C. Hardness Testing Background theory and information Often carrying out a tensile or toughness test to obtain the mecha

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C. Hardness Testing Background theory and information Often carrying out a tensile or toughness test to obtain the mechanical properties of a material is not convenient or cost effective. In addition, these tests also destroy the material being tested. A hardness test avoids these problems. Hardness can be defined as "the resistance of a material to indentation" and is a quick and practical way of estimating the strength and wear resistance of a material. Early hardness tests were based on the hardness of natural minerals with a scale being based solely on the ability of one material to scratch another that was softer. This scale was termed the "Mohs Scale" of hardness and it ranged from 1 at the soft end for tale (soft chalk-like substance) to 10 at the hard end for natural diamond. The modern approach to determining the hardness of a metal is to press a very hard indenter into the surface of the material being tested and then to measure the size, or depth, of the resulting indentation to give a hardness number. For a soft the material the indentation will be larger and deeper, and consequently the hardness number will lower. From these tests hardness is defined as the load applied (in kg) divided by the projected area of the indentation
Both of these tests have the same basic approach, the only difference being the shape size and the indenter material. Other hardness test methods also exist. In any indentation hardness test the hardness number (H) is obtained by dividing the applied indenter load (in kg) by the surface area of the indentation (impression). F HE A Note: This equation must not be confused with stress or pressure since the surface area of the indentation is not perpendicular to the applied force, and in the following equations "F" is measured in kg and "d" (or "D) is measured in millimetres. Thus the hardness number (Hvor HB) for these two types of hardness tests can be evaluated as follows: I VICKERS HARDNESS NUMBER (HV) 2Fsin 68 A+B ну where du d' 2 BRINNEL HARDNESS NUMBER (HB) HD D-V|D? - d?) 2F Hв [VID d
OTHER HARDNESS TESTS Knoop Hardness Test (HK) - for measurement over small areas. Janka Hardness Test - for wood. Rockwell Hardness Test (HR) - principally used in the USA. Shore Hardness Test - for polymers. Barcol Hardness Test - for composite materials. Meyer Hardness Test - not often used. VICKERS BRINELL F INDENTERS Diamond indenter Steel sphere indenter D Sure of depression created by the tool is a measure o hardness INDENTATIONS A Polished surface A=B B PROFILES 136° TOTT HARDNESS TESTING INDENTER SHAPES Fortunately we no longer need to calculate the Hy and He values by hand for most modern machines display these values automatically. Alternatively they can be read from tables provided with the hardness testing machines.
TASK C (15 marks) 1. A Brinell hardness test is performed on steel using a 10 mm indenter with a load of 3000kg. A 3.2 mm impression is measured on the surface of the steel. Calculate the He value obtained from this test (show all calculations in detail). [7.5 marks) 2. A Vickers hardness test was carried out on brass using a 10kg load and produced an indentation with diagonals (A = B) with a mean length of 0.51mm. Calculate the HV value obtained from this test. [7.5 marks)