433 research outputs found
Structural Health Monitoring of Composite Aircraft Structures Using Fiber Bragg Grating Sensors
Get PDFAircraft industry is continually striving towards reducing the acquisition, operation and maintenance costs. Usage of advanced composite materials in primary aircraft structures have resulted in significant weight savings owing to their higher specific strength and specific stiffness. Composite structures, in spite of their inherent advantages, are prone to various damages. To detect and repair various structural damages that can occur during the service life of the aircraft, a thorough inspection schedule is implemented through conventional visual and Non Destructive Evaluation methods. Such scheduled inspections lead to considerable increase in maintenance cost & down-time of the aircraft. An online structural health monitoring (SHM) system consisting of well-designed sensor networks incorporated in the structure along with necessary hardware and software can provide information about the structure, thereby leading to reporting of flaws or damages in real time. Such a system can provide inputs for condition based maintenance which can result in reduced maintenance cost. This paper presents the work carried out at CSIR-National Aerospace Laboratories towards developing a flight-worthy SHM system and its demonstration on an unmanned aerial vehicle (UAV). Sensor selection, characterization, instrumentation design, algorithm development towards damage detection & load estimation at lab level and implementation of the technology on a UAV are discussed in this paper
Predicting fracture of laminated composites
No full textThe compressive strength of currently used carbon fiber-reinforced plastics is generally 30–40% lower than the tensile strength due to fiber microbuckling, thus it is recognized that the compressive strength is often a design-limiting consideration. The zones of compressive stresses can appear in composite structures even under tensile loads. They could be due to the presence of holes, cut-outs and cracks, or generated by impact. It has been revealed that a possible mechanism of failure initiation is fiber or layer microinstability (microbuckling) that might usually occur in regions where high stress gradients exist, for instance, on the edge of a hole or near free edges. A better understanding of the compression failure mechanisms, specific only to heterogeneous materials, is crucial to the development of improved composite materials. The task of deriving Three-Dimensional [“3-D”] analytical solutions to describe the compressive response has been considered as one of great importance. Such solutions, if obtained, enable to analyze the behavior of a structure on the wide range of material properties, and kinematic and loading boundary conditions, without the restrictions imposed by simplified approximate methods
Aerospace engineering requirements in building with composites
No full textThe growing use of composite material has arisen from their high specific strength and stiffness when compared to the more conventional materials, and the ability to tailor their structure to produce aerodynamically more efficient structural configurations. In this introductory chapter, it is argued that fiber-reinforced polymers, especially carbon fiber-reinforced plastics can and will, in the near future, contribute more than 50% of the structural mass of an aircraft. Of course, affordability is the key to survival in aerospace, whether civil or military and therefore, effort should be devoted to low-cost manufacturing methods in addition to analysis and computational simulation of the manufacturing and assembly process. The simulation of the structural performance should not be neglected since they are intimately connected. Virtual reality models in engineering prior to manufacturing to identify potential problems will make Industry 4.0 and the smart factory for composites a reality. Industry 4.0 focuses on data-driven manufacturing, where in the future, billions of machines, systems, and sensors will communicate with each other and share information, physical systems connected to digital twins, the Industrial Internet of Things (IIoT). This will not only enable companies to make design and production significantly more efficient, but it will also give them greater flexibility when it comes to tailoring production to meet market requirements
Damage detection in composite materials using lamb wave methods
No full textCost-effective and reliable damage detection is critical for the utilization of composite materials. This paper presents part of an experimental and analytical survey of candidate methods for in situ damage detection of composite materials. Experimental results are presented for the application of Lamb wave techniques to quasi-isotropic graphite/epoxy test specimens containing representative damage modes, including delamination, transverse ply cracks and through-holes. Linear wave scans were performed on narrow laminated specimens and sandwich beams with various cores by monitoring the transmitted waves with piezoceramic sensors. Optimal actuator and sensor configurations were devised through experimentation, and various types of driving signal were explored. These experiments provided a procedure capable of easily and accurately determining the time of flight of a Lamb wave pulse between an actuator and sensor. Lamb wave techniques provide more information about damage presence and severity than previously tested methods (frequency response techniques), and provide the possibility of determining damage location due to their local response nature. These methods may prove suitable for structural health monitoring applications since they travel long distances and can be applied with conformable piezoelectric actuators and sensors that require little power
Analysis of delamination in laminates with angle-ply matrix cracks
No full textThe failure process of composite laminate under quasi-static or fatigue loading involves sequential accumulation of intra- and interlaminar cracking. Matrix cracking parallel to the fibres in the off-axis plies is the first damage mode observed. It triggers development of other harmful resin-dominated modes such as delaminations. In this chapter, analytical modelling of crack-induced delaminations in composite laminates subjected to general in-plane loading is presented and discussed. A two-dimensional shear lag analysis is used to determine ply stresses in a representative segment and the equivalent laminate concept is applied to derive expressions for mode I and mode II and the total strain energy release rate associated with uniform local delaminations. These expressions could be used with appropriate fracture criteria to estimate the onset of local delamination in an already cracked off-axis laminate. Dependence of strain energy release rate on crack density, delamination area and ply orientation angle in unbalanced symmetric laminates is examined and discussed, and the effect of crack-induced delamination on the laminate stiffness is predicted
Modelling damage in laminate composites
No full textThe failure of Glass- and Carbon-Fiber Reinforced Plastic [“GFRP” and “CFRP”] laminates subjected to static or cyclic tensile loading acting in the plane of reinforcement, and also under thermal fatigue, is a complex process. It involves sequential accumulation of various types of intra- and interlaminar damage, which gradually lead to the loss of the laminate's load-carrying capacity. The main damage mechanisms, exhibited in composite laminates, are matrix cracking, delamination, fiber debonding, and fiber breakage. Damage mechanisms in composite laminates can be studied theoretically following two approaches. Using the continuum damage mechanics approach, various types of damage are accounted for via the damage tensor. A composite is described as a continuum with mechanical properties depending on the damage tensor. Using the damage micromechanics approach, stress analysis of the damaged composite is carried out in the explicit presence of damage. Various types of damage are analyzed directly with the aim to predict their onset and growth, and also their effect on the properties of the laminate. While for homogeneous isotropic materials it is possible to obtain exact solutions within the linear elasticity theory, stress analysis of damaged composite laminates is approximate in the majority of cases. If interaction between various types of damage is especially complex, stress field can only be determined by numerical methods such as the finite-element method
Compressive fracture of layered composites caused by internal instability
No full textThis chapter revisits the exact three-dimensional (3-D) approach to study internal instability in layered media based on the model of a piecewise-homogeneous medium, in which the behaviour of each component of the material is described by the 3-D equations of solid mechanics. The results of this approach can be used as a benchmark for simplified models. The chapter begins with the development of a unified computational procedure for numerical realisation of the 3-D analytical method as applied to various constitutive equations of the layers, different loading schemes (uniaxial or biaxial loading) and different precritical conditions (large or small precritical deformations). It contains many examples of calculation of critical stresses/strains for particular composites as well as analyses of different buckling modes. The second part of the chapter deals with the application of a developed model to investigation of compressive behaviour of stiffened thin-skinned composite panels with stress concentrators
Damage detection in composite materials using frequency response methods
No full textCost-effective and reliable damage detection is critical for the utilization of composite materials. This paper presents part of an experimental and analytical survey of candidate methods for the in situ detection of damage in composite materials. The experimental results are presented for the application of modal analysis techniques applied to graphite/epoxy specimens containing representative damage modes. Changes in natural frequencies and modes were found using a laser vibrometer, and 2-D finite element models were created for comparison with the experimental results. The models accurately predicted the response of the specimens at low frequencies, but coalescence of higher frequency modes makes mode-dependant damage detection difficult for structural applications. The frequency response method was found to be reliable for detecting even small amounts of damage in a simple composite structure, however the potentially important information about damage type, size, location and orientation were lost using this method since several combinations of these variables can yield identical response signatures
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