Hardness

1
Hardness

• UAA School of Engineering
• CE 334 - Properties of Materials
• Lecture 9

2
Hardness Various Definitions

1. Indentation hardness
2. Rebound hardness
3. Scratch hardness
4. Wear hardness
5. Machinability

3
Indentation Hardness

• Resistance to permanent indentation under static
  or dynamic loads
• Examples?

Rebound Hardness

• Energy absorbed under impact loads
• Examples?

4
Scratch Hardness

• Resistance to scratching
• Examples?

Wear Hardness

• Resistance to abrasion
• Examples?

5
Machinability

• Resistance to cutting or
• drilling
• Examples?

Other Hardness?
6
Hardness Continued....

• The fundamental physics of hardness is not yet
  clearly understood.
• All hardness measures are functions of
  interatomic forces.
• There is no single measure of hardness has been
  devised that is universally applicable to all
  materials.
• Hardness is arbitrarily defined.

7
Relationship of Hardness to Other Mechanical
Properties

• No method of measuring hardness uniquely
  indicates any other single mechanical property.
• Some hardness tests seem
• to be more closely associated
• with tensile strength, others
• with ductility, etc.

8
Indentation Hardness Testing of Metals

• Brinell Hardness Test (ASTM E 10) - Commonly
  used.
• Rockwell Hardness Test (ASTM E 18) - Commonly
  used. Indentor and loads are smaller than with
  the Brinell test.
• Vickers Hardness Test (ASTM E 92) - Similar to
  Rockwell. Uses a square-based diamond pyramid
  for the indentor.
• Knoop (Tukon) Hardness Test - used for very thin
  and/or very small specimens.

9
Hydraulic Brinell Machine

• Pump handle
• Table for specimen
• Brinell ball

10
Features of Hydraulic Brinell Machine

• Position of specimen
• Brinell ball
• Oil pressure applied by pump
• Force measurement

11
Brinell Ball and Indentation

• Applied force P
• Ball diameter D
• Indentation diameter d
• Depth of indentation
• Thickness of specimen

12
Brinell Hardness

• A Load applied to a 10mm diameter ball.
• Note real diameter in the machine 10.35mm
• Measure diameter of the indentation to the
  nearest 0.02 mm under a microscope.
• Compute the Brinell Hardness Number (Bhn)
• D diameter of ball (mm) D 10mm
• d diameter of indentation (mm)
• P applied load (unit kg)

13
Important Bhn Variables I

• Applied load units kg
• 1500 kg can be used for 48ltBhnlt300
• 1000 kg can be used for 32ltBhnlt200
• 750 kg can be used for 24ltBhnlt150
• 500 kg can be used for 16ltBhnlt100

14
Important Bhn Variables II

• Rate of Load Application
• Inertia may cause momentary overload
• Need time for plastic flow of material.
• For ferrous metals The full load is applied for
  a minimum of 15 seconds, and 30 seconds interval
• For soft metals 30 seconds, 60 seconds interval

15
Important Bhn Variables III

• Thickness of Specimen

Thickness of specimen (mm) Minimum Brinell hardness for which a Brinell test may safely be made Minimum Brinell hardness for which a Brinell test may safely be made Minimum Brinell hardness for which a Brinell test may safely be made
Thickness of specimen (mm) 500 kg load 1,500 kg load 300 kg load
2 79 238 476
4 40 119 238
6 26 79 159
8 20 60 119
10 16 48 95
Specimen is too thin
Too large indentation
Too small Bhn
Not acceptable
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Important Bhn Variables IV

• Proximity to edge or other test locations

• If an indentation is made too near the edge of
  the specimen. It may be both too large and
  unsymmetrical.
• The distance of the center of the indentation
  from the edge or from the center of adjacent
  indentations ? 2.5 times the diameter of the
  indentation.

?2.5d
d
?2.5d
?2.5d
17
Standard Brinell Numbers

• Steel, Aluminum, Copper (Brasses)
• Big range for each material

18
Dynamic Hardness Tests

• Shore Scleroscope (ASTM E 448) - Measures the
  rebound of a small pointed
• device dropped from a 254mm height.
• Schmidt Hammer - Measures rebound of a spring
  loaded hammer. The test has been correlated with
  concrete compressive strength.

19
Dynamic Hardness Testing Principles

• The methods of calculating the energy absorbed
  are questionable.
• The results are arbitrary.
• As is true in arbitrary testing procedures, it is
  important follow the procedure meticulously and
  take accurate measurements in order to have
  results that are comparable.

20
Brinell Hardness Lab

• Student performed lab
• Check the set up oil leak?
• Force meter check the unit kg?
• Report discussion all Important Bhn Variables
• Compare with the standard
• values
• Have fun!