Monitoring-structuresfor timely failure prevention.SOLUTIONS with Effective, Practical AdviceMonitoring-structures is a much important need in general, because continuing safe operation of constructions and structures has to be assured until they are eventually demolished. It is especially relevant for aging structures, built in the past according to less stringent requirements and approaching their design service-life limit. (Sponsored Links) They are also urgent for those structures which sustained possible damages from unexpected overloads like severe earthquakes, their real condition needing assessment before being cleared for secure usage. In this context the term structure is intended in its broadest meaning, including anything built by human beings, and whose collapse would endanger life and/or property. Two short notes in our Practical Welding Letter introduce special fatigue sensors, developed to provide advance alert of progressing damage in structures while in service. The first, a link titled Fatigue monitoring in welded structures was published in section 5 of Issue #021 for May 2005. The second, on the same subject, was published in section 11 of Issue #030 for February 2006. Click on PWL#030 to see it. We dealt in the past with Monitoring-structures in an Article on Remaining Life Assessment published (7) in Issue 56 of Practical Welding Letter for April 2008. Click on PWL#056 to read it. Classic methods used for Monitoring-structures to detect cracks, corrosion, and fatigue in old constructions have definite shortcomings that undermine the efforts to ensure their safety. Furthermore current inspection techniques are very costly, often requiring to shut down normal operations or activities during inspection. In particular non destructive inspections are work intensive for specialized personnel, are bound to depend on subjective judgement, and are time consuming. In some cases, provided all external surfaces can be examined, Monitoring-structures is performed by visual inspection only, which may be critical, inasmuch as those are usually the most stressed locations. However, fatigues and failures start at micro crack levels and propagate in due time into major catastrophic events. Many cracks are difficult to spot by visual inspection until they become large enough to be a potential hazard. In the last few decades, much progress was achieved with advances in emerging technologies as new materials, new sensors, electronic communication, and information processing were developed in view of providing the capability of Monitoring-structures by application of local devices for nondestructive evaluation of structural conditions. Much research was devoted to develop non-intrusive active sensors to be applied on existing aging aerospace metal and composite structures, and for monitoring the onset and progress of structural damage (fatigue cracks and corrosion). The current trend, extended to concrete constructions, to bridges and high rise buildings, to pressure vessels and power generating equipment, is poised to find and implement cost effective means for Monitoring-structures, by implanting or attaching suitable sensors within any structure. The purpose is to monitor in real time their output and, by duly elaborating it with suitable software, to obtain clear indication of distress whenever there is still time to intervene or at least to shut down and to evacuate the structure and its surroundings before catastrophic collapse. One method of Monitoring-structures which has shown promising results for such applications is the impedance-based technique. Applications using Ultrasonic and Acoustic Emission sensors have also demonstrated reliability. TWI pioneered the introduction of a fatigue sensor designed to sustain the same cyclic stressing of the structure it is to monitor, and to develop in time a crack to be detected electrically. Developers plan to derive from its behavior the proportion of the fatigue design life that has been used. Optical Fibers provide practical sensing capabilities in many applications, especially for bridges, where experimental applications were tested in practice. In conclusion, with the continuing improvement of the hardware and of the analytic software, it is quite safe to forecast that the use of suitable sensors will be increased for Monitoring-structures as they are erected. For old ones, studies implementing the application of fracture mechanics methods and the local testing of actual mechanical properties will continue to provide risk analysis results and recommendations as to the safe residual life available. A review of sensors and of recent publications presenting experimental applications of Monitoring-structures, was published in our Mid Month Bulletin No. 45 of Practical Welding Letters, released at mid January 2010. Click on Structure Monitoring Sensors to see it. Any questions or comments or feedback? Write them down and send them to us by e-mail. Click on the Contact Us button in the NavBar at top left of every page.
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