Achieving consistent, repeatable results requires consistent, repeatable testing conditions. The major sources of variability when testing stiff materials, such as composites, are gripping and alignment.
Proper gripping of stiff materials or coupons is best done with a powered grip. Powered grips are most often driven by hydraulics or pneumatics, and offer the most repeatable conditions since the operator is not directly involved. Manual grips are subject to variability due to different operators applying different forces when tightening the grip onto the specimen. Furthermore, it is best that these powered grips are rigidly mounted and preloaded. Flexible couplings or loose joints can easily cause bending strains outside of NADCAP or ASTM bounds, and introduce a significant amount of scatter in results.
Finally, the location of the specimen in the grips also plays a significant role in producing repeatable results. Round specimens are less of an issue, since the "Vee" in the grip face positively locates and centers the specimen. Flat specimens, however, require specimen alignment stops to ensure that they are located in exactly the same place for every test. We recommend that you should reset these alignment devices with every change in specimen geometry.
Rigid load strings, powered grips, and specimen location alignment devices are all technologies which, when combined with careful testing technique, can go a long way to maximizing measured mechanical properties, and minimizing variability.
Proper gripping of stiff materials or coupons is best done with a powered grip. Powered grips are most often driven by hydraulics or pneumatics, and offer the most repeatable conditions since the operator is not directly involved. Manual grips are subject to variability due to different operators applying different forces when tightening the grip onto the specimen. Furthermore, it is best that these powered grips are rigidly mounted and preloaded. Flexible couplings or loose joints can easily cause bending strains outside of NADCAP or ASTM bounds, and introduce a significant amount of scatter in results.
Finally, the location of the specimen in the grips also plays a significant role in producing repeatable results. Round specimens are less of an issue, since the "Vee" in the grip face positively locates and centers the specimen. Flat specimens, however, require specimen alignment stops to ensure that they are located in exactly the same place for every test. We recommend that you should reset these alignment devices with every change in specimen geometry.
Rigid load strings, powered grips, and specimen location alignment devices are all technologies which, when combined with careful testing technique, can go a long way to maximizing measured mechanical properties, and minimizing variability.
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