A methodology for detection of crack initiation in adhesively bonded joints under constant and variable amplitude fatigue loading

The use of adhesively bonded joints is increasing in the lightweight design; however, their fatigue performance in safe-life design is challenging. Therefore, a combination of damage-tolerant design and Structural Health Monitoring (SHM) is often considered. The present work deals with a new approach for detection of crack initiation in adhesively bonded single lap joints (SLJ) subjected to constant and variable amplitude fatigue loading. For this purpose, the so-called backface strain method is used, in which a strain gauge for damage monitoring is placed at a highly sensitive point in the overlap area of SLJs. A striking converging-diverging trend in strain amplitude curves was attained in the tests subjected to constant amplitude loading, which was in a good agreement with the onset of crack initiation observed with video-microscopy. Additionally, this methodology was demonstrated on the SLJs subjected to variable amplitude loading using an additional farfield (outside of the overlap area) strain gauge, which showed a high potential for detection of crack initiation.