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Developing and Defining an Effective Approach for Detection of Multi-site Damage (MSD) in Metallic Aircraft lap Joint Skins

Alem, Behrouz | 2017

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 50653 (45)
  4. University: Sharif University of Technology
  5. Department: Aerospace Engineering
  6. Advisor(s): Abedian, Ali
  7. Abstract:
  8. Today, considering the structural health monitoring system is approximately essential for aerospace structure due to more safety and reduction of maintenance costs. In this manner, aircraft designers and manufactures have recently attempted to usage of the monitoring system on their new products to maintain the superirority of them among the competitors. Furtheremore, installation of these systems on the aircraft will also becom one of the requirements of the aviation regulation in near future which must be complied.The lap joints are one of the major and inevitable components of the metallic airframe structure and the multisite damage such as fatigue cracks are the high probable damages initiated and propagated around the rivet holes of this structures. Usually, because of inaccessibility or time consuming of disconnecting the components of these structures, their monitoring is particularly important. Therefore, this study was designed to develop an effective method for monitoring the lap joint structures by using elastic wave propagation technique with embedded wafer active sensors.Since damage identification results based on the Lamb-waves propagation approach can be influenced by varying environmental and operational conditions, development of a robust monitoring system with no need of the prior measured data of the structure, has gained much attention recently. In this research, the main focus is on the establishment of a monitoring method for lap joint structure based on the reference free approaches. Therefore, four reference free techniques (time reversal, mode conversion, Instantaneous baseline, and relative baseline) are first described and their strengths and weakness, especially for damage detection in complex structure, are investigated by analysis of the damage identification results. The instantaneous baseline damage detection technique is one of the promising methods that overcome the mentioned obstacles. For this, the material properties, electromechanical characteristics, and the geometric features must be identical. Also, sensor distances in similar paths and the geometric dimensions of the transducers must be the same. So, implementing the instantaneous baseline damage identification in complex structures is rather complicated due to the inherent non-homogeneities involved. To understanding the important of issue, the effects of two uncertainties, related to the manufacturing and assembly tolerances, on the instantaneous baseline method damage identification results are examined. To ease the complexity, this thesis proposes a semi-instantaneous baseline damage identification approach by modifying an existing instantaneous baseline method.In this method, an active sensing network is mounted on the lap joint and a robust and effective feature called energy ratio change, is extracted from the collected time-domain signals using the wavelet transform. The introduced “Identicality Coefficient (IC)” for all the sensing paths in pristine condition of the structure are obtained and used to remove any inequalities that may occur to the signals of each paths. To save the time and cost, the wave propagation in the structure is simulated numerically by using the finite element (FE) method and ABAQUS software. The FE simulation has been verified with two different experimental models (a simple aluminum plate and a lap joint structure).The proposed method is used to detect and characterize multi-site damages such as fatigue cracks that initiate around the rivet holes of lap joint structures. The obtained results show that this method can detect fatigue cracks around the lap joint rivet holes of both upper and lower panels and estimate the crack size
  9. Keywords:
  10. Structural Health Monitoring ; Lamb Wave ; Piezoelectric Sensor ; Lap Joint ; Fatigue Crack ; Damage Detection ; Instantaneous Baseline Method

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