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In order to facilitate a discussion of gripping technology, the effect of three types of grips on the results of rapid loading as well as long-term creep loading tension tests is assessed. As background, a review of combined stresses and friction effects is presented. Monitoring the effective gage length of the specimen under test shows how movement of the specimen in the grips can influence the apparent stress-strain behavior, and how initial use of some extensometry may serve to establish active gage lengths for repetitive testing. Creep-rupture results are affected by both stress concentrations and excessive specimen movement in the grips. There is evidence that a stick-slip resonance phenomenon in the grip can influence creep strain and creep rupture results, particularly data scatter. If the grips used to produce the creep-rupture results are not the same as the grips used to establish the ultimate tensile strength (UTS) then it may be invalid to express the creep-rupture results as a percentage of UTS. In this case, there are three distortions that can arise with two of them leading to overly optimistic estimates of the long term creep-rupture strength.
tension testing, creep testing, creep-rupture testing, effective gage length, horn grips, capstan grips, pneumatic clamps, coefficient of friction, polyester, yarn, time-temperature superposition, stepped isothermal method
Principal Scientist and President, Texas Research International, Inc., Austin, TX
Vice President and Division Manager, TRI/Environmental. Inc., Austin, TX
Research Engineer/Scientist, TRI/Environmental, Inc., Austin, TX