Experimental investigation of screwed joints capabilities for the CFRP composite laminates

Abstract

The load-carrying capability of the screwed joints for the carbon fiber reinforced polymer (CFRP) composite laminates was investigated by compression and torsion tests. To this end, the M6, M8, M10, and M12 metric internal threads were obtained by tapping directly into the CFRP composite laminates. In order to avoid material damage, the milling and tapping operations were implemented using CNC controlled machine tools. Quasi-static compression and torsion tests were carried out on the simply tapped and Helicoil reinforced screwed joints in the universal tensile-press test machine under uniform test conditions. The applied force and torque values respect to the screw displacements were obtained. Based on the experimental outcomes, the load-carrying capability and failure mechanisms of screwed joints were evaluated. The SEM graphics were utilized to explain the failure mechanism of threads. The load-carrying capability of joints increased with increasing the joint size. Helicoil reinforced specimens showed significantly higher maximum failure load and torque values compared to simply tapped specimens. Although, Helicoil inserts were effective in improving load-carrying of screwed joint, analysis of Helicoil performance proved that the effectiveness of Helicoil decreased with an increase in metric screw size. The computed stripping strength values indicated that Helicoil reinforced screwed joint can be an alternative to riveted and adhesively bonded joints as a detachable joining technique for thick CFRP composite laminates.