Gripping a specimen correctly is important if you want to acquire accurate test data. To most test operators, this is second nature.
- Make sure the grips and specimen are well aligned
- Make sure the grips and grip jaws are suitable for the material being tested and the test loads expected
- Make sure the grips and grip jaws are clean and undamaged.
However, often overlooked is ensuring the specimen is correctly inserted into the jaws, particularly when using wedge-type grips. You should insert the specimen so that it is centered and that it contacts the full length of the grip jaws. Further, the grip jaws themselves, once gripping the specimen, should not protrude below the lower face of the grip. If they do, then you should select a smaller grip for the test. See the illustration below. If a test is performed with a partially clamped specimen or with the grip jaws protruding below the lower grip face, the grip jaws can experience severe twisting loads leading to their failure