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Written by Administrator
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Thursday, 05 February 2009 05:33 |
Check List for Greater
Accuracy of Test Results
for Charpy Impact Testing  One of the most common tests of the physical characteristics of metals is the notched Charpy impact test as specified by ASTM E23 Standard Test Method for Notched Bar Impact Testing of Metallic Materials. In simple terms the test places a specimen, horizontally, at the center of a set of support anvils and a striker on a pendulum then strikes, and breaks, the specimen. The energy absorbed by the specimen in the breaking process is known as the breaking energy and can be converted into an indication of a materials impact resistance using such units as foot-pounds or joules. While use of the data generated from a test for designing a part is not necessarily recommended, it still provides reasonable service as a quality control tool. For some materials, the results of these kinds of impact tests, when correlated with temperature variations and service experience, can help to accurately predict the likelihood of brittle fracture. |
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Last Updated on Thursday, 05 February 2009 05:37 |
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Written by Administrator
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Thursday, 05 February 2009 05:32 |
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In order to do a notched Charpy test, a precise notch must be cut into the specimens. The purpose of the notch is to serve as a stress concentrator. The notch is probably the most critical part of specimen preparation and there is a tight tolerances defined by ASTM E23 on the type of the notch, the depth of the notch (actually the material remaining under the radius of the notch), the angle of cut and the radius at the base (or apex) of the notch. Research has shown that the notch in the specimen is perhaps the greatest source of variability of test data. Specimens can be notched using a milling machine or a commercially available, specifically designed, notching machine. ASTM E23 does not define the specific operation of the notching machine, but most involve adjusting the cutter height on the machine so that it cuts a notch that leaves 8 mm of material remaining under the apex of the notch and a radius of curvature at the apex of 0.25 mm. While the notch depth, the included angle of the notch and the radius of curvature are critical, only the measurement of the notch depth is relatively simple and can be easily done on specimens prior to testing. The angle of the notch and the radius of the angle at the base of the notch are more difficult to measure. Therefore, specimens must be periodically sampled for inspection under magnification. A method of identifying the individual specimens within the sample should be devised. Measure and record the width of each individual specimen in the area of the notch using a micrometer or similar instrument. A notch depth verification device, which consists of an analog dial indicator positioned above an anvil, is commonly used to measure the depth of the notch. A digital indicator may also be used and is particularly useful if the actual depth of the notch is to be determined or if it is necessary to switch between measuring in traditional US units and metric units. However, for those laboratories interested in doing a go/no-go type determination, analog dials are very convenient because they can be equipped with follower arms that serve to mark the upper and lower limits of the depth tolerance. |
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Written by Administrator
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Thursday, 05 February 2009 05:31 |
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There is occasion for samples to be tested at alternate temperatures, typically lowered temperatures, so that an analysis of embrittlement can be performed. In these cases the ASTM standard states that the sample has be at the test temperature for a minimum of five minutes prior to testing. Additionally the locating tongs, used to put the sample in place on the support anvils, must be kept at the test temperature. When the system is ready, the sample must be removed from the cooling medium, placed on the anvils and the test performed within 5 seconds of removing the sample from the cooling medium. An alternative to this is a heating and/or cooling system that can bring the sample to test temperature on the support anvils – in fact the anvils are parts of the heating and cooling system, however, this can be an expensive option. This option allows accurate placement of the test specimen, rapid cooling or heating of the sample and, since there is no time constraint imposed on completion of the test, a subsequent faster throughput of accurately tested samples can be achieved. |
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Written by Administrator
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Thursday, 05 February 2009 05:31 |
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Instruments for testing metals have been manufactured commercially since the 1900's, and there are several manufacturers of testing equipment that meet the requirements of ASTM E23. Over the years, the basic physical characteristics of the test machine have stayed the same, but the means of data acquisition have changed significantly and, as a result, many types of equipment, ranging from analog dial & pointers to digital readouts to computer interfaces, are found in various labs throughout the country. Just because equipment is old doesn't mean that it is obsolete. It doesn't mean it is acceptable for use either since wear and tear take their toll over the years and some machines were manufactured before strict adherence to the specifications in ASTM E23 was deemed to be critical. There is no hope of ever getting accurate results if the equipment isn't operating according to the specification, therefore it is strongly recommended that the impact tester is calibrated and/or verified by an accredited calibration service at regular intervals. |
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