STRUCTURAL HEALTH MONITORING OF FIBER REINFORCED AND SANDWICH COMPOSITES WITH EMBEDDED FIBER BRAGG GRATING SENSORS
Esat Selim Kocaman
Materials Science and Engineering, MSc. Thesis, 2015
Assoc. Prof. Mehmet Yildiz (Thesis Advisor), Assoc. Prof. Melih Papila
, Assoc. Prof. Bahattin Koç
Date &Time: January 5th, 2015 – 9.30 AM
Keywords : Fiber reinforced polymer composites, fatigue, sandwich composites, Fiber Bragg Grating optical sensors, structural health monitoring.
Scope of this thesis is the investigation of fiber reinforced and sandwich composites under different loading conditions using embedded FBG sensors and evaluate the feasibility and performance of these optical sensors to achieve structural health monitoring. To this end, three different works were conducted.
First part covers the investigation of the practical problems regarding the sensor integration into fiber reinforced composites and signal acquisition quality during the fatigue loading. Obtaining a clear and accurate signal is highly important in terms of reliability of the measure strains. When FBG sensor experiences a non-uniform strain field along its gagelength, this can result in splitting of the reflected spectrum acquired from the sensor. In order to evade this problem, this work proposes certain practical factors that require consideration need in order to obtain reliable data acquisition as dynamically varying strain field in the proximity of the FBG sensor can result in intermittent loss of signal during fatigue.
For the second part, performance and behavior of FBG sensors embedded inside glass reinforced composites are studied under constant strain, low-cycle fatigue loading conditions in order to assess the mechanical energy, strain distribution and evolution along the specimen. Understanding FBG response under low-cycle fatigue conditions is important in terms of applicability of these sensors to monitor structures that are exposed to repetitive high dynamic loads. It is shown that strains from the sensors located in different locations can decrease and significantly deviate from each other as low-cycle fatigue progress notifying the distinction between the global and local response of the material. Furthermore, this work also proposes important considerations for the implementation of strain-controlled fatigue tests which is also crucial for proper characterization and understanding of the material behavior.
In the last part, failure modes of foam core sandwich composites are investigated by evaluating three different failure modes i.e. facing indentation, compressive facing and core shear failure using embedded Fiber Bragg Grating (FBG) sensors. The understanding of the strain evolution induced by flexural loading and the FBG response to different damage states in sandwich composites is crucial in terms of applicability of these sensors into real structures. Exploiting how sensors respond to a particular damage mode by tracking the wavelength shift and spectrum information, failure detection strategy is developed for damage characterization to perform condition monitoring of sandwich structures.