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Fiber Bragg-Grating Sensor Array for Health Monitoring of Bonded Composite Lap-Joints Mahmoodul Haq, Anton Khomenko, Lalita Udpa, and Satish Udpa
Abstract The detection and characterization of defects in adhesively bonded composite joints is of special interest for determining the load carrying capacity and structural integrity of resulting components for automobile, marine and aerospace applications. Embedded fiber Bragg-grating (FBG) sensors are being increasingly used to monitor the bonded region in adhesive joints as they do not affect the intrinsic bonding properties. This paper presents a highly reliable system that uses embedded FBG sensors for health monitoring in glass-fiber composite joints. Particularly, an array of strategically placed FBG sensors characterizes the extent and location of defects in the joints studied. Experimental data from the embedded FBGs can be further used to develop experimentally validated simulations (EVS) which can be used as a design tool and also to evaluate residual capacity of damaged joints. Preliminary results demonstrate potential of the developed technique for a wide variety of bonded joints with similar and dissimilar adherends.
22.1
Introduction
Composites are being increasingly used in aerospace, marine, and automotive sectors because of their light weight, high corrosion resistance, and excellent mechanical properties at elevated temperatures [1, 2]. Adhesively bonded joints are gaining popularity in place of conventional fasteners as they provide light weight designs, reduce stress concentrations, enable joining of dissimilar materials, and are often cheaper than conventional fasteners. Bonded joints provide larger contact area than bolted joints thereby providing efficient stress distribution enabling higher efficiency and improved fatigue life [3–5]. Nevertheless, the quality of adhesively bonded joints depends on various factors including manufacturing techniques, manufacturing defects, physical damage and deterioration due to accidental impacts, moisture absorption, improper handling, etc. These factors can significantly affect the strength of resulting bonded joints and a successful monitoring technique that can provide information about the adhesive layer and its resulting joint is essential. Strain distribution inside the adhesive layer can provide valuable information on the damage initiation, progression and health deterioration for in-service structures. Additionally, if the sensors can be incorporated prior to manufacturing, it can enable quality control of the resulting manufacturing processes. Ideally, the strain measuring sensors must be embedded in the adhesive and should not influence the intrinsic behavior of the adhesive or act as a damage initiator. In the past decade FBG sensors have become a promising tool to address this issue as they can be easily embedded inside the adhesive layer without affecting the resulting bonding properties [6–8]. In this study, an array of FBG sensors was embedded into the adhesive layer of single-lap joints. Similar to the study in reference [9], the array consists of several FBG sensors placed in the inspected area transverse to the joint direction, thereby allowing monitoring of the entire joint. Proposed technique allows identification of the defect location and its severity. Since FBG sensors can measure the strain distributions only in local area, the difference between measurements of reference FBG sensors in healthy region and those in vicinity of defected area provides us information about the defect position and severity. Also, since reference measurements within the same bonded area are used, the need for separate baseline or control data is not required.
M. Haq (*) • A. Khomenko • L. Udpa • S. Udpa Composite Vehicle Research Center, Michigan State University, 2727 Alliance Drive, Lansing, MI 48910, USA e-mail:
[email protected] G.P. Tandon et al. (eds.), Experimental Mechanics of Composite, Hybrid, and Multifunctional Materials, Volume 6, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-00873-8_22, # The Society for Experimental Mechanics, Inc. 2014
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Using this technique defects can be successfully located and their severity can be estimated based on the measurements of strain distributions in healthy and defective areas under static load. Also, strain distributions inside the adhesive during every step of manufacturing process can also be monitored. Moreover, accurate information about the adhesive obtained from these sensors can be used to develop and validate realistic and robust numerical simulations. Once validated experimentally, such simulations can be used as a powerful design and residual-life prediction tools. A similar effort on development of experimentally validated simulations through experimental data from embedded sensors has been attempted using single FBG sensors in Pi/T-joints [10]. This work follows the recommendations from [10] and includes an array of sensors instead of a single sensor. Furthermore, the array of strategically embedded FBG sensors can also be combined with other NDE modalities to develop reliable and robust data fusion techniques for precise structural evaluations.
22.2
Specimen Manufacturing and Sensor Installation
In this section, the principle of FBG sensor operation, strategic placement of sensors, details of materials used and the manufacturing process for adherends and single-lap joints are provided.
22.2.1 FBG Sensor Principle and Strategic Embedding of FBG Sensors in Array Fiber Bragg-grating is a sandwich-like distributed reflector with periodically changed refractive index that is embedded into the optical fiber [11]. Such a structure acts as an optical filter that transmits the entire spectrum of the light source and reflects back the resonant, Bragg wavelength. Bragg wavelength is given by the following Eq. 22.1: λB ¼ 2neff Λ
(22.1)
where neff is the effective refractive index of the core and Λ is the period of Bragg-grating. The spectrum bandwidth of back reflected radiation also depends on the effective refractive index of the fiber core and Bragg-grating period. As one can see the perturbation of Bragg-grating period results in the shift of Bragg wavelength: the strain response arises due to both the physical elongation of the sensor and the change in fiber effective refractive index due to photoelastic effects, whereas the thermal response arises due to the inherent thermal expansion of the fiber material and the temperature dependence of the refractive index. Wavelength-encoded nature of the FBG output provides a built-in self-referencing capability for the sensor. Since the wavelength is an absolute parameter, the output does not depend directly on the total light levels, losses in the connecting fibers and couplers, or source power. Moreover, the fiber and sensor have relatively small dimensions; therefore embedding in the composite structure does not affect the intrinsic properties of the host. These advantages of FBG sensors along with its immunity to electromagnetic interference, lightweight, high sensitivity, and ease in implementing multiplexed or distributed sensors make it very appealing for many areas of NDE applications, such as strain and temperature, vibration measurements, etc. [12]. However, FBG provides status of the region in the vicinity of sensor. Therefore in order to evaluate the entire area of interest an array of FBG sensors is used [9]. Each FBG sensor in the array can use the readings of other sensors as reference, which increases the robustness and adds the redundancy to the proposed technique. This will allow to overcome the thermal response of the FBG sensors and to compensate global strains of the in-service composite structure.
22.2.2 Glass-Fiber Composite Adherend Manufacturing The vacuum assisted resin transfer molding (VARTM) technique was used to manufacture the composite adherends for the lap joints. The reinforcement used for the adherend was Owens Corning ShieldStrand S, S2-glass plain weave fabric with areal weight of 818 g/m2. The distribution medium was AIRTECH Advanced Materials Group Resinflow 60 LDPE/HDPE blend fabric. The resin used was two part toughened epoxy, namely SC-15 obtained from Applied Poleramic SC-15 resin. A steel mold (609.6 mm 914.4 mm) with point injection and point vent was used to fabricate 508.0 mm 609.6 mm plates. After the materials were placed, the mold was sealed using a vacuum bag and sealant tape. The mold was then infused under vacuum with a pressure of 1 atm. The resin infused panel was cured in a convection oven at 60 C for 2 h and post cured at 94 C for 4 h.
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Fig. 22.1 (a) Healthy lap-joint with side-placed baseline FBG sensor A, (b) lap-joint with manufacturing defect, two reference side-placed FBG sensors C and D and the signal FBG sensor B and (c) actual photo of the lap-joint with manufacturing defect and 3 FBG sensors
Fig. 22.2 Manufacturing of composite single-lap joints with embedded FBG sensors
22.2.3 Composite Single-Lap Joint Manufacturing and Sensor Placement In this study, lap-joints were made from the glass-fiber reinforced plates manufactured by the above described process. The bonding area was sand-blasted and cleaned with acetone. Pseudo-disbond, or pseudo-damage representing a manufacturing flaw, was created by placing a teflon sheet of 15 mm 15 mm at the center of the bonded area. Two single-lap joints were manufactured: (a) baseline joint with no initial disbond, and (b) pseudo-disbond joint with disbond in the center of bonded area (see Fig. 22.1). The manufacturing of single-lap joints with embedded FBG sensors and pseudodisbond is represented in Fig. 22.2. Each lap joint has a bonding area of 50.8 mm 50.8 mm. The adhesive bond-line thickness was ensured to be 0.76 mm per ASTM D5868 by placing Teflon-coated steel strips at the end of the bonded area. These spacers were placed strategically such that it did not influence the resulting performance, and were removed prior to testing. The FBG sensors employed in this study had a 10 mm grating length, varying Bragg wavelengths and were obtained from Micron Optics, Inc, Atlanta, USA. It should be noted that the initial Bragg wavelength of the sensor does not affect strain coefficient which is 1.2 pm/με for each FBG sensor. The tensile and fracture properties of adhesive were obtained experimentally per ASTM standards. The experimentally obtained adhesive properties along with the experimental data from the sensors can be further used to develop realistic and accurate numerical models. These models are a part of a parallel effort by the authors and are currently in development and are not included in this paper.
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Fig. 22.3 Spectral response of the embedded in composite single-lap joint FBG sensor: (a) after infusing the resin in composite single-lap joint and (b) after the resin cured in composite single-lap joint
For the first healthy/baseline composite single-lap joint FBG sensor A with Bragg wavelength 1,532 nm was placed 7.5 mm away from the side edge; edge effects were found to be negligible in preliminary simulations. The second composite single-lap joint contained the manufacturing defect and three FBG sensors. Two reference sensors C and D with Bragg wavelength 1,572 nm were placed 7.5 mm away from the side edge, similar to FBG sensor A in the baseline joint. The central, FBG sensor B with Bragg wavelength 1,512 nm was placed in the middle, on the same level with reference sensors C and D. After embedding the array of strategically placed FBG sensors in composite single-lap joints FBG responses were monitored throughout every step of manufacturing process. It was observed that after resin cures the spectral response of FBG sensor is changed from narrow-band single peak to wide-band multi-peak spectrum. An example of the actual spectral change for reference side-placed sensor D is shown in Fig. 22.3. This may be caused by non-uniform stress distribution in adhesive layer due to resin shrinkage during the curing. It is known that in the vicinity of the defect the spectrum of FBG response splits to separate peaks that start to shift independently [13]. In this study we propose to use multi-peak feature of the FBG spectrum: peak shifts were tracked individually and corresponding strain values compared for the same FBG sensor and other FBG sensors. It is expected that for baseline sensor A and reference sensors C and D the variation of strain values should be lower than those of signal sensor B.
22.3
Composite Single-Lap Joint Evaluation
In this section, the experimental setup used for static tension-shear testing of composite single-lap joints and the experimental methodology are provided.
22.3.1 Experimental Setup and Methodology The experimental test setup is shown in Fig. 22.4. Testing was performed as per ASTM D5868 in displacement control with a rate of 0.25 mm/min. The displacement and applied load from MTS were recorded. Additionally, an external laser extensometer (LE-05 Epsilontech Laser Extensometer) was used to obtain precise relative displacements between the adherends. Spectral responses of FBG sensors embedded in composite single-lap joints were logged in PC using Micron Optics Optical Sensing Interrogator sm125-700. All measurements from FBG sensors and extensometer were synchronized with MTS data.
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Fig. 22.4 Experimental setup for composite single-lap joint evaluation
Fig. 22.5 Internal strain dependence on the applied load for baseline FBG sensor A
22.3.2 Experimental Results In the first set of experiments, healthy lap joint with baseline FBG sensor A (see Fig. 22.1a) was evaluated. Static tensileshear test was performed. The displacement and internal strain dependence on the applied load were recorded. To monitor internal strains the shift of the spectrum consisting of two peaks was tracked. The relationship between measured internal strain and applied load from both peak shifts is shown in Fig. 22.5. As one can see the deviation of internal strain values is insignificant therefore the entire spectrum shifts uniformly. In the second set of experiments lap joint with manufacturing defect, two reference FBG sensors C and D and signal sensor B (see Fig. 22.1b) was evaluated. The displacement and internal strain dependence on the applied load was measured similar to the baseline joint. To monitor internal strains the shift of the spectral peaks for all FBG sensors was tracked and recorded. Spectral response of reference FBG sensors C and D had one and three peaks respectively and central FBG sensor B spectrum contained five peaks. The comparison of internal strains at varying applied loads calculated from spectral peak shifts for reference FBG sensors C and D is shown in Fig. 22.6a. A magnified, zoomedin region of Fig. 22.6a is shown in Fig. 22.6b. Similar to the baseline FBG sensor A, the deviation of internal strain values is insignificant for reference sensors hence the entire spectrum shifts uniformly. This suggests that the manufacturing defect does not affect the local strain distribution in the vicinity of reference FBG sensors. Moreover the responses of reference sensors are identical which means they can be used to compensate thermal response of the Bragg-grating and global strains of the composite single-lap joint. However spectral response of signal from FBG sensor B is different. One can see that the manufacturing defect affects the local strains: it adds more non-uniform stress distribution in adhesive layer and causes the peaks in the spectrum to shift separately and the spectrum begins to spread out (see Fig. 22.7a). This feature can be used as the health indicator of the composite structure. For comparison spectral responses from reference FBG sensors C and D and signal FBG sensor B are plotted together in Fig. 22.7b. It is obvious that in the vicinity of defects, the spectral response of the FBG sensor changes significantly: it does not shift uniformly and local strain values increase substantially. The overview of measured results is represented in Fig. 22.8. The displacements of healthy and defected composite single-lap joints measured by laser extensometer at different applied loads are shown in Fig. 22.8a. Actual measurements do not start from zero as the grip closure introduces an initial compressive load of approximately 1,000 N. Local strain dependence on the applied load for baseline, reference and signal FBG sensors are represented in Fig. 22.8b. Initially at very low loads all FBG sensors behave similarly. As the applied load increases the response of the signal sensor B with manufacturing defect starts to deviate from reference and baseline FBG sensors. As it is well known, defects introduces
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Fig. 22.6 (a) Internal strain dependence on the applied load for reference FBG sensors C and D and (b) zoomed-in region
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Fig. 22.7 (a) Internal strain dependence on the applied load for signal FBG sensors B and (b) comparison of internal strain dependence on the applied load for reference FBG sensors C and D and signal FBG sensors B
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Fig. 22.8 (a) Comparison of the displacement versus load curves for healthy and defected single-lap joints and (b) comparison of strain versus load curves for baseline, reference and signal FBG sensors
stress concentrations that are very well captured by FBG sensor B that is in direct vicinity of the defect. The baseline and reference FBG sensors still agree very well. Upon further load increase (loads greater than 1,500 N), the response of reference sensors C and D is also influenced by the defect and starts to deviate from baseline response of sensor A. Overall, the variation of strains in the defective joint, namely, strains in B relative to C and D provide valuable information on the defect within that joint, and the location of the defect. Moreover, comparisons of strains from sensor
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A corresponding to baseline, pristine joints and sensors C, D and B reveal the variation of strains in a defective joint at various loading conditions. A similar approach can be extended to a wide array of sensors to fully characterize the location and extent of defects.
22.4
Conclusions
An array of strategically embedded FBG sensors in the composite lap-joints with and without manufacturing defects was successfully implemented. The response of various sensors throughout the manufacturing process and during experimental static tension-shear loading conditions was studied. The spectral response of the sensors changed from a narrow-band single peak to a wide-band multi-peak during the curing process, which is attributed to the non-uniform stresses introduced due to resin shrinkage. During mechanical loading, the multi-peaks were found to shift together in pristine joints, while the peaks shift separately in the vicinity of the defect. This feature can be used as a stand-alone indicator for presence of defect. Additionally, the strategic placement of an array of sensors within the defective joint allowed for temperature, global strain compensations, and eliminated the need for additional baseline data. Furthermore, the effect of stress concentrations in the vicinity of defects was well captured by the array of sensors, with the sensor closest to defect being active at lower loads while the sensors away from the defect active at higher loads. Overall, this technique shows potential in the detection of defect location inside the adhesives and allows estimation of damage severity. Since the strain distributions are dependent on various factors including joint geometries, failure modes, operative conditions, the distance between sensor and defect, the sensor arrangement, monitoring parameters, etc., further research on robust diagnosis methods based on fiber Bragg-grating sensors is needed. Also, statistically significant experimental tests need to be performed before any strong conclusions are made. Nevertheless, the proposed technique shows great potential in defect detection and health monitoring of bonded joints. The proposed system can be further enhanced by combining with other NDE modalities to create reliable and robust data fusion based techniques for health monitoring of composite structures. Acknowledgements This work was supported by US Army under TACOM/MSU Cooperative Agreement No. W56HZV-07-2-0001 and under ARL/MSU Cooperative Agreement No. ARL CA #W911NF-11-2-0017. The authors also acknowledge the cross-disciplinary collaboration with Dr. Alfred Loos and Dr. Nicholas Gianaris of Composite Vehicle Research Center, Michigan State University.
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