End-grip configurations for axial loading of composite tubes

Abstract

Testing of high-strength, tubular, composite specimens in uniaxial and biaxial tension require end-grip fixturing to transfer large axial loads into the specimen. Two gripping configurations were analytically and experimentally evaluated to determine their affect on stress distribution within the specimen test section. The first was a bonded grip in which the tubular specimen is adhesively bonded in a deep-slotted aluminum end tab. The second was a threaded grip with glass cloth/epoxy overwrap on the specimen ends that thread into an aluminum, split-collar end tab. Large axial loads can be transmitted to the specimen with either design; however, each introduces axial-stress concentrations into the composite sample. The magnitude and distribution of stresses (strains) with axial position, which are a function of biaxial-stress ratio, were computed by finite-element analyses and verified experimentally with surface-strain measurements. This study illustrates the importance of utilizing analytical tools to examine the effect of end grip/specimen interaction on stress distribution within the gage section and on test data.