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150 THE DISTRIBUTOR’S

150 THE DISTRIBUTOR’S LINK ROB LaPOINTE HARDNESS TESTING SMALL FASTENERS: USING MACROHARDNESS & MICROHARDNESS TECHNIQUES from page 130 the width of the indentation. For Vickers indentations, both diagonals are measured, and the average is used to calculate the hardness value. For Knoop, only the long axis is measured and used to calculate the hardness value. See Figure 10 for a diagram of Vickers and Knoop indentations. ASTM E18 (macrohardness) and ASTM E384 (microhardness) both require testing instruments to be verified periodically using a certified test sample. E18 requires Rockwell instruments to be verified each day of use and E384 requires Vickers and Knoop instruments to be verified weekly. FIGURE 6 HARDNESS TESTING ANVIL CLEANED AND READY FOR MEASUREMENTS Since it is the indenter travel distance that is converted into hardness, the measurement will indicate a softer value than is correct for the material being tested. Microhardness testers use a micrometer eyepiece or video vision system to observe and measure the magnified indentation. Figure 7 shows the view through a micrometer eyepiece. Notice the movable lines that mark Indentation Spacing The spacing of indentations relative to one another and to the edge of the sample is critical. When a hardness indentation is made, the material is compressed under and around the indentation. This compression creates a swell of material around the indentation and can change the hardness of the material in that location. Insuring there is sufficient space between indentations guarantees that each measurement is representative of the material and not biased by a previous test. ASTM E18 for Rockwell hardness requires there to be 3 indentation diameters between the centers of indentations and 2.5 indentation diameters between the center of an indentation and the edge of the material, see Figure 8. ASTM E384 for microhardness, requires there to be 2.5 indentation diagonal widths both between the centers of indentations and the edge of the material for Vickers. FIGURE 7 VICKERS MICROHARDNESS INDENTATION WITH MICROMETER EYEPIECE MEASUREMENT FIGURE 8 ROCKWELL INDENTATION SPACING REQUIREMENT PER ASTM F606/F606M CONTINUED ON PAGE 158

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