On the detection of fatigue damage prior to surface indication

The aim of this study was to investigate the failure mechanism of riveted aluminium lap joints when exposed to constant load amplitude fatigue. Tests were conducted on lightweight aircraft grade aluminium sheet (2024-T4). Visual observations of the failure behaviour and lifetime records were supplemented by real-time, in-situ structural integrity (SI) measurements. Structural Monitoring Systems Ltd. (SMS) equipment, which owns the patent rights to the Comparative Vacuum Monitoring (CVM) method, provided the SI measurements of the specimens during their fatigue life. SMS sensors were installed either on the surface or within the lap joint specimens. Observation of fatigue loading from the perspectives of the two differently installed SMS sensors has provided insight into the failure mechanism. Failure of the riveted lap joint is found to occur initially by a process of rivet shank/rivet hole fretting damage and damage accumulation within the anti-chaffing material prior to crack initiation within the aluminium plates and/or rivets. Major fatigue damage was detected at 50-70% of the fatigue life, despite visual indication being absent. The benefits of this technology to the aviation industry are large, as early detection of fatigue damage improves safety and allows increased flexibility in maintenance schedules. CVM is applied to galleries containing a low vacuum and atmospheric pressure. The atmospheric pressure is often contained with a second set of galleries that alternate with the low vacuum galleries. If a flaw is not present, the low vacuum remains stable at the base value. If a flaw develops, air will flow from the atmospheric galleries through the flaw to the vacuum galleries. A transducer measures the rate of flow between the galleries and therefore gives an indication of flaw size. CVM is rapidly gaining acceptance as a technique for crack initiation detection, automated crack propagation measurement, localised yield stress determination and composite disbonding indication. The aviation industry has shown the greatest interest, with fatigue laboratories also including SMS technology in their testing equipment.