Esat Selim Kocaman

Prediction of fatigue response of composite structures by monitoring the strain energy release rate with embedded fiber Bragg gratings

Composite materials are becoming increasingly more valuable due to their high specific strength and stiffness. Currently, most components are operated for a number of service cycles and then replaced regardless of their actual condition. Embedded fiber Bragg gratings are under investigation for monitoring these components in real time and estimating their remaining life. This article presents research conducted on a novel technique for prediction of the remaining life of composites under fatigue loading using embedded fiber Bragg grating sensors. A prediction is made of the remaining life at every cycle based on data collected from the sensors and the previous loading history.

The performance of embedded fiber Bragg grating sensors for monitoring failure modes of foam cored sandwich structures under flexural loads

In this study, failure modes of foam core sandwich composites are investigated by using embedded Fiber Bragg Grating sensors. Sandwich specimens with Fiber Bragg Grating sensors, embedded inside the face sheet, are manufactured using vacuum infusion process and then subjected to a static and a cyclic loading under the three-point bending mode. Different failure modes are monitored utilizing the wavelength shift and the spectrum of Fiber Bragg Grating sensors. It is shown that the responses of the Fiber Bragg Grating sensor differ depending on damage modes thereby making structural health monitoring of sandwich structures possible.

Monitoring the Damage State of Fiber Reinforced Composites Using an FBG Network for Failure Prediction

A structural health monitoring (SHM) study of biaxial glass fibre-reinforced epoxy matrix composites under a constant, high strain uniaxial fatigue loading is performed using fibre Bragg grating (FBG) optical sensors embedded in composites at various locations to monitor the evolution of local strains, thereby understanding the damage mechanisms. Concurrently, the temperature changes of the samples during the fatigue test have also been monitored at the same locations. Close to fracture, significant variations in local temperatures and strains are observed, and it is shown that the variations in temperature and strain can be used to predict imminent fracture. It is noted that the latter information cannot be obtained using external strain gages, which underlines the importance of the tracking of local strains internally.

An experimental study on the effect of length and orientation of embedded FBG sensors on the signal properties under fatigue loading

Abstract Fiber Bragg grating (FBG) sensors provide excellent capability for the structural health monitoring (SHM) of load-bearing structures by allowing for local internal strain measurements within structures. However, the integration of these sensors to composite materials is associated with several challenges that have to be addressed to have the correct strain measurement and in turn to perform reliable SHM. One of the most important issues is the presence of uneven strain fields around FBGs, which significantly affect the response of the sensors and hence the reliability of the acquired data. The uniformity of the strain fields around sensors is important for dependable data acquisition; however, to generate such a condition, tow width-to-FBG length relationship, optical fiber configuration with respect to reinforcement fiber orientation, and crack density resulting from fatigue loading are very important factors that have to be considered. In this paper, these issues are addressed by investigating the signal properties of FBG sensors with 1 and 10 mm lengths embedded within the composite specimens during the manufacturing process. After fatigue testing of the specimens, it is shown that 1-mm-long FBGs embedded in-line with adjacent reinforcement fibers with tow widths of ∼2 mm provide much more reliable signals than 10-mm-long FBGs embedded perpendicular to adjacent tows.

Poisson’s Ratio as a Damage Index Sensed by Dual-Embedded Fiber Bragg Grating Sensor

Monitoring the health of glass or carbon fiber reinforced polymer under dynamic loading conditions is still a challenge. When metals are tested under dynamic loading conditions, usually a single crack is a source of the failure in the material. On the other hand, composite materials include several damage modes such as transverse cracking, delamination and splitting under the dynamic loading and all of them contribute ultimate failure of the material. Due to the complexity of the damage in composite materials, it is very hard to estimate health or damage state of composite materials. In this study, we propose the usage of a novel embedded biaxial Fiber Bragg Grating sensor system to track the evolution of Poisson’s ratio which can be employed as a reliable damage index in composites. The fatigue experiments on specimen made of biaxial glass fiber infused with resin transfer molding system have shown that signal from novel embedded biaxial sensor system can be easily collected to evaluate Poisson’s ratio. The current study also indicates that the evolution behavior of Poisson’s ratio is consistent with other fatigue parameters such as temperature, force and strain energy that show very rapid change in the first region of fatigue with respect to number of cycles.

Effect of fiber densities on impact properties of biaxial warp-knitted textile composites

In this study, an appropriate fabric weight content controlled by the density of the warp and weft fibers is determined for biaxial warp-knitted composites referring to mechanical test results. Six different types of composite panel with two different fabric weights (813 and 1187 gr/m2) and with three different stacking sequences [90we/0wa/90we/0wa]s, [90wa/0we/90wa/0we]s, and [90wa/0we/90we/0wa]s are fabricated by using Resin Transfer Molding method. Having produced composite panels, drop weight impact tests are conducted on specimens. Microstructural characterization of impact tested materials is performed using optical microscope. The results of this study reveal that composites with biaxial warp-knitted preforms with lower weft and warp fiber densities (thin-ply) could absorb higher impact energies compared to those with higher weft and warp fiber densities (thick-ply).