229 research outputs found

    Prediction of delamination in glass fibre reinforced composite materials using elasto-plastic modelling

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    Glass Fibre reinforced composite (GFRC) has been used for numerous structural applications in Aerospace, Chemical, Automotive and Civil infrastructure fields over a hundred of years. Due to this reason, understanding the intricate fracture behaviour of GFRC materials is crucial and essential for designing critical structural components. Voids and micro-cracks are considered as imperfections in Glass Fibre Reinforced composites. Much research has been undertaken on approaches to calculate and evaluate the effects of the imperfections on mechanical properties. However, it is an established fact that the micro-mechanical approach alone is not sufficient to understand a complete damage accumulation process during delamination. The damage mechanism which largely affects the performance of GFRC structures is commonly known as 'delamination'. Since the delamination is invisible, and hard to detect with ordinary non-destructive evaluation methods, therefore it is considered as a hidden killer which can cause catastrophic failure without any prior warnings. Due to this reason, research work on delamination modelling, damage detection and self-healing materials have been the highly placed research topics for more than five decades. Unfortunately there are a number of unresolved problems in delamination damage modelling and prediction, and few grey areas regarding application of Structural Health Monitoring systems to monitor delamination damages. This thesis has proposed to study the insight into the cause of delamination damage and its propagation mechanisms, by analytical modelling and experimental verifications. Within this research project, extension of the work by Tsukrov and Kachanov (2000) – “An innovative Elasto-plastic model” has been undertaken to evaluate, investigate and model the onset and propagation of delamination damages. Mode I, Mode II as well as Mixed Mode I/II delamination damage analysis has been utilised to study the proposed model predictions for GFRC structures for both in-plane and out-of-plane load applications. The proposed model has been validated using the Double Cantilever Beam (DCB), End Notch Flexure configurations (ENF) and Cracked Lap Shear (CLS) experiments on 0/90-glass woven cloth specimens. For the validation process, the procedures stipulated by ASTM standards were employed. It was observed that there were significant discrepancies between calculated fracture energies using standard procedures and the proposed model. Interestingly these observations have revealed some inconsistencies associated with the standard method for strain measurements that majorly controls the fracture energy calculations. This research project has demonstrated and evidently proven the accuracy of the proposed model predictions using the strain measured with embedded Fibre Bragg Grating (FBG) sensors, located inside the sample in proximity of the crack tip. The extended use of FBG strain measurement has created a breakthrough in Structural Health Monitoring (SHM) of composite structures. Non-availability of a suitable damage prediction model is an issue for accurate damage monitoring process. The proposed model has also demonstrated the potential for its integration with Structural Health Monitoring (SHM) systems. Additionally, Thermoplastic Stress Analysis (TSA) has been employed to monitor delamination. The potential for integration of FBG sensors and TSA techniques has been experimentally demonstrated during this project and, it is another breakthrough in SHM field as a result of this research. In addition to analytical model, a detailed Finite Element model was also created on ABAQUS commercial software. The cohesive elements with state variables (SDV) and UMAT codes were used for FEA simulations. Interestingly, the FEA results have shown an excellent correlation with the experimental results. Finally, this thesis has evidently proved the validity of the proposed model and integration of model with SHM system based on FBG sensors and TSA techniques. The outcomes of the thesis have provided a novel and innovative damage prediction model and a breakthrough technology for SHM systems

    The use of a CFBG sensor for detecting damage in composite laminates and adhesively bonded joints

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    Reliable in-situ damage detection techniques which can detennine the existence and location of damage in composite materials and structures are critical for the effective use of these materials. In this work, embedded chirped fibre Bragg grating (CFBG) sensors have been shown to be successful for both detection and location of matrix cracks in composite laminates and disbond detection in bonded composite joints. In all the cases, the CFBG reflection spectra were predicted using commercial software and agreed well with the experimental results. In the matrix cracking work, single matrix cracks in cross-ply GFRP (glass' fibre reinforced plastic) laminates were detected and located using a CFBG sensor embedded within the 0° plies, near the 0/90 interface. The CFBG sensor showed an approximately sinusoidal variation of the intensity of the reflected spectrum at the position of the crack, enabling both crack development and crack position to be identified. It was shown that the precise position of the cracks does not correspond with the bottom of a dip in the reflected spectrum, as has previously been thought. Disbond initiation and progression from either end of a composite bonded joint was monitored by embedding the CFBG sensor in one of the GFRP adherends, with the low wavelength end ofthe sensor positioned at the cut end ofthe adherend. A shift in the low wavelength end of the spectrum to lower wavelengths indicated disbond initiation and movement of a perturbation in the reflected spectrum towards higher wavelengths indicated disbond propagation. In a related fashion, disbond initiation and propagation was detected from the high-wavelength end ofthe spectrum (adjacent to the other cut end of the adherend). With the aid of a parametric study based on a closed-form solution for the strain field in the bonded joint (available in the literature), it has been shown that the sensitivity ofthe CFBG sensor in detecting the disbond depends mainly on the position of the sensor within the adherend and the strain distribution in the adherend. Finally, artificial manufacturing defects were introduced into GFRP-GFRP bonded joints using Teflon inserts and it has been demonstrated that the location of the defects is possible using the CFBG technique.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

    Classification of composite damage from FBG load monitoring signals

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    <p>This paper describes a new method for the classification and identification of two major types of defects in composites, namely delamination and matrix cracks, by classification of the spectral features of fibre Bragg grating (FBG) signals. In aeronautical applications of composites, after a damage is detected, it is very useful to know the type of damage prior to determining the treatment method of the area or perhaps replacing the part. This was achieved by embedding FBG sensors inside a glass-fibre composite, and analysing the output signal from the sensors. The glass-fibre coupons were subjected to mode-I loading under tension-compression and static tests, in order to induce matrix cracks and delamination damages respectively. Afterwards, using wavelet features extracted from spectral measurements of the FBG sensors, classification of the damage type was carried out by means of support vector machines as a general classification tool with a quadratic kernel.</p

    Mechanism of Subordinate Peak Skewing of FBG Sensor during Cracks Propagation Monitoring on Aluminum Alloy Structure

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    This study investigates the variety of the spectra features of fiber Bragg grating (FBG) around the crack tip during fatigue crack propagation. The study results reveal that the turning of the subordinate peak is significantly associated with crack lengths and corresponds to strain gradient along the FBG. Meanwhile, the strain distribution sensed by the FBG changes with the sensing section of the grating. FBG sensors could observe the monotonic plastic zone ahead of the fatigue crack tip. The cubic strain is distributed along the grating, with monotonic plastic zone propagation at the initial and terminal part of the grating, at approximately a 30% ratio of the entire grating. However, the monotonic plastic zone is sensed by the FBG, at ±15% bias of the grating center, with the quadratic strain gradient pattern along the grating. In particular, when the initial and terminal parts of the grating experience highly inhomogeneous strain distribution, the spectrum distortion occurs

    Strain state detection in composite structures: Review and new challenges

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    Developing an advanced monitoring system for strain measurements on structural components represents a significant task, both in relation to testing of in-service parameters and early identification of structural problems. This paper aims to provide a state-of-the-art review on strain detection techniques in composite structures. The review represented a good opportunity for direct comparison of different novel strain measurement techniques. Fibers Bragg grating (FBG) was discussed as well as non-contact techniques together with semiconductor strain gauges (SGs), specifically infrared (IR) thermography and the digital image correlation (DIC) applied in order to detect strain and failure growth during the tests. The challenges of the research community are finally discussed by opening the current scenario to new objectives and industrial applications

    The response of embedded FBG sensors to non-uniform strains in CFRP composites during processing and delamination

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    From airplanes to sailboats to bridges, composite materials have become a significant part of our everyday structures. With increasing demand, these materials are pushed to their limits to improve structural efficiency. As a consequence, research and development must continually improve products and provide support for the end user who will need to know the characteristics of their new material. Progress made in the area of optical fibre sensing has opened new avenues for measuring and monitoring fibre-reinforced polymer (FRP) composites, since they can be embedded directly into the composite during manufacturing. These globally noninvasive sensors can provide internal strain and temperature measurements from the moment processing starts until the final failure of the part. The goal of this research is to develop and demonstrate fibre optic sensing techniques that can characterize the internal strain state of FRP composites. In particular, this work focuses on measuring three-dimensional, non-uniform strain fields in carbon fibre-reinforced polymers (CFRP) using fibre Bragg grating (FBG) sensors. Although FBG sensors are becoming widespread for simple uniaxial strain measurements, their response to complex, non-homogeneous strain fields is still difficult to interpret. To illustrate advances in both experimental techniques and the interpretation of measured FBG data, two main areas of composite monitoring are addressed. They include the study of residual strain evolution and of delamination cracking, which both produce non-homogeneous strain fields. Unidirectional carbon fibre-reinforced polyphenylene sulphide (AS4/PPS) laminates are observed during processing to measure residual strain progression, and then later subjected to Mode I double cantilever beam delamination tests. These thermoplastic composite specimens are also produced in a cross-ply configuration, for the purpose of residual strain monitoring. In each laminate, a long-gauge length (20-35 mm) FBG is embedded parallel to the reinforcing fibres, and centred along the length of the plate. Results of polarization sensitive FBG monitoring indicate characteristic material state changes such as the glass-transition and the melting temperatures. These measurements take advantage of both the transverse and longitudinal strain sensitivity of the FBG. When transverse strains are unequal they induce birefringence in the FBG (defining a fast and a slow axis), which results in a split of the normally bell-shaped reflected spectrum. An evolution of this birefringence is monitored during cooling, culminating in average residual transverse strain differences in the embedded FBGs of 230 ΌΔ and 410 ΌΔ for unidirectional and cross-ply specimens respectively. Based on the wavelengths measured along the fast polarization axis of the fibre, (observed to be less sensitive to transverse strains) cross-ply specimens exhibit absolute longitudinal residual strains in the order of -350 ΌΔ. Small longitudinal strain values are the result of the low coefficient of thermal expansion of the carbon reinforcing fibres. An important step forward in FBG monitoring is taken by measuring the absolute values of the three unequal principal strains in a composite material without making assumptions about the state of strain in the FBG (i.e. diametric loads, plane stress, axisymmetry, etc.). For this purpose, a polarization controlled, hybrid FBG-Fabry PĂ©rot optical sensing technique is developed to measure residual strain evolution. The Fabry PĂ©rot sensor used in this hybrid method is only sensitive to longitudinal strains, thus providing the additional data required to solve the three-dimensional strain state directly. To better understand the state of residual strain in the composite material, a temperature dependent thermoelastic finite element model is employed to investigate the strain accumulation during cooling. By comparing modelled results to the data from the optical fibre, it is shown that the mould influences the residual strain development during cooling, and that some of these strains are released after demoulding. Examination of the simulated and experimental curves indicates that the final residual strain state observed with the FBG is close to that of a freely cooling composite plate. Since the embedded optical fibre is a local inclusion, its strain state is not necessarily that of its host material. In this work, finite element models are used to determine the stresses and strains developed in the surrounding composite material. Near the optical fibre, there is a perturbation of the strain field that extends to a distance of three fibre diameters. In the far-field of cross-ply specimens, tensile transverse stresses reach half the matrix fracture strength. This may help to explain matrix cracking observed on the surface of these specimens. The second portion of this study is aimed at the measurement of non-uniform longitudinal strains superimposed on an already three-dimensional residual strain state. A polarization adapted optical low coherence reflectometry (OLCR) technique takes distributed measurements of the local Bragg wavelengths for a given polarization axis. For a constant state of birefringence, one can relate the distributed wavelengths to the non-uniform longitudinal strains along the length of the FBG sensor. Delamination cracking in double cantilever beam specimens creates a non-uniform strain field ideally suited to illustrate this type of measurement. At increasing crack lengths, the distributed wavelengths (proportional to axial strain) are measured by an FBG embedded parallel to the delamination plane. The long gauge length of the sensor provides a sufficiently large set of data so that the crack position and growth direction can be distinguished. The strains retrieved from these experiments are further employed to determine the stress distribution caused by the fibres bridging the delamination crack. The combination of FBG measurements with inverse identification via finite element modelling is a new technique for determining bridging laws from static delamination specimens. Results of this work indicate that the maximum bridging stress is approximately 2.5 MPa and that the fibre bridging zone length ranges from 20-50 mm. Comparisons of bridging laws determined using this method and a J-integral approach are made using a second finite element model that includes cohesive elements. Simulations of advancing delamination cracks highlight the sensitivity of the force-displacement response of the specimen to differences in bridging laws. Through the advances in FBG-based methods outlined in this thesis, significant progress is made in the area of non-homogeneous strain detection in fibre-reinforced composites. This allows for improved characterization of three-dimensional residual strain states and the non-uniform strain distributions caused by delamination cracking

    The Public Service Media and Public Service Internet Manifesto

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    This book presents the collectively authored Public Service Media and Public Service Internet Manifesto and accompanying materials.The Internet and the media landscape are broken. The dominant commercial Internet platforms endanger democracy. They have created a communications landscape overwhelmed by surveillance, advertising, fake news, hate speech, conspiracy theories, and algorithmic politics. Commercial Internet platforms have harmed citizens, users, everyday life, and society. Democracy and digital democracy require Public Service Media. A democracy-enhancing Internet requires Public Service Media becoming Public Service Internet platforms – an Internet of the public, by the public, and for the public; an Internet that advances instead of threatens democracy and the public sphere. The Public Service Internet is based on Internet platforms operated by a variety of Public Service Media, taking the public service remit into the digital age. The Public Service Internet provides opportunities for public debate, participation, and the advancement of social cohesion. Accompanying the Manifesto are materials that informed its creation: Christian Fuchs’ report of the results of the Public Service Media/Internet Survey, the written version of Graham Murdock’s online talk on public service media today, and a summary of an ecomitee.com discussion of the Manifesto’s foundations

    Structural Health Monitoring Damage Detection Systems for Aerospace

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    This open access book presents established methods of structural health monitoring (SHM) and discusses their technological merit in the current aerospace environment. While the aerospace industry aims for weight reduction to improve fuel efficiency, reduce environmental impact, and to decrease maintenance time and operating costs, aircraft structures are often designed and built heavier than required in order to accommodate unpredictable failure. A way to overcome this approach is the use of SHM systems to detect the presence of defects. This book covers all major contemporary aerospace-relevant SHM methods, from the basics of each method to the various defect types that SHM is required to detect to discussion of signal processing developments alongside considerations of aerospace safety requirements. It will be of interest to professionals in industry and academic researchers alike, as well as engineering students. This article/publication is based upon work from COST Action CA18203 (ODIN - http://odin-cost.com/), supported by COST (European Cooperation in Science and Technology). COST (European Cooperation in Science and Technology) is a funding agency for research and innovation networks. Our Actions help connect research initiatives across Europe and enable scientists to grow their ideas by sharing them with their peers. This boosts their research, career and innovation

    Structural health monitoring damage detection systems for aerospace

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    Acoustic emission detection using optical fibre sensors for aerospace applications

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    Structural Health Monitoring (SHM) ensures the structural health and safety of critical structures covering a wide range of application areas. This thesis presents novel, low-cost and good-performance fibre Bragg grating (FBG) based systems for detection of Acoustic Emission (AE) in aircraft structures, which is a part of SHM. Importantly a key aim, during the design of these systems, was to produce systems that were sufficiently small to install in an aircraft for lifetime monitoring. Two important techniques for monitoring high frequency AE that were developed as a part of this research were, Quadrature recombination technique and Active tracking technique. Active tracking technique was used extensively and was further developed to overcome the limitations that were observed while testing it at several test facilities and with different optical fibre sensors. This system was able to eliminate any low frequency spectrum shift due to environmental perturbation and keeps the sensor always working at optimum operation point. This is highly desirable in harsh industrial and operationally active environments. Experimental work carried out in the laboratory has proved that such systems can be used for high frequency detection and have capability to detect up to 600 kHz. However, the range of frequency depends upon the requirement and design of the interrogation system as the system can be altered accordingly for different applications. Several optical fibre configurations for wavelength detection were designed during the course of this work along with industrial partners. Fibre Bragg grating Fabry-Perot (FBG-FP) sensors have shown higher sensitivity and usability than the uniform FBGs to be used with such system. This was shown experimentally. The author is certain that further research will lead to development of a commercially marketable product and the use of active tracking systems can be extended in areas of healthcare, civil infrastructure monitoring etc. where it can be deployed. Finally, the AE detection system has been developed to aerospace requirements and was tested at NDT & Testing Technology test facility based at Airbus, Filton, UK on A350 testing panels
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