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Hardness. UAA School of Engineering CE 334 - Properties of Materials Lecture # 9. Hardness: Various Definitions. Indentation hardness Rebound hardness Scratch hardness Wear hardness Machinability. Indentation Hardness. Resistance to permanent indentation under static or dynamic loads

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Hardness

UAA School of Engineering

CE 334 - Properties of Materials

Lecture # 9

Hardness and Fracture


Hardness:VariousDefinitions

  • Indentation hardness

  • Rebound hardness

  • Scratch hardness

  • Wear hardness

  • Machinability

Hardness and Fracture


Indentation Hardness

  • Resistance to permanentindentation under static or dynamic loads

  • Examples?

Rebound Hardness

  • Energy absorbed under impact loads

  • Examples?

Hardness and Fracture


Scratch Hardness

  • Resistance to scratching

  • Examples?

Wear Hardness

  • Resistance to abrasion

  • Examples?

Hardness and Fracture


Machinability

  • Resistance to cutting or

    drilling

  • Examples?

Other Hardness?

Hardness and Fracture


Hardness: Continued....

  • The fundamental “physics” of hardness is not yet clearly understood.

  • All hardness measures are functions of interatomic forces.

  • There is nosingle measure of hardness has been devised that is universally applicable to all materials.

  • Hardness isarbitrarilydefined.

Hardness and Fracture


Relationship of Hardness to Other Mechanical Properties

  • Nomethod of measuring hardnessuniquelyindicates any other single mechanical property.

  • Some hardness tests seem

    to be more closely associated

    with tensile strength, others

    with ductility, etc.

Hardness and Fracture


IndentationHardness 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.

Hardness and Fracture


Hydraulic Brinell Machine

  • Pump handle

  • Table for specimen

  • Brinell ball

Hardness and Fracture


Features of Hydraulic Brinell Machine

  • Position of specimen

  • Brinell ball

  • Oil pressure applied by pump

  • Force measurement

Hardness and Fracture


Brinell Ball and Indentation

  • Applied force: P

  • Ball diameter: D

  • Indentation diameter: d

  • Depth of indentation

  • Thickness of specimen

Hardness and Fracture


Brinell Hardness

  • A Load applied to a 10mmdiameter 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)

Hardness and Fracture


Important Bhn Variables I

  • Applied load: units: kg

    • 1500 kg can be used for 48<Bhn<300

    • 1000 kg can be used for 32<Bhn<200

    • 750 kg can be used for 24<Bhn<150

    • 500 kg can be used for 16<Bhn<100

Hardness and Fracture


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

Hardness and Fracture


Important Bhn Variables III

Thickness of Specimen:

Specimen is too thin

Too large indentation

Too small Bhn

Not acceptable

Hardness and Fracture


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.

d

2.5d

2.5d

2.5d

Hardness and Fracture


Standard Brinell Numbers

  • Steel, Aluminum, Copper (Brasses)

  • Big range for each material

Hardness and Fracture


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.

Hardness and Fracture


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.

Hardness and Fracture


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!

Hardness and Fracture


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