On the Residual Strain and Stress of Standard Block for Hardness due to the Indentation Hardness Test

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• The requirement for the standard block for hardness is that it has a designated hardness over the whole surface and must reproduce the same value at each test.<BR>In the indentation hardness test, however, plastic deformation is also produced inevitably by repeating tests on the same block, and it affects the standard value of hardness. It is important to know how long the initial designated hardness may be retained.<BR>The author measured, on the Rockwell hardness test block, 1) the change of hardness, 2) the residual strain and 3) the change of residual stress on the surface due to repetition of test, using electric wire strain meter, optical flat and X-ray method, and obtained the followings as the test results:<BR>(1) In the process of successive indentation, three stages of hardness change were observed; until fifty times of indentation, two groups were observed for hardness, one which increases slightly, and the other, which shows no changes, and thereafter there appears no change in hardness till one hundred and fifty times of indentation. Over that number of indentation, it is likely that the hardness will increase.<BR>(2) Over the back side of the block, residual compressive strain was observed It was greater for softer materials, and greater as well when the standard block was of smaller thickness.<BR>(3) On the testing surface, residual compressive stress was observed. The stress increased as the number of indentation increases.<BR>(4) The reliable value of standard hardness is obtained after indentations of fifty times. The block, subjected to indentation of more than one hundred and fifty times, gives an incorrect value of hardness.

• 社団法人 日本材料学会の論文

社団法人 日本材料学会 | 論文

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