ANALYZING AND REPORTING TENSILE DATA ON FORCE MEASUREMENT SYSTEMS The L.S. Starrett Company 9 78-249-3551 www.starrettmetrology.com/force When testing the properties of a material or component, a range of options are available for examining, reporting and tracking tensile data. In addition to reporting just the most common load and distance data for a tensile test performed on a force or material measurement system, there is more information that can be incorporated to improve traceability, readability, and context to measurements. These might include graphs, tolerances, comments, and test criteria. A visual comparison of graphical data from a force measurement test can serve as a quick, straight forward tool to compare tests to one another. These visual checks can often help operators identify issues. Many software options have the ability to overlay and recall previously run tests with known good parts that can be used as a reference guide, much like a “golden part”. Expanding on this, some force and material testing software have interactive graphs, so when problem areas are identified they can then be analyzed in greater detail and annotated. These visual checks are a good aid but can be subjective and vary from operator-to-operator. This variability can be removed by adding tolerances to the data collection portion of the test method. In addition, the tolerances can then be included in reports, so the performance expectations are clearly expressed. When multiple tolerances are used, the result can be reduced to pass/fail in a report summary. Testing Tolerances Tolerances are often based on material properties, supplier or customer specifications, or historical data of known good and bad parts. In general, force testing applications have tolerances that are often expressed plus or a minus force value (lbf, kgf, N) or distance (in, mm). Spring tolerances differ in that they are commonly expressed in terms of percentage of a nominal value. E.g. 40lbf ±5% at 0.750 inch length.
Testing software can aid in converting tolerance % to loads to prevent mathematical or transcription errors by the operator.
Applying tolerances to individual points is a common method used. In material measurement applications it is also common to see tolerances on stress (PSI, MPa) or strain (%) values. These tolerance points allow operators in a production environment to quickly determine if a part passes or fails without detailed analysis that might otherwise require quality personnel to interpret the data.
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