Determination of the strain distribution in adhesive joints using Fiber Bragg Grating (FBG) (original) (raw)
Related papers
Fiber Bragg-Grating Sensor Array for Health Monitoring of Bonded Composite Lap-Joints
Conference Proceedings of the Society for Experimental Mechanics Series, 2013
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.
2016
Structural health monitoring (SHM) systems are recognized as a key element in a modern and efficient approach to structural lifecycle management. With novel applications of advanced materials, such as extensive use of fibre reinforced composites on aircraft, further advances in monitoring and inspection techniques are required. This is especially relevant for repairs on primary (i.e. load bearing) military aircraft composite structures, where traditional non-destructive inspection methods alone may be insufficient for certification of the repair. Optical fibre sensor systems with continuous in-flight monitoring are considered a promising approach to providing the necessary information on the status of the repaired area. Fibre optic sensor systems commonly involve the use fibre Bragg grating (FBG) sensors bonded to or embedded in the structure. As an element in a certified repair strategy, the limitations of the load transfer from the structure to the sensor must be known. Hence, extensive analysis and testing must be done before introducing any adhesive system/optical fibre combination as part of the SHM system. This paper presents experimental test results obtained for the interface properties of optical fibres with polyimide or Ormocer® coating embedded in an adhesive. Three different adhesives, which are all relevant for military aircraft primary composite structural repairs, are included in the test program. A modified fibre pull-out test setup is applied for estimating the interfacial shear strength (IFSS) for each of the six combinations of adhesive and optical fibre, at two different nominal embedded lengths of 0.5 mm and 1.0 mm. For the polyimide coated fibre, the IFSS values obtained for the three adhesives from fibre pull-out do not differ significantly. The highest IFSS values are obtained at a nominal embedded length of 0.5 mm. The optical fibre with Ormocer® coating seems to have the strongest bond to the adhesives, but results in core pull-out instead of fibre pull-out. The obtained IFSS values for the core/coating interface of the Ormocer® coated fibre are not directly comparable with the IFSS values obtained for the polyimide coated fibre, but they still demonstrate the load transfer properties and the potential of the optical fibre for obtaining accurate strain measurements. As an overall conclusion, the polyimide coated fibre is considered the most appropriate for use in a fibre optic sensor system for SHM.
Response of fiber Bragg gratings bonded on a glass/epoxy laminate subjected to static loadings
Composite Structures, 2015
Fiber Bragg gratings (FBG) may be used to monitor strain over the surface of a structure as an alternative technology to conventional strain gauges. However, FBG bonding techniques have still not been established to yield satisfactory surface measurements. Here, two adhesives were investigated, one with low viscosity and the other with high viscosity for bonding FBGs on glass/epoxy sandwich skins. First, instrumented elementary specimens were tested under tension. FBG strain results were analyzed together with digital image correlation (DIC) measurements. The influence of the bonding layer on the measured strain and on the integrity of the sensor was investigated by considering different regions of interest. Next, an instrumented structural sandwich beam was tested under four-point bending. FBG rosettes were compared to conventional strain gauge rosettes. The high viscosity adhesive demonstrated behaviors that affected FBG accuracy. Brittleness of the bonding layer and poor interface adhesion were observed using DIC and X-ray tomography. By contrast, the low viscosity adhesive demonstrated satisfactory results. The FBG strain measurements appeared to be consistent with those of DIC. The accuracy is also adequate as the FBGs and the conventional strain gauges had similar results in three directions, under tension and under compression.
International Journal of Adhesion and Adhesives, 2022
Similarly to other industrial areas, there is a strong interest for the use of bonded FRP (Fiber Reinforced Polymers) repair or reinforcement for steel structures in the case of offshore applications. However, the reliability of the adhesively bonded (FRP) shall stand as high as steel renewal, this requires additional developments, in particular, a complete understanding of the repair mechanical strength which depends on material and interfacial properties. Fracture mechanics is an interesting approach to assess the risk to undergo interlaminar fracture or steel to adhesive interfacial disbonding failure. The experimental determination of the required design values for this an approach (critical toughness) are generally obtained using common tests such as Double Cantilever Beam (DCB), End Notched Flexure (ENF) or Mixed Mode Bending (MMB) tests. These tests require a precise crack length monitoring that is currently carried out using visual observation or Digital Image Correlation (DIC) on the flank of the sample. This may induce error in crack length measurement especially if the crack doesn't remain straight during the test. The paper presents a study of crack front monitoring by a distributed optical fiber as an alternative to the standard techniques to monitor crack front and to determine the critical toughness in mode I and II through respectively, DCB (Double Cantilever Beam) and ENF (End Notched Flexure) tests. Firstly, the issues related to the use of this continuous optical fiber are raised (insertion, precision resolution, measurement noise, exploitation methodologies). Then, some experimental investigations on ENF and DCB tests are presented and analyzed using the proposed methodology to monitor crack propagation using the optical fiber strain measurement. The obtained results are compared, focusing on the proper determination of the critical toughness of the adhesive. These results show that an optical fiber bonded on the surface of the sample can be used to measure and follow the crack propagation during the test which simplifies and adds precision to the standardize critical toughness computation method.
Composite Structures, 2015
This work addresses the experimental identification of mode I cohesive law of wood bonded joints. The approach combines the double cantilever beam (DCB) test with both digital image correlation (DIC) and embedded fibre Bragg grating (FBG) sensors. The spectrum geometric mean of the FBG reflected spectral response was determined, and the wavelength evolution was used to define the fracture process zone (FPZ) development phase. This evaluation allowed a consistent selection of experimental range of over which the identification procedure of mode I cohesive law is build up. Mode I crack length, Resistancecurve and cohesive law parameters are characterised and discussed. The strain energy release rate (G I) is determined from the P-d curve by the compliance-based beam method (CBBM). The crack tip opening displacement (w I) is determined by post-processing displacements measured by DIC. The cohesive law in mode I (r I-w I) is then obtained by numerical differentiation of the G I-w I relationship.
2008
This paper presents a method for the health monitoring of composite joints based on strain measurements using distributed embedded fiber Bragg grating (FBG) sensors. Secondary bonded woven E-glass/vinyl ester composite doubler plate joints were subjected to fatigue tension loading to induce stable crack propagation. A finite element (FE) model was developed to correlate experimental strain measurements prior to cyclic loading with the numerical predictions and to determine the sensitivity of the sensors to changes in longitudinal strain due to crack growth. Initial quasi-static tension tests demonstrated satisfactory correlation between the strains from the FBG sensors and FE predictions. The changes in longitudinal strain distribution during the fatigue tests were correlated with crack growth. A progressive shift in the strain distribution in the vicinity of the crack was observed. The experimental results demonstrated that a strain-based methodology can be utilized to detect crack propagation in this type of composite joints.
Dynamic Strain Measurement Using Improved Bonding Fiber Bragg Grating
This study proposes an improved bonding method for attaching a fiber Bragg grating (FBG) on surface of a structure to measure its dynamic strains. The improved bonding fiber Bragg grating is called IBFBG for short in this study. The bonding method provides the chance to let the IBFBG and resistance strain gauge measure the longitudinal strain at the same point. This study use two steel cantilever beams, one with rectangular cross section and the other with circular, to demonstrate the ability of the IBFBG in dynamic strain measurements. Their dynamic strains are induced by impacts of a steel ball in free-fall transversely or in pendulum axially, respectively. The obtained strain time histories from both the IBFBG and strain gauge are nearly proportional in amplitudes; in addition, the Fourier spectra of those time histories show the IBFBG recognizes the natural frequencies of the cantilever beams up to 20000 Hz.
Materials, 2021
To date, a method of attaching a FRP (fiber-reinforced polymer) to concrete members with epoxy has been widely applied to increase the strength of the member. However, there are cases in which the adhesion of the epoxy deteriorates over time and the reinforcing effect of the FRP is gradually lost. Therefore, monitoring whether or not the reinforcing effect is properly maintained is needed in order to prevent a decrease in the structural performance of the member improved by FRP reinforcement. In this regard, this study examines FRP with OF (optical fiber) sensors to monitor the reinforcing effect of FRP in concrete structural members. In particular, this paper seeks to determine an appropriate adhesion length when FBG (fiber Bragg grating) based OF sensors are externally bonded to FRP strips with epoxy resin. To this end, a tensile test was carried out to evaluate the sensing performance according to the adhesion length. In addition, an analytical approach was performed and the resu...
Metals and Materials International, 2014
This work investigated issues for an efficient and reliable embedding and use of Fiber Bragg Grating (FBG) sensors for strain monitoring of composite structures with particular regard to the manufacturing process of components in the nautical field by means of the vacuum bag technique in autoclave. CFRP material laminates with embedded FBGs were produced and the effect of the curing process parameters on the light transmission characteristics of the optical fibers was initially investigated. Two different types of coating, namely polyimide and acrylate, were tested by measuring the light attenuation by an Optical Time Domain Reflectometer. Tensile specimens were subsequently extracted from the laminas and instrumented also with a surface-mounted conventional electrical strain gage (SG). Comparison between the FBG and SG measurements during static tensile tests allowed the evaluation of the strain monitoring capability of the FBGs, in particular of their sensitivity (i.e., gage factor) when embedded.
Open Journal of Composite Materials, 2013
A joint combining riveting and bonding is considered in terms of structural performance if the composite structure has a mismatched stiffener. The transfer loading is correlated with high performance aerospace joints to increase delamination resistance in the out-of-plane direction. However, combined joints (rivet/bonded) will create a bearing area that induces another potential damage source aside from secondary bending moment on the edge of the stiffener. Another problem is that the structure is difficult to be inspected by using conventional methods because of limited accessibility. The use of embedded fiber Bragg grating (FBG) technology in the structure as a strain sensor can potentially solve the problem in structures that have a stiffness mismatch. The FBG can be used to detect and characterize delamination before it reaches a critical stage. The model used to represent this problem is a thin composite stiffened skin under two load cases: tension and three-point bending. Finite element modeling using a traction versus separation theory is performed to determine the critical area on the specimen for placement of the FBG before manufacturing and testing. Experiments were presented to determine the distribution of load in a combined joint under both loading cases using ideal loads to create a secondary bending moment and bearing loads in the stiffness-mismatched structure. In this research, the FBG successfully detected and characterized the delamination caused in both loading cases. In addition, FBG can predict the delamination growth quantitatively. A spectrum graph of the FBG results can be used to replace the conventional mechanical graph in composite structural health monitoring in real applications.