Aramid cords are synthetically prepared and are stronger per weight than steel. Due to this high strength nature, finding the correct grips for use in a tensile test can be quite challenging. Besides slipping within the grip, cords may also exhibit internal slippage in the jaw, where one fiber is slipping past the others. This kind is much harder to detect and can lead to jaw breaks and/or inaccurate results. For this application study, we tested an aramid cord using our standard pneumatic cord and yarn grips and compared the results to those obtained with pneumatic aramid cord and yard grips to determine which is more accurate and suitable.
For this test, we used a 68TM-50 dual column testing frame, configured with a 5 kN static load cell, and 1.75 kN pneumatic cord and yarn grips. The gauge length of the specimen was set to 250 mm. We conducted the test at a crosshead speed of 250 mm/min, per ASTM D7269, and data was acquired at an interval of 50 ms into Bluehill® Universal software. To check for slippage, we attached flags of tape to the ends of the cord and monitored them to make sure there was no relative movement of the flag with respect to the grip. The 1.75 kN cord and yarn grips were then replaced with 2 kN pneumatic aramid cord and yarn grips for the second test.
Looking at the differences in the results between the regular and aramid cord and yarn grips, it seems that the cords gripped with the regular grips exhibited internal slippage in the jaw. This means that the fibers within the jaw slipped past one another, which jeopardizes the accuracy of the test results. When internal slippage occurs, the cord is weakened for obvious reasons. In order to correct for this, higher air pressure can be used, but this could lead to jaw breaks, further jeopardizing the accuracy of the test results.
The aramid cord and yarn grips did not exhibit symptoms of internal slippage. Due to the unique surface finish of the jaws, they are able to grip the strong aramid cord in such a manner that limits internal slippage and provides more accurate and consistent results.