrated by considering an automobile fender.
The movement of the fender is strain. The strains in this case are elastic and no permanent displacement of the fender results because of the application of the load.

4. Modulus of Elasticity of Selected Metals E,GPa (psi x 106)
Copper (Cu) (17)
Silver (Ag) (11)
Bronze (16)
Aluminum (Al) (10)
Steel (FeC alloys) (30)
Magnesium (6.5)
Brass (16)
Lead (2)

5. Tensile Testing
The tensile test has been traditionally the standard test to measure the modulus of elasticity (or stiffness) of metals and alloys.
Nondestructive tests for stiffness determinations have been developed.

6. Tensile Testing
Stiffness can be estimated from ultrasonic measurements.
The speed with which the waves of sound move within a metal increases as the stiffness of the metal increases.

7. Tensile Testing
Because of the ease with which ultrasonic measurements of stiffness can be made using ultrasonics.
The technique may be substituted for the tensile test as a means of measuring stiffness in metals and alloys.

8. Tensile Testing
The yield strength and the stiffness of a sample are determined from data gathered during the initial stages of the tensile test.
The ultimate strength and elongation to failure are determined from data gathered much later in the test.

9. Tensile testing movie

10. Movie on hardness testing

11. Tensile Testing Summary
The mechanical properties determined from a standard tensile test can be divided into three basic areas:
Strength
Stiffness
Ductility

12. The Hardness Test
The hardness of a metal has been defined as the resistance of a metal to plastic deformation (usually by indentation).
This resistance is generally measured by indentation testing techniques
Scratch hardness and rebound hardness can also be measured.

13. Indentation Hardness Testing
There are three common types of indentation hardness measurements:
Brinell
Rockwell
Vickers or Knoop

14. Indentation Hardness Testing
In all three of these techniques an indenter is forced into the sample of interest by the application of a specific load.
The depth or width of the impression left by the indentation is evaluated to establish a measure of the hardness.

15. Brinell Hardness Test
J. A. Brinell proposed the first widely accepted and standardized indentation hardness test.
The Brinell Hardness Test, is performed by a machine that presses a 10 mm (0.4 in.) diameter steel or tungsten carbide ball into the material’s surface under a standard load.

16. Brinell Hardness Test
The diameter of the impression is measured under 10x magnification to allow calculation of the Brinell hardness number (abbreviated HB or BHN).

17. Brinell Hardness Standard indenter loads are:
3000 kg (6600 lb.)
500 kg (1100 lb.)
The Brinell hardness number is expressed in terms of the load (P).
P is divided by the surface area of the indentation.
This relationship is where D is the diameter of the ball and d is the diameter of the indentation.

18. Brinell Hardness
Tables giving HB as a function of d are provided with most Brinell hardness testing equipment.
Therefore it is not necessary to perform a calculation.
Because of the size of the Brinell impression, Brinell hardness tests are used for large samples.

19. Rockwell Hardness Test
The Rockwell tester uses a much smaller indenter than the Brinell tester and therefore can be used on smaller samples.
A Rockwell hardness test is made by slowly elevating a specimen against an indenter until a small or minor load has been applied.
This minor load is approximately 10 kg (22 lb.), and serves to position the indenter with respect to the face of the sample.

20. Rockwell Hardness Test
A major load of either 60, 100, or 150 kg (132, 220, or 330 lb.) is then applied by releasing a loaded lever system.
The major load forces the indenter into the sample.
It is then released and the depth of indentation is measured.
The deeper the indentation, the softer the material.

21. Rockwell hardness There are nine major Rockwell hardness scales.
Each scale uses a specific combination of major load and indenter profile.

22. Rockwell hardness The three primary indenter profiles are:
Cone-shaped diamond.
A (1/16 in.) diameter ball.
A (1/8 in.) diameter ball.
The scales correspond to the specific combination of indenter and major load.

23. Rockwell hardness
The original and most commonly used scales are the Rockwell C and Rockwell B scales.
The hardness values for these (and all other Rockwell) scales lie between 0 and 100 units.
Values between 10 and 80 is considered more accurate than values at either extreme.
There is no theoretical correspondence between the hardness values determined on the various scales.

24. Knoop and Vickers Hardness Tests
The Knoop hardness, is one type of the so-called “microhardness tests”.
The term microhardness relates to the size of the indentation made by the test.
The typical Rockwell indentation is much smaller than the typical Brinell indentation.
Both of the indentations are visible to the unaided eye.

25. Knoop and Vickers Hardness Test
Microhardness indentations are typically too small to be seen without the aid of a microscope.
The techniques for microhardness measurements are similar to those described for the other hardness tests.
The exception is that the applied force and indenter are both very small, and the resulting indentation is tiny.

26. Microhardness movie

27. Knoop and Vickers Hardness Test
There are two semi-standard types of micro hardness tests:
The Knoop micro hardness test which uses a rhombohedral-based pyramid indenter.
The Vickers micro hardness test which uses a square-based pyramid indenter.

28. Scratch hardness testing
Scratch hardness testing is primarily of interest to mineralogists.
The technique uses a scale, called the Mohs’ scale, of one to ten to rate the ability of materials to scratch each other (from talc = 1 to diamond = 10).

29. Scratch hardness testing
Copper and hardened steel have Mohs’ scale hardnesses of 3 and 7, respectively — and most metals fall within this range.
Because of its inability to sharply distinguish between various hardness conditions, the scratch hardness test is not very useful for metals.

30. Rebound Hardness Testing
The dynamic or rebound hardness is measured by dropping a hardened steel tup on the surface of the materials of interest and measuring the height to which the tup rebounds.
The harder the material, the higher the rebound.

31. Hardness Testing
Hardness tests are quick, convenient and may even be nondestructive.
However, hardness testing does not establish the behavior of the metals and alloys under the sudden application of a load.
The behavior of metals and alloys under the sudden application of a load information, is obtained through an impact test.

32. The Impact Test
The impact test measures the ability of a metal or alloy to absorb energy during sudden loading.
The value of the impact test was brought to the forefront of mechanical testing technology during World War II.
This was due to the brittle fracturing nature of welded Liberty ships.

33. WWII Liberty ship

34. The Impact Test
Over 1000 of the approximately 4000 Liberty ships built, developed cracks.
Some of the ships broke completely in half.
The cracks in other ships were less severe and ranged in size from cracks that caused complete disability to cracks that were simply repaired.

35. Impact Testing Most of the cracks developed when the ships were cold.
Because of such cracking and failures, an extensive research program was undertaken to determine the cause of the brittle behavior.
Brittle fracture of steel had been recognized for years before the Liberty ship failures, but such failures had not received significant attention.

36. Impact Testing
Tanks, pipelines, bridges and pressure vessels had all experienced such failures.
Today the list could be expanded to include cranes, towers, and oil drilling equipment.
Many classes of steel become brittle and fracture easily at low temperatures under certain conditions.
That fact is not obvious from a standard tensile test.

37. Impact Testing
The effect of temperature on the stress-strain behavior of a low carbon steel suggests that the steel gets stronger as the temperature decreases.
Therefore, did not explain the easy fracture of the Liberty ships during cold weather.

38. Impact Testing
Failure analysis of the Liberty ship failures, concluded that three basic factors contributed to the brittle fractures.
These factors were:
Notches or flaws in the metal.
Low temperatures.
Rapid or sudden rates of loading.
The test developed to evaluate effects of these factors on various metals and alloys is the impact test.

39. Impact Testing
The impact test operates by positioning a heavy mass or weight at some distance above the test sample.
The mass or impact head is then released and drops the distance h1 to strike or impact the sample.
The energy of the head when it strikes the sample is h1 W, where W is the weight of the head.

40. Impact Testing
W (head weight) is generally measured in kilograms (pounds) and h1 in meters (feet) the energy of the impact is given in Joules (foot-pounds).
The impact head then fractures the sample and rises to a height of h2.
The energy required to break the sample is termed the impact energy of the sample.