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Ch 27.11 hardness test

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Ch 27.11 hardness test

  1. 1. Hardness Test Definition: Hardness is a very important but hard to define property of materials. “Hardness is the resistance offered by a material to indentation.”
  2. 2. IES-1992 Hardness as a function of carbon content is shown in (a) Fig-A (b) Fig-B (c) Fig-C (d) Fig-D
  3. 3. IAS-2001 With the increase of percentage of carbon in the steel, which one of the following properties does increase? (a) Modulus of elasticity (b) Ductility (c) Toughness (d) Hardness
  4. 4. Moh’s Scale of Hardness Material Moh’s hardness Talc 1 Gypsum 2 Calcite 3 Fluorite 4 Apatite 5 Orthoclase 6 Quartz 7 Topaz 8 Corundum 9 Diamond 10
  5. 5. Brinell Hardness test  Most commonly used tests.  A 10 mm tungsten carbide or hardened steel ball intended.  Gradually applied load.  Load applied at right angle to the specimen surface  Indentation diameter made on the specimen is measured. Contd…   2 2 2 / 2 ; Load P BHN kgf mm Surface Area of Indentation D D d D P Kgf D and d in mm       
  6. 6.  It cannot be used on very hard or very soft material.  It is best if the thickness of the material is at least 10 times the depth.  Tensile strength = k×BHN MPa k = 3.296 for alloy steel = 3.342 for Plain Carbon steel.  For steel, flexural endurance limit e 1.75 BHN MPae     VHN BHN BHN Rc   x 10
  7. 7. IES 2010 The hardness of lathe bed material should be measured by (a) Rockwell tester (b) Brinell hardness tester (c) Shore Scleroscope (d) Vickers hardness tester
  8. 8. IES-1992 A carbon steel having Brinell Hardness number 100 should ultimate tensile strength closer it (a) 100 N/mm2 (b) 200 N/mm2 (c) 350 N/mm2 (d) 1000 N/mm2
  9. 9. Rockwell Hardness test  A sphere – conical diamond cone of 120º angle and a spherical apex of radius 0.2 mm is used  Depth of indentation t is measured.  Gradually applied load,  Load is applied at right angle to the specimen surface.  R = 100 – 500 × t  Depending on the load used a scale A, B, C, etc. used.  Rockwell test should not be used on thin materials (generally less than 0.16 mm, on rough surface or on materials that are not homogeneous).
  10. 10. IAS-1999 A measure of Rockwell hardness is the (a) Depth of penetration of indenter (b) Surface area of indentation (c) Projected area of indentation (d) Height of rebound
  11. 11. Vickers hardness test (i) A square based diamond pyramid having 136º between the opposite faces is used. (ii) Average Diagonal of the indention (D) is measured. (iii) Even a light load will produce plastic deformation that so why we may use VHT on very hard material likes Tungsten Carbide. p VHN D  2 p in kgf1.854 , D in mm.
  12. 12. The Scleroscope (i) Based on the rebound height of a ball from the specimen. (ii) Harder the material, higher is the height to which the ball rebounds. (iii) Scleroscope hardness numbers are comparable only among similar materials. A comparison between steel and rubber would not be valid.
  13. 13. Knoop Test (Micro Hardness Test)  Various micro hardness tests have been developed for application where it is necessary to determine the hardness of a very precise area of material, or where the material or surface layer is exceptionally thin.  Special machine have been developed for this purpose. The location for this test is select in the high magnification.  A small diamond penetrator is then loaded with a predetermined load ranging fm 25g to 3000g.
  14. 14. IES-1992 Which of the following would you prefer for checking the hardness of very thin sections? (a) Hebert cloud burst test (b) Shore's Scleroscope (c) Knoop hardness test (d) Vickers hardness test
  15. 15. Durometer  When testing soft, elastic materials, such as rubbers and non rigid plastics, a durometer is use Herbert Cloudburst hardness test:  It is used to know the uniformity of hardness over a surface.
  16. 16. IES-1992 Herbert cloudburst Hardness test is conducted to know (a) Uniformity of hardness over a surface (b) Softness of non-metallic components (c) Hardness of non-metallic components (d) Hardness at specified depth inside the surface.
  17. 17. Charpy and Izod test  Toughness of a material is measured by means of impact tests.  Notched bar prepared as per standard from the test material, is held in a vice and a weight is allowed to swing from a known height in such a way that it hits the notched bar in its path and breaks it.  Since the material has absorbed some amount of energy during its fracture, the swinging mass loses part of its energy and therefore will not be able to reach the same height from where it started.  The loss in height (h) multiplied by the weight represents the energy absorbed by the specimen during fracture, which can be directly measured from the indicator on the tester. Contd…
  18. 18.  In the Charpy impact test, the specimen is held between two grips whereas in the Izod impact test the specimen is held at one end like a cantilever .  The standard test specimens used in the izod and Charpy impact tests are made of bars with a square cross section of 10 mm.  The impact resistance is dependent upon the material composition as well as the heat treatment process given to it.  The annealed materials normally would have better toughness than the corresponding normalized or quenched specimens. Coarse-grained structure would tend to have higher ductility compared to fine grain structure and consequently better toughness.
  19. 19. Sheet Formability Tests  During the various sheet metal forming processes, the stresses and strains developed in the material are quite complex. So, any measure of the formability of the sheet metal on the basis of material properties derived from a simple tension test( for example, % elongation as an index of ductility of the material) is not very accurate.  So cupping tests have been developed to evaluate the formability of the sheet metals.
  20. 20. GATE-2006 Match the items in Column I and Column II. Column I Column II P. Charpy test 1. Fluidity Q. Knoop test 2. Micro hardness R. Spiral test 3. Formability S. Cupping test 4. Toughness 5. Permeability (a) P - 4, Q - 5, R - 3, S – 2 (b) P - 3, Q - 5, R - 1, S - 4 (c) P - 2, Q - 4, R - 3, S – 5 (d) P- 4, Q - 2, R - 1, S – 3
  21. 21. IES-2002 Assertion (A): The hardness test is a slow, expensive method of assessing the mechanical properties of materials. Reason (R): The hardness is a function of yield stress and the work hardening rate of material. (a) Both A and R are individually true and R is the correct explanation of A (b) Both A and R are individually true but R is not the correct explanation of A (c) A is true but R is false (d) A is false but R is true
  22. 22. The End

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