Assessing the Effect of Pre-Tightening Force on Damage Mechanism of Single-Lap GLARE Bolted Joints Using Acoustic Emission

Pre-tightening force is a critical factor affecting the performance of the bolted joint. Although its effect on the double-lap bolted joint of fiber metal laminate (FML) has been identified, that on the single-lap case with loading eccentricity remains unclear. In this study, single-lap glass fiber reinforced aluminum laminate (GLARE) bolted joints under different pre-tightening forces were tested. Various mechanical responses were considered to fully expose the effect of pre-tightening force, including bearing stiffness and its degradation, as well as damage and ultimate bearing strength. To reveal the bearing damage mechanism and thereby understand the mechanical responses clearly, the deformation process and damage morphologies were characterized optically. Particularly, the acoustic emission (AE) technique was employed to collect the in situ damage information. Based on the AE results, global damage evolution was initially considered to disclose the joint's damage process macroscopically and to evaluate the appropriateness of AE parameters. Thereafter, individual damage evolution after clustering was utilized to reveal the progression of each damage. It was found that matrix cracking and Al/GFRP delamination were the two primary damages. Increasing pre-tightening force could suppress matrix cracking in the early loading stage, but it hardly worked on Al/GFRP delamination. Then, only the bearing stiffness and damage bearing strength were improved; however, the ultimate bearing strength was not. The findings are anticipated to clarify the pre-tightening force dependence of the damage mechanism of single-lap GLARE bolted joint and to provide suggestions for improving the joint's performance, that is, enhancing the matrix in GFRP and Al/GFRP bonding quality.