טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBracha Ido
SubjectHealth Monitoring of Airborne Bolted Joints by Local
Dynamic Effects
DepartmentDepartment of Design and Manufacturing Management
Supervisors Professor Emeritus David Durban
Dr. Raruch Karp
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract



Large aerospace structures like space shuttles, civilian and military aircraft, and helicopters, are costly operated complex systems. Aerospace platforms are commonly subjected to extreme operational environments characterized by rapidly changing harsh conditions. Most aerospace structures consist of simple structural elements such as beams, plates and cables attached and assembled together using structural joints.

Structural joints are characterized by local stress concentrations and geometrical defects, such as cracks or holes, and therefore prone to failure, in particular under adverse conditions. Joint damage presents a potential hazard that may lead to catastrophic failure of the entire structure; proper structural functionality relies mainly on the reliability of the joints. Structural health monitoring (SHM) is an emerging field aimed to improve readiness and reliability of existing structures, reduce their maintenance cost, and increase the cost effectiveness and performance of newly designed structures. That interdisciplinary field combines several scientific and engineering disciplines, culminating in measurement systems, which are part of the structure, providing continuous information on structures’ condition. That field is based on well-established practices, enhanced by recent studies on structural behavior and by advanced sensors and smart materials.


Recent analytical and experimental studies on implementation of local dynamic effects provide a promising avenue for new and efficient SHM procedures. The underlying idea is based on the dynamic version of Saint Venant's principle manifested by the emergence of evanescent waves, in response to dynamic excitation. It has been demonstrated that those waves carry important data that reflects the health status of the end conditions. “End conditions” are considered here as a measure of a joint clamping condition with possible defects. The monitoring method is based on the existence of local dynamic effects as the key indication of the structure integrity since changes in the joint condition are reflected by the evanescent waves near the joint location. It is expected that small irregularities due to the presence of bolts, and damage due to loosening, can be detected upon comparative inspection (with respect to a definite baseline) of near field waves. The chosen joint configuration was an airborne bolted joint attaching an avionics chassis to the airframe wall. Using numerical analysis and experimental tools the structural response to dynamic (impact and harmonic) excitation was examined.


A clear evidence of joint condition deterioration was revealed by observing significant changes of the dynamic structural response in the vicinity of the bolt, while the remote structural response is unaffected by small variations in the joint condition. The monitoring method provides a promising ground for detection of damage with increased sensitivity in bolted joints due to bolt loosening.