There are different kinds of optical fiber sensors. They are relatively free from environmental attack and have been used for general structural health monitoring. They can be embedded inside the bond to leave the external surface smooth, which is essential for aerodynamic structures. Optical fibers are known to have excellent fatigue endurance. In addition, NDE cannot reliably and readily detect kissing bonds, which are zero-volume fabrication defects with little or no strength. It has been pointed out that non-destructive inspection for defect techniques are mainly helpful at the fabrication stage for quality assurance and at the late stage of failure where explicit defects have formed but are ineffective for bonds weakened by degradation during the service stage. Non-ultrasound techniques such as electromechanical impedance spectroscopy using external or embedded piezoelectric sensors, thermography, and shearography have also been proposed. These include ultrasonic techniques such as the traditional pulse-echo or through transmission methods, as well as the more advanced guided wave, acoustic microscopy, and electromagnetic acoustic transducer. Non-destructive examination (NDE) techniques are available to detect defects in adhesive joints. ![]() Some means to evaluate the structural health of an adhesive joint are required to ensure safe operation. On the other hand, service loading conditions such as impact, occasional overload, and long-term fluctuating loading may induce joint degradation/damages that lead to eventual structural failure. However, a severe weakness of adhesive joining is that it cannot be disassembled to inspect its integrity. In composite structures, they provide additional merits in helping to avoid fiber discontinuity and possible edge delamination due to hole drilling. Adhesive joints significantly reduce stress by spreading load transfer over a large area, leading to better stiffness, better strength-to-weight ratios, and markedly reduced fatigue problems. ![]() They help to keep the structure surface smooth and avoid localized bearing load at stress-concentrating holes or material degradation due to localized heating. With these refined techniques, we hope to shed some light on the relative merits and limitations of the two techniques.Īdhesively bonded joints have a number of advantages over traditional bolt or weld joints. The extent of damage is revealed by a fluorescent penetrant and correlated with the monitoring signals. The whole FBG spectrum, instead of a single peak, will be used, and a quantitative parameter to describe spectrum changes is proposed for monitoring purposes. In view of the above shortfalls, measurements are made at some low and fixed loads to preclude any unwanted effect due to the applied load. ![]() Degradation of adhesive and inter-facial defects will lead to non-uniform strain that may chirp the FBG spectrum, causing complications in the peak shifting measurement. Moreover, most FBG works primarily relied on the peak shifting phenomenon for sensing. Most of the previous works took measurements under an applied load, which by itself will affect the monitoring signals without the presence of any damage. Both techniques will be employed and compared in the current work to monitor damage development in adhesive joints under tensile and cyclic fatigue loading. ![]() Embedded fiber Bragg grating (FBG) sensors and the resistance of carbon nanotube (CNT)-doped conductive joints have been proposed to monitor the structural integrity of adhesive joints. Conventional non-destructive examination techniques do not readily reveal joint degradation before the formation of explicit defects. Adhesive joints possess a number of advantages over traditional joining methods and are widely used in composite structures.
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