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Hardness

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

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  1. Hardness UAA School of Engineering CE 334 - Properties of Materials Lecture # 9 Hardness and Fracture

  2. Hardness:VariousDefinitions • Indentation hardness • Rebound hardness • Scratch hardness • Wear hardness • Machinability Hardness and Fracture

  3. Indentation Hardness • Resistance to permanentindentation under static or dynamic loads • Examples? Rebound Hardness • Energy absorbed under impact loads • Examples? Hardness and Fracture

  4. Scratch Hardness • Resistance to scratching • Examples? Wear Hardness • Resistance to abrasion • Examples? Hardness and Fracture

  5. Machinability • Resistance to cutting or drilling • Examples? Other Hardness? Hardness and Fracture

  6. 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

  7. 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

  8. 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

  9. Hydraulic Brinell Machine • Pump handle • Table for specimen • Brinell ball Hardness and Fracture

  10. Features of Hydraulic Brinell Machine • Position of specimen • Brinell ball • Oil pressure applied by pump • Force measurement Hardness and Fracture

  11. Brinell Ball and Indentation • Applied force: P • Ball diameter: D • Indentation diameter: d • Depth of indentation • Thickness of specimen Hardness and Fracture

  12. 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

  13. 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

  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 Hardness and Fracture

  15. Important Bhn Variables III Thickness of Specimen: Specimen is too thin Too large indentation Too small Bhn Not acceptable Hardness and Fracture

  16. 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

  17. Standard Brinell Numbers • Steel, Aluminum, Copper (Brasses) • Big range for each material Hardness and Fracture

  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. Hardness and Fracture

  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. Hardness and Fracture

  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! Hardness and Fracture

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