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Session:
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13P - Poster Session
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Date & Time:
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Wednesday August 9, 10:00 AM - 10:45 AM
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Paper Title:
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Calibrating Linear Distances For Brinell, Vickers and Knoop hardness testing systems using Image Analysis Systems
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Speaker:
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Robert Ellis, Quality Manager
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CoAuthors:
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Speaker Info
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Company:
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David L Ellis Company, Inc.
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Address:
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Box 592
, 310 Old High Street Acton, MA, 1720, United States
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Phone:
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978-897-1795
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Fax:
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978-897-0844
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Email:
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dlellisco@aol.com
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Abstract:
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Image analysis has grown rapidly in industry for determining linear distances in Brinell, Vickers and Knoop hardness testing. Its ease of use and consistent results from one operator to another within a laboratory has made it a popular tool. In general, Image Analysis removes some of the operator influence that occurs while using manual test systems. Under ideal conditions, the repeatability using the image will be much smaller than manual systems. This assumes that the system settings are the same on a daily basis as well. Comparing one image system at one location may or may not be the same. Again settings may be different as well as the manufacturer's method of determining the indent size. The apparent quick and easy calibration techniques can give the user false security that the system is correct. Since linear distances are a key component of the Brinell , Vickers and Knoop test, it is very important to determine if the system is correctly measuring the linear distance. Small errors in the recording of linear distances can equate to very large differences in hardness values. Typical calibration methods may not give the user a correct bias. Currently, the traditional method of using a stage micrometer is used to determine the optical linear part of the hardness tester. Although the stage micrometer can be traceable to a national laboratory, it is flat compared to hardness indents. This does not adequately address the nature of hardness indents. Hardness indents are not flat and exhibit pile up at the edge of the indent where the linear distance is measured. In some cases where the material is very soft, the pile up can be very large. A large indent should exhibit less percentage of error for having same linear error readout over larger distance but this is not necessarily true. This phenomenon of pile-up causes more error for larger size indent than one would think. Verification using a stage micrometer is not at the same level as a hardness indent. Direct verification can best be accomplished by measuring traceable indents from a national laboratory or a secondary laboratory traceable to national laboratory . In addition, lighting and focus can play a significant role in calibrating an Image analysis system used for hardness. These verification / calibration methods are necessary so that Image analysis systems can maintain national and international hardness standards.
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