Advanced Composite Structures Research Articles

Impact damage identification in composite skin-stiffener structures based on modal curvatures

The feasibility of a vibration-based damage identification approach for impact damage in two advanced composite skin-stiffener structures is investigated in this paper. Mode shape curvatures combined with the modal strain energy damage index (MSE-DI) algorithm are utilized to identify the damage. Special attention is paid to the effective application of this vibration-based methodology by investigating the relation between the damage location, the structural design, and the dynamic behavior. The performance of a 1D and 2D formulation of the MSE-DI algorithm is compared for several damage scenarios. Experiments demonstrated the capabilities of the MSE-DI algorithm to detect, localize, and roughly quantify the size of barely visible impact damage in advanced composite structures. It is concluded that the method is particularly effective for health monitoring of skin-stiffener connections. The most effective results were obtained by considering the 1D formulation in the direction of the stiffeners for the stiffener mid-section and perpendicular to the stiffeners for the stiffener run-out. The method remained inconclusive in the case of pure skin-related damage. The results obtained in this study contribute to the development of guidelines for vibration-based structural health monitoring of advanced composite skin-stiffener structures. Copyright © 2015 John Wiley & Sons, Ltd.

Study of mould design and forming process on advanced polymer-matrix composite complex structure

Advanced carbon fibre-reinforced polymer-matrix composites are widely applied to aviation manufacturing field due to their outstanding performance. In this paper, the mould design and forming process of the complex composite structure were discussed in detail using the hat stiffened structure as an example. The key issues of the moulddesign were analyzed, and the corresponding solutions were also presented. The crucial control points of the forming process such as the determination of materials and stacking sequence, the temperature and pressure route of the co-curing process were introduced. In order to guarantee the forming quality of the composite hat stiffened structure, a mathematical model about the aperture of rubber mandrel was introduced. The study presented in this paper may provide some actual references for the design and manufacture of the important complex composite structures.

Interfacial thermal properties and size-effect in thermo-mechanical characterisation of nanocomposites

Carbon nanostructure-based multifunctional materials have found various applications in advanced composite structures primarily due to their enhanced mechanical characteristics like high strength-to-weight ratio and unique thermal and physio-chemical properties of the nanostructures. The effect of size of the nanoparticle is found to significantly affect the overall thermo-mechanical properties of the nanocomposite material. To understand and develop efficient composite systems with the desired thermal characteristics, it is necessary to develop an accurate thermal transport model that considers these ‘size parameters’ along with the various parameters that influence the thermal properties. The novelty of this study is the development of a multiscale mathematical framework and simulation strategy to estimate the thermal transport properties of nanostructure-based composites by considering the size-effect obtained from the thermal transport phenomenon together with their interfacial thermal properties obtained from atomistic simulations.

해저터널용 복합신소재 배수복합관 부재의 정적거동에 관한 실험적 연구

In order to design an advanced composite materials drainage pipe structures for an undersea tunnel, mechanical properties for the lamina types of the structural member must be predetermined. It is also reported that the size effect of the specimen is significant. In this study the tensile tests for the lamina types of the structural member are conducted at the room temperature () and the seawater temperature (). In addition, the mechanical properties are predicted by theory based on the rule of mixtures and elasticity solution technique. The predicted mechanical properties are compared with test results obtained by a test method. In the design of an advanced composite materials drainage pipe structural members for an undersea tunnel, the used mechanical properties must be applied at the room temperature with considering the modified factors. These are to be offered the datum for the design an advanced composite materials drainage pipe structures for an undersea tunnel.

Evaluation of the sensitivity and fatigue performance of embedded piezopolymer sensor systems in sandwich composite laminates

It has been claimed that embedding piezoceramic devices as structural diagnostic systems in advanced composite structures may introduce mechanical impedance mismatches that favor the formation of intralaminar defects. This and other factors, such as cost and their high strain sensitivity, have motivated the use of thin-film piezopolymer sensors. In this paper, we examine the performance of sandwich composite panels fitted with embedded piezopolymer sensors. Our experiments examine both how such thin-film sensors perform within a structure and how the inclusion of sensor films affects structural performance. Strain-controlled tests on sandwich panels subjected to three-point bending under wide-ranging static and dynamic strains lead us to conclude that embedding thin piezopolymer films has no marked reduction on the tensile strength for a wide range of strain loading paths and magnitudes, and that the resilience of the embedded sensor is itself satisfactory, even up to the point of structural failure. Comparing baseline data obtained from standard surface-mounted sensors and foil gauges, we note that whereas it is possible to match experimental and theoretical strain sensitivities, key properties—especially the pronounced orthotropic electromechanical factor of such films—must be duly considered before an effective calibration can take place.

Self-sensing properties of smart composite based on embedded buckypaper layer

Advanced polymer matrix composite is a brittle material, and local fibre, matrix fracture and bonding delamination will cause the catastrophic failure of the composite. So, the strain and damage structural health monitoring of the composite material and its structures is very important for its applications. Based on the piezo-resistivity effect, micro-pore structure and resin infiltration properties of buckypaper, in this study, we develop a self-sensing composite based on embedded buckypaper, which only acts as a sensing layer to monitor the strain and damage of the composite. The strain and damage of the composite are reflected by the resistance change in the embedded buckypaper. The strain sensing properties of the buckypaper/epoxy film with different buckypaper aspect ratios 3:1, 6:1, 9:1, 12:1 and 15:1 were studied with voltammetry method, and the results indicated that there were two-stage relation between resistance change and strain, and under low strain level (~2000 µε), the strain sensitivity was nonlinear and unstable, but in the high strain level, the strain sensitivity was high with strain factor about 6.2, which was independent of the dimension of buckypaper layer. The self-sensing composite made of glass fibre–reinforced composite with embedded buckypaper shows the same strain sensing properties as buckypaper/epoxy film; at same time the damage revolution of the composite could be monitored by resistance development. The scanning electron microscope characterization demonstrated that the resin can penetrate into buckypaper of the composite and form strong bonding interface. The self-sensing composite can be applied in further large-scale advanced polymer composite materials and structures structural health monitoring (strain monitoring and damage detecting).

복합신소재 구조물의 형상비에 따른 고유진동수의 영향

In this paper. the effects of the aspect ratio on the natural frequency of the advanced composite road structures is studied. The advanced composite structures are too difficult for such design engineers for construction and some simple but accurate enough methods are necessary. Some laminate orientations have decreasing values of , , and stiffnesses as the ply number increases. The plate aspect ratio considered is from 1 to 5. Most of the road structures have large aspect ratios, for such cases further simplification is possible by neglecting the effect of the longitudinal moment terms.

Reliability and accuracy of embedded fiber Bragg grating sensors for strain monitoring in advanced composite structures

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.

On the blast response of sandwich aerospace composites

Aircraft floors are a typical example of an advanced sandwich composite structure. Although not structurally loaded due to flight loads, they play an important role on passenger survivability. Additionally, due to their connection with the lower aircraft structural components, in the case of an extreme loading scenario, such as an explosion in the aircraft cabin, it is possible to transfer loads beyond the operational levels to the airframe. The present paper aims to compare the contribution of different core configurations to the accelerations exerted to the frame due to blast loading by numerical analysis. Two different core materials are investigated, bearing similar densities, namely aluminium and Nomex paper honeycomb. The hydrodynamic code LS-DYNA is used in order to model the fluid structure interaction associated with blast loading by an ALE scheme and strain rate dependent material properties are incorporated. The skins of the considered aircraft sandwich floor are made of GFRP. Accelerations are extracted on the connection points of the specified floor. Moreover, the damage modes of the structure’s constituents are considered (skin, core) and direct comparisons are made on blast energy absorption capacity.

Extraction and processing of real time strain of embedded FBG sensors using a fixed filter FBG circuit and an artificial neural network

Fibre Bragg Grating (FBG) sensors have been used in the development of structural health monitoring (SHM) and damage detection systems for advanced composite structures over several decades. Unfortunately, to date only a handful of appropriate configurations and algorithm sare available for using in SHM systems have been developed. This paper reveals a novel configuration of FBG sensors to acquire strain reading and an integrated statistical approach to analyse data in real time. The proposed configuration has proven its capability to overcome practical constraints and the engineering challenges associated with FBG-based SHM systems. A fixed filter decoding system and an integrated artificial neural network algorithm for extracting strain from embedded FBG sensor were proposed and experimentally proved. Furthermore, the laboratory level experimental data was used to verify the accuracy of the system and it was found that the error levels were less than 0.3% in predictions. The developed SMH system using this technology has been submitted to US patent office and will be available for use of aerospace applications in due course.

Prediction of Obsolete FBG Sensor Using ANN for Efficient and Robust Operation of SHM Systems

ncreased use of FRP composites for critical load bearing components and structures in recent years has raised the alarm for urgent need of a comprehensive health mentoring system to alert users about integrity and the health condition of advanced composite structures. A few decades of research and development work on structural health monitoring systems using Fibre Bragg Grating (FBG) sensors have come to an accelerated phase at the moment to address these demands in advanced composite industries. However, there are many unresolved problems with identification of damage status of composite structures using FBG spectra and many engineering challenges for implementation of such FBG based SHM system in real life situations. This paper details a research work that was conducted to address one of the critical problems of FBG network, the procedures for immediate rehabilitation of FBG sensor networks due to obsolete/broken sensors. In this study an artificial neural network (ANN) was developed and successfully deployed to virtually simulate the broken/obsolete sensors in a FBG sensor network. It has been found that the prediction of ANN network was within 0.1% error levels.

복합신소재구조물의 고유진동수에 대한 하중크기의 영향

본 논문에서는 고유진동수를 구하기 Simple Iteration Method을 제시하였다. 이 방법은 임의의 단면과 지점을 갖고 임의의 하중을 받는 보나 탑의 진동모드와 관련된 고유진동수를 간편하면서도 정확하게 계산할 수 있는 획기적인 방법이다. 이 방법에는 공진상태에서 관성력에 기인한 부재의 처짐 모드를 구하게 된다. 진동해석을 위하여 처짐의 영향을 고려한 다양한 방법이 검토되었다. 이러한 목적으로 본 논문에서는 유한차분법을 사용하였다. 고유진동수에 대한 <TEX>$D_{22}$</TEX> 휨강성의 영향을 철저하게 검토하였다. 본 논문에서는 구조 요소의 하중 분포 또는 상이한 단면에 따른 고유진동수에 대한 영향을 연구하였으며 그 결과를 제시하였다. 이 방법은 첨단복합재료를 포함한 2차원 문제에도 적용할 수 있다. Simple Iteration Method for calculating the natural frequency is presented in this paper. This method is simple but exact method of calculating natural frequencies corresponding to the modes of vibration of beams and tower structures with irregular cross sections and arbitrary boundary conditions. This method consists of determining the deflected mode shape of the member due to the inertia force under resonance condition. Finite difference method is used for this purpose. The influence of the <TEX>$D_{22}$</TEX> stiffness on the natural frequency is rigorously investigated. In this paper, the influence of the loading sizes, different cross section on the natural frequency of vibration of some structural elements is presented. This method extends to two dimensional problems including advanced composite material structures.

Validation of damage modelling in composite fuselage structures under high velocity impact

The paper discusses finite element (FE) simulation methods in use for predicting impact damage in advanced composite structures. Before these simulation tools are accepted by the industry for design and certification of composite aircraft structures, composites damage models have to be developed, implemented in commercial FE codes and validation studies at specimen and substructure level have to be performed. For vulnerability analysis, the DLR uses meso-scale composites ply damage models with an energy-based delamination failure criteria in explicit FE codes to study damage progression in composite structures under impact. The methods developed are summarised in the paper and applied to predict impact damage in stiffened aircraft panels and novel foldcore sandwich structures from hard debris. Issues concerned with use of such computational methods for certification by analysis are discussed.

Strain monitoring with embedded Fiber Bragg Gratings in advanced composite structures for nautical applications

Methods to effectively and reliably embed Fiber Bragg Grating sensors in the composite material have been investigated in this work, with particular regard to a sailing boat mast manufacturing process by means of bagging technique in autoclave.Small samples were produced in order to investigate the effect of the curing process parameters on the light transmission characteristics of the embedded optical fibers. Polyimide and acrylate coated optical fibers were tested measuring the relative coefficient of attenuation by Optical Time Domain Reflectometry. In particular, carbon fiber reinforced epoxy laminas with embedded FBGs were manufactured and specimens for tensile tests extracted from the laminas. Each specimen was instrumented with FBG and conventional electrical strain gage bonded on the surface. The comparison between the FBG and SG measurements recorded during static tensile tests permitted to assess the strain monitoring capability of the FBGs and their sensitivity and accuracy.

Progressive failure analysis for the interaction of interlaminar and intralaminar failure modes in composite structures with an initial delamination

AbstractThis paper is concerned with the development of a failure initiation and progressive failure analysis (PFA) method for advanced composite structures. The present PFA model is capable of predicting interactive out-of-plane and in-plane failure modes observed in fiber reinforced composite laminates including interlaminar behavior and matrix microdamage at the mesoscale. A probability analysis tool is coupled with the PFA to account for uncertainty in modelling parameters caused by material variability and manufacturing inconsistencies. The progressive damage response of a laminated composite panel with an initial delamination is studied and used to demonstrate the PFA modelling framework that is presented here.

Properties of 5-layer angle-interlock Kevlar-based composite structure manufactured from vacuum bagging

The proposed research aims to manufacture, fabricate and evaluate the 5-layer ‘through-the-thickness’ angle-interlock composite structure from Kevlar. Results show that composites developed from angle-interlock fabric, exhibit different and far better tensile properties in the weft direction as compared to the warp direction. It is much harder to break the fibres in the weft direction than in the warp direction for this particular construction of Kevlar-based composite. This behaviour of composites can lead to better products while developing advanced textile-based composite structures.

Proposed Parameter Investigation Based on Design of Experiment for Woven Fabric Composites Deformation

Advanced composite laminate and honeycomb core sandwich structure depict process induced geometrical and dimensional deformations after end of curing process. These shape deformations are unpredictable and contribute to fit, form and functional error during an assembly stage. Often a conventional trial–and–error method is deployed to correct the geometrical shape of the mould tool prior to mass production, which is very costly, uneconomical and time consuming. Alternatively a better method is sought to predict shape deformations considering the elements of material properties, tool–part interaction and processing factors. Based on previous studies, there is still lacking of experimental data and studies on the effect of weavings styles and honeycomb core material properties in affecting shape deformations of flat and L–angled composite parts. Hence, it is proposed to perform a design of experiment with a fractional factorial of 28–4, Resolution IV, to investigate the parameters that affect the shape deformations of composite parts.

Resin Flow of an Advanced Grid-Stiffened Composite Structure in the Co-Curing Process

The soft-mold aided co-curing process which cures the skin part and ribs part simultaneously was introduced for reducing the cost of advanced grid-stiffened composite structure (AGS). The co-curing process for a typical AGS, preformed by the prepreg AS4/3501-6, was simulated by a finite element program incorporated with the user-subroutines ‘thermo-chemical’ module and the ‘chemical-flow’ module. The variations of temperature, cure degree, resin pressure and fiber volume fraction of the AGS were predicted. It shows that the uniform distributions of temperature, cure degree and viscosity in the AGS would be disturbed by the unique geometrical pattern of AGS. There is an alternation in distribution of resin pressure at the interface between ribs and skin, and the duration time of resin flow is sensitive to the thickness of the AGS. To obtain a desired AGS, the process parameters of the co-curing process should be determined by the geometry of an AGS and the kinds of resin.

Advanced composite sandwich structure design for energy absorption applications: Blast protection and crashworthiness

This paper describes an experimental investigation on the response of composite sandwich structures with tubular inserts to quasi-static compression. The performance parameters, namely the peak load, absorbed crash energy, specific energy absorption; average crushing load and crush force efficiency were evaluated. The composite sandwich specimens were fabricated from glass fiber, polystyrene foam and epoxy resin. The primary mode of failure observed was progressive crushing with the composites exhibiting high energy absorption capabilities and high crushes force efficiency. The mechanism of progressive crushing of the sandwich structures and its relation to the energy absorption capabilities was deliberated. Furthermore, a statistical analysis was performed to investigate the effects of the design variables and also to determine if there were interactions between these variables. Such information is vital in the design of polymer composite sandwich structures as energy absorbers.

X-ray damage characterisation in self-healing fibre reinforced polymers

Realising autonomous healing in advanced composite structures requires a detailed understanding of the damage profile to be repaired. Quantifying the damage volume and mapping its through-thickness location is key to ensuring that the delivery infrastructure can supply sufficient healing to critical locations whilst maximising coverage and minimising structural cost. In this study micro-X-ray computer tomography (μCT) was used to determine the damage volume in quasi-isotropic carbon fibre reinforced plastic (CFRP) laminates subjected to low velocity impacts. The laminates incorporated a layer of hollow glass fibres (HGFs) at either the 3rd or 13th interface for the purpose of delivering a self-healing agent. Analysis of the μCT data indicated that HGF inserted at interface 3 (near back face) altered the through-thickness damage map whilst visualisation of the HGF at both interfaces indicated low levels of HGF fracture.