טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentPevsner Pavel
SubjectDamage Detection in Composites Using High Frequency Range
Vibration and Embedded Optic Fibers
DepartmentDepartment of Aerospace Engineering
Supervisors Professor Emeritus Tanchum Weller
Professor Avraham Berkovits


Abstract

This research introduces two advanced approaches to damage detection in composites: a) one in which very high frequency vibration technique is used to excite the structure, and b) one in which damage is "visualized " with the aid of embedded optic fibers. The use of these methods for monitoring and assessing structural integrity , “health monitoring”, was evaluated.

Contrary to methods which rely on exciting the structure by relatively low frequency vibration and therefore are capable of detecting damage of the order of global structure dimensions only, the present study focuses on local damage detection  by application of relatively high frequencies, 10 - 30 kHz and higher, for structure excitation. Both theoretical and experimental investigations were performed . The proposed high frequency method succeeded in detecting the position and extent of a damaged region of  ≈ 2 cm2  in a composite plate .

Damage in optic fibers embedded in composites generates heat when laser light is transmitted through the fiber. The rise in temperature can be detected by an infrared camera . Analytical and numerical tools for evaluating the temperature rise in the neighborhood of broken embedded fibers have been developed. The results of these calculations have indicated that the hot spot generated by the light from the broken fiber could be satisfactorily  detected in test by use of an infrared camera .Experiments with an  impact damaged graphite-epoxy plate have shown that the heat energy emitted by broken fibers embedded into, or bonded to the composite plate was adequate to change the local temperature so that it could be accurately identified by an infrared camera . The size of the damage which can be detected by this method depends on the density of embedded optic fibers .