Structural health monitoring of frp composites using fbg sensors

Use of composite materials have become prevalent in aerospace, marine, energy and automotive industries. The well known advantages of composites make them very attractive for complex load bearing structures. However, there is still a long way to fully exchange metallic materials with composites; because unknown factors like long term fatigue behavior necessitates relatively higher safety factors that should be applied for composite structures during the design phase. This study was performed to develop structural health monitoring (SHM) methods using Fiber Bragg Grating (FBG) sensors, which may reduce safety-oriented design perspective by providing continuous information about the integrity of structure during the service. An FBG sensor behaves like an optical filter that reflects a specific wavelength of light back to the light source. The reflected light's peak wavelength is dependent on the distance between gratings. Once the distance between gratings change due to a mechanical load, peak wavelength of reflected light shifts. This property of FBG sensors make them perfect for strain measurement. Advantages such as small diameter, immunity to electromagnetic fields, multiplexed usage opportunity and embeddable nature of FBG sensors make them unique for the use of SHM applications. Embedding FBG sensors into composite materials is not a straight-forward task, since it is a delicate process especially for manufacturing technique like Resin Transfer Molding (RTM) method which utilizes a closed mold. Specimens manufactured by RTM method should be designed in such a way that the fiber optical cable must be taken out from the end of the specimen in one piece and should be protected during the entire manufacturing process. In the beginning of this study, testing the specimens that contain embedded FBGs were performed by special apparatus because of the method that was used to take the fiber optics out of the specimen. Studies showed that additional apparatus was complicating the experimental procedure and most of the time caused immature failure of the specimens during fatigue tests. To overcome this, initial research was focused on developing an effective method for taking the optical sensor out of the specimen that makes both manufacturing and testing more feasible. In this context, L shaped geometry was developed for specimens that contain FBG sensors which resolved all complications related to testing of FBG embedded specimens by eliminating apparatus use. Embedded FBG sensors are ideally suited for continuous monitoring of composites under cyclic loading conditions, which can be very useful for SHM applications. Remaining useful life (RUL) of composite structures under fatigue loading, which have been investigated by many researchers in literature, was utilized in this thesis study for FBG embedded composites by calculating released strain energy at each load cycle by using FBG sensor data. Initially fatigue constants of GFRP material derived experimentally by testing sensorless specimens and calculating strain energy release rate (SERR) in a wide variety of strain ratios. Following to that FBG embedded GFRP specimens were manufactured and tested under high cycle fatigue conditions. Research results showed that FBG sensors are very effective to obtain strain from the interior of structure during fatigue experiments. It has been seen that shorter sensor lengths are more reliable in terms of signal quality as an embedded sensor since long FBGs are prone to peak splitting due to the variable strain fields within the composites. Sinusoidal strain behavior for each load cycles were clearly obtained using embedded FBGs at 4Hz of cyclic loading. Predicting the remaining life under some assumptions was successfully performed by analyzing the FBG data. As an interesting outcome, it was revealed that the local behavior of composites monitored by embedded FBG sensors differs from the global observations made by external sensors. It should be noted that FBG sensors stayed intact until the fracture of specimens, which proves the method's usability for actual applications. Two FBG sensors on the same optical fiber were used as a multiplexed sensor and embedded into specimens to utilize a biaxial sensor configuration. This method was enabled to collect strain data both in axial (i.e. loading) and transverse directions using embedded FBG sensors under cyclic loading conditions. By analyzing the fatigue strain data of both FBGs, Poisson's ratio of GFRP specimens was utilized at each load cycle. Poisson's ratio degradation trend of tested specimens was revealed using embedded FBG data during fatigue loading. It has been seen that Poisson's ratio of composite materials can be simultaneously monitored using embedded FBGs during the entire life of the structure, which contains critical data about the current state of the structure in terms of integrity. Both extensometer and displacement controlled test types were used during the experiments. It has been revealed that the local strain trends of composites observed by FBG sensors was different than the global behavior throughout the gage length obtained by extensometer and LVDT in a similar manner with previous study. Strain-gage sensors were also used during the experiments and it has been seen that these sensors are not as durable as FBG sensors especially for cyclic loading conditions. More than 3.5 millions of load cycles were performed in a low strain fatigue experiment and both FBG sensors were working flawlessly when the test was ended intentionally without any visible damage in the specimen. Temperature effect on FRP composites and embedded FBG sensors was also studied within the scope of this thesis study. FRP composites manufactured by RTM and hand layup with Vacuum Curing&Debulking Table (VCDT) methods were mechanically compared under cold, room and elevated temperature conditions. Products of RTM method were performed better for all temperature conditions. It has been seen that cold temperature (i.e. -40C) was increased while elevated temperature (i.e. 70C) was deceresed the ultimate strength of the composite materials compared to room temperature conditions. Overall, the mechanical behavior of both specimen types did not heavily differentiate especially considering the fiber volume fraction difference. Surface quality of the specimens was totally different for both techniques. Hand layup specimens manufactured using VCDT were containing many bubbles and the surfaces of them were heavily porous while RTM products' surfaces were perfectly smooth without any porosity and visible bubbles. Results revealed that wet hand layup technique was not suitable for embedding FBGs since the sensors are very sensitive to discontinuities within the hosting material. Moreover, porous nature of wet hand lay up products were not appropriate for fatigue testing since it would reduce the fatigue life substantially. Therefore, it was decided to investigate elevated temperature behavior of FBG embedded specimens manufactured by RTM method under fatigue loading. Experiment was performed at 50C with a 0.27 peak strain ratio. Specimen was tested more than 2 million cycles under these conditions and both specimen and FBG sensor were stayed intact without any failure. The data of FBG sensor was analyzed for SERR calculations and no any adverse effect of elevated temperature was noticed on FBG data. FRP composites containing open holes were also investigated from the SHM point of view. GFRP specimens were manufactured that contains central hole notch with a 4mm of diameter. Static experiments were performed on specimens without sensors in the beginning to understand the mechanical behavior. Following to that, three specimens manufactured with biaxially embedded FBG sensors. Axial and lateral sensors were located at 4mm, 7mm and 10mm away from the center of the holes of each specimen. Opening the holes using CNC for all specimens were monitored by the embedded FBGs. Release of the residual strains resulted by impregnation was observed within the analyzed data during the drilling process. Specimen that contains FBGs at 7mm from the hole center was tested statically. Both sensors were collected strain data around the hole notch as expected till failure of the composite. Other two specimens that contains biaxially located FBG sensors were fatigue tested. Experiments revealed that the sensors at 2mm distance from the hole edge (i.e. 4mm from the center) were able to survive only about 28-40 \% of total fatigue life due to the progressive damage caused by cyclic loading. Other specimen's FBG sensors, which were located at 8mm distance from the hole edge (i.e. 10mm from center),were survived during the entire fatigue life of the specimen (i.e. around 50000 cycles). Analyzed FBG data was indicated that a very unstable strain behavior exists around the hole during the experiment. It can be stated that monitoring damage dependent strain field around an open hole was the most challenging environment for embedded FBG sensors since they were directly exposed to the growing cracks in the vicinity of the hole edges. Nevertheless, embedded sensors were able to perform their monitoring function under cyclic loading condition at a designated distance from the hole edges. FBG sensor behavior in open hole composites was also studied using FEM analysis. Unidirectional carbon/epoxy composite specimens were considered as host material about 1mm thick, which were used for fatigue monitoring with FBG sensors and performed very well. After performing experiments for obtaining material constants, three FEM models were run to simulate behavior of FBG sensors embedded into open hole specimens under tensile loading. Results showed that a significant amount of axial strain (i.e approx. 900 microstrain) variation may develop along an FBG sensor which is located near the hole and aligned with center point of specimen in longitudinal direction. The revealed uneven strain field in the vicinity of an open hole is already a challenging environment for an FBG sensor even ignoring the growing damage in its surroundings under cyclic loads. These results clarify complicated behavior of FBG sensors that were embedded into open-hole specimens and tested under fatigue loading in previous studies. Overall, different techniques were investigated that effectively utilize FBG sensors in this study for static and fatigue monitoring of FRP composites. It can be stated that the used methods of monitoring FRP composites with embedded FBG sensors have created a strong basis for future SHM systems.