Abstract A basic review of the traditional method ofdamage detection and some issues will be listed in the introduction below.Moreover, an idea is going to be presented and aimed to use several types ofembedded optical strain sensor to set an advanced technique called in-situstrain-based fibre optic damage detection assessment system (FODDAS) which canaddress and improve detection of damage in the composites. Classic fibre opticsensors and fully distributed sensor network system will be discussed in the literaturereview. In additional to, fracture of the optical fibre is supposed to be identifiedas an approach related sensor network to detect damage, and an example fordetecting matrix crack by the harm of optical fibre will be pointed at the lastpart as well as advantages of the method. 1. Introduction Fibre reinforced composites are consistedof and composited of reinforced fibrematerials, such as glass fibre, carbon fibre, aramid fibre, and the matrixmaterials through winding, composites moulding or extrusion moulding process.However, there are some unexpected damages in the fibre reinforced composites.
For instance, micro-crack is a kind of those damages in the fibre reinforcedcomposites, which are created greatly under impact conditions and fatigue,should be healed before fast crack propagation and catastrophic failure happen.Therefore, it is essential that the methods and technologies should be createdor improved to monitor the damage in the composites. Due to the barely visibleimpact damage usually existing in the composites, a competent technology ofnon-defect test for damage determination and continuous material structuralintegrity monitoring has been considered as the professional method to detectdamage. At present, most conventionalNDT technologies, such as visual inspection, thermography, ultrasonic C-Scanand X-Ray radiography are restricted as they have to test structural componentsof complex geometry which are taken out of service for the considerable lengthof time because of post-damage check and evaluation. For continuous and in-situ monitoring of integritystructures, a method of using strain gauges shows great further potential.However, strain gauges are susceptible to electrical and electromagnetic effectin addition to the damage. As for the issues, how to find a feasible method toimprove damage detection techniques have to start from following representativemodes of damage in the composites. According to course book of sensors and composites, there are twotypical modes of damage in the fibre reinforced composites.
Figure 1 shows aseries of crack situations, which are happening when a unidirectional fibrereinforced composite (Figure 1 a) is suffered from tensile loading. In thiscase, the failure strain of matrix is presumed to be importantly higher thanthat of the fibres. Therefore, with increasing tensile loading on thecomposites (Figure 1 b), a reinforced fibre will break at its weakest positionanywhere along its length. Obviously, a redistribution of stress will takeplace with additional stress being brought to bear on nearby fibres. Thisfailure mode is a characteristic process in most fibre reinforced composites.Other processes (Figure 1 c) which will take place in the vicinity of brokenfibre are interfacial debonding or matrix yielding.
The last process (Figure 1d) represents the position where the fibre debonding shown in the Figure 1 chas been transformed to a longitudinal crack along the direction of fibres and become a planethickness crack as well.Figure 1: schematic illustrations of damage mode 1 Above mode is the same complexity as when the failurestrain of fibre is higher than that of the matrix, which shows in Figure 2. Figure 2: schematicillustrations of damage mode 2 Basically, the consequences of all types of damagein smart structures are part or whole changes in strengths and stiffness. Fibreoptic strain sensors can measure these changes through one of the opticalproperties, such as intensity, wavelength, phase or state of polarization.Additionally, fibre optic strain sensors can be integrated into a presentcomposite structure and form the kind of superb smart structure so as to accessthe interior of material where other sensors or devices cannot probe.
The researchwas directed by Bhatia V (1995) and gathered some results, which have indicatedthat embedded fibre optic sensor carried through the same way as eithernon-embedded sensors in terms of failure stresses in tension and compression,notwithstanding the stress and strain concentrations around the embedded fibreoptic sensor. As fordetecting damage, embedded sensors should have appropriate mechanical bondingwith the host composites structures so as to existing the same strain gradientsituation as host composite structures. The present essay intends to focus on usingembedded fibre optic strain sensors to establish a technical system, which candevelop in-situ damage detection and assessment systems. The major issuesneeded to be solved are an in-situ detection of complex structural damage anddamage state, which will be addressed via in-situ strain-based fibre opticdamage detection assessment system (FODDAS). 2.Literature Review As forimproving damage detection, the FODDAS should be enhanced via various majortypes of fibre optic sensors and related optical properties which are helpfulto measure mechanical strain. The general optical fibre consists of a centresilica core covered by an annular silica cladding with an outermost protectivecoating, as shown in figure 3.
Main fibre optic strain sensors are categorisedinto intensity, interferometric and polarimetric sensors in terms of whichoptical properties are inflected by the external loading. Due to their fibrouscharacterisation, they can defiantly measure axial strains. For measuringmechanical strains, there are two aspects of requirements to fibre optic strainsensors. One is that fibre strain sensors should be too adjacent the damage togain the reliable data and responses. Another point is that they have toguarantee an adequate strain resolution, in case that the positions of thedamage are far away from them. In many applications (Tay A K,1990), one-loopserpentine fibre optic sensors were used to increase the degree of sensitivity.Furthermore, optical fibres have a critical value of failure strain as the samelevel as that of the reinforced fibres of the host composites. Obviously, if thefailure of the optical fibres is used to indicate the damage, indeed, it isunnecessary for the strain measurement.
Figure 3Schematic of the general optical fibre 2.1 Intensity sensors Aseveryone knows, the simplest fibre optic sensor is based on a kind oflight-intensity modulation in a multi-mode optical fibre. In its workingprocess, the light wave with greater intensity is going to spread, Meanwhile,when the optical fibre is under strained state, it will reflect losses of lightintensity.
An advantage is that these sensors are simply to establish and donot present complex instrumentation and signal processing. However, they canonly gather limited data and information about the location of damage. For thisdrawback, if a large number of intensity sensors can be used in a completesystem or a network (Kulman R,1987. McBride R,1998) for detecting significantdamage, they could be useful and practical because of the low cost in theco-operation system or network.
2.2 Fibre Bragg Grating (FBG) sensor Thefibre Bragg grating sensor is based on a single mode optical fibre, and has asuit of periodic reflective Bragg gratings along with the length of theindividual fibre (Udd E,1996), as illustrated in figure 4. When an embedded FBGsensor is impacted by the external load, the changes in the wavelength of thegratings could be defiantly associated with mechanical strain. During theproducing of FBG sensors, the pitch spacing of every grating is able to controlindividually so that the wavelength shift of a set of individual FBG sensorscan be tracked simultaneously for multiplexing. According to an existing research(Liu T,1998), the positive aspects of FBG sensors are easy to use wildly,unaffected by discrepancy calibrations, and having a high value of failurestrain, up to 2%. Additionally, the sensors are also easy for multiplexing. Onthe other hand, the potential drawbacks that the measured strain could bethree-dimensional features so as to that a detailed analysis of the output isnecessary to correctly work out axial strains, and that they are very sensitiveto the influence of temperature variations. Figure 4Diagram of Bragg Grating Sensor 2.
3Polarimetric sensor Thepolarimetric sensor is a special optical fibre based on an elliptical core duelmode with birefringent polarisation-maintaining. Basically, the birefringencewill be generated by the remanent strain field across the core in addition tothat could be induced by an asymmetry character of the core geometry. Tospecifically, the localised sensing region of a polarimetric sensor usually ismade by either two in-line splices (Figure 5 a), or one in-line splice with areflecting mirror in the end (Figure 5 b). It is worth noting that this type ofsensor costs too much and has a complex sensing system. Moreover, low axialstrain sensitivity and three-dimensional feature of measured strains willsometimes get the process of detection into trouble. whereas the excellentcharacter of high transverse sensitivity will bring the sensors more help to confirmthe location of impact.Figure 5Schematic of (a) in-line splice and (b) single-ended polarimetric sensor 2.4 Fibre optic sensor system or network Toroundly inspect the damage in the smart composites, it is necessary that themultiple strain value is measured and determined.
As for how to achieve thecomplex and multivariable measurements, mentioned fibre optic detection damage sensorsystem (FODDAS) is able to provide such technique. According to a research (CulshawB,1985), A direct way to set a sensor network system is easily using enoughnumber of orthogonal arrays of intensity optical fibre at different plyinterfaces to form a composite structure. Consequently, it is possible that theimpact location, damage severity and distribution could be estimated as opticalfibre across the depth of the smart structures. An existing research (RogersA,1999) results showed a type of multiplexing technique calledquasi-distributed fibre optic sensor network system.
As the Figure 6 shows, theindividual sensor in the network system can be posed into either seriestopology (see Figure 6 (a)), parallel topology as fleshed out figure 6 (b) or acombination of both as presented in figure 6 (c). Another idea which thesensing location can be at any points along with the direction of the opticalfibre called fully distributed sensor network system. When the external loadcauses the optical fibres change such as pressure or tensile in the extent of thereflected signal, the mechanical strain will be detected by mentionedmultiplexing techniques. In general, this type of sensor network system isconsiderable to provide guarantees in the further damage assessment and detection. Figure 6Schematic of multiplexed sensor system in (a) serial topology, (b) paralleltopology and (c) serial and parallel combined arrangement 2.5 Method of detectingdamage Damage usually exists in the form of three states,such as matrix cracks, delamination and fracture of reinforced fibres. One ofthe most straightforward methods of detecting damage is based on whether theembedded sensing fibres are to fracture or not. In essence, fracture of opticalfibre approach is a reliable technique to inspect damage at present.
Forexample, matrix crack or fracture of reinforcing fibre could lead a part orcomplete fracture, especially, of the nearby optical fibre. This feature isvery useful in some applications where it is possible to know whether thecomposite is under sustained impacts above the thresholds (Martin A,1997).Usually, the fibre does not have to fracture completely, whereas it sufferssome level of breaking after the impacts. In a sense, the broken fibre willemit light because of the fracture, then the light can be detected bymonitoring devices. It is no denying to say that this approach can only fixwith one-time damage led by a transverse loading. However, the nature of thismethod depends on the magnitudes of local normal stress, so there is no need toconcern about the quality of interfacial bonding between the fibre and host composite.The distinct advantage is that it can be applied to the smart composite withcomplex geometry and do not to be required to be in the all-time standby modeso as to reduce the cost of monitoring devices.
3. Conclusions So far, many efforts to improve the methods ofdamage detection have been proposed and discussed. Although traditional approachesto detect damage, such as visual inspection and thermography, are basictechniques and pioneer in the area of damage detection, they still have the obviousweak aspect which cannot be applied to detect such damage with complexstructural geometry. FODDAS is built in the structures of laminated composites andis based on the embedded fibre optic strain sensors, achieving multiple strainmeasurements. These sensors have been enhanced to the position where they have realisedcapabilities to be the enable technique in the development of such networksystems. The core part of this technology has concentrated on detection ofdamage caused by loading impacts embodied in a typical damage mode of matrixcrack. Moreover, FODDAS must be custom-built and have the specific structure notonly with many different types sensors or networks but also with assessmentmodels, further formulated for different application field. All in all, to makea progress on the FODDAS and bring it to be a viable and practical technology,more conclusive research and experiments must be carried in the near future.
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