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

Abstract

ABSTRACT 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.