Carbon Fiber Composite Structures Research Articles

Use of inorganic materials to enhance thermal stability and flammability behavior of a polyimide

While a great variety of high temperature polyimide materials exist, these materials are being subjected to higher and higher use temperatures in oxidative and environmentally aggressive environments. There is a limit to the extent one can take a polyimide before it will oxidize and subsequently suffer property degradation, thermal decomposition, and structural failure. Therefore, we instead sought to use materials which do not oxidize (inorganic materials) to enhance the polyimide composition and perhaps move the properties of the organic polymer more into the realm of ceramics while maintaining polyimide composite weights and processing advantages. In this paper we present results of the combination of inorganic micron sized particles with and without carbon nanofibers to produce a variety of highly inorganic particle filled polyimides. These polyimides were tested for thermal stability and flammability in resin pellet form and as a protective coating for a carbon–fiber composite structure. Our results demonstrate that the resin with inorganic particles exhibited significant reductions in flammability by themselves, but minimal flammability reduction when used as a thin coating to protect a carbon–fiber composite. Further, the gains in thermal stability are limited by the thermal stability of the polyimide matrix, suggesting that more work is needed in measuring the limits of inorganic fillers to improve thermal stability. Still, the results are promising and may yield polyimide systems useful for providing resistance to damage from high heat flux exposures/fire risk scenarios.

Fabrication and crushing behavior of low density carbon fiber composite pyramidal truss structures

A new method for fabricating carbon fiber composite pyramidal truss cores was developed based on the molding hot-press technique. In this method, all the continuous fibers of composite are aligned in the direction of struts and thus, the truss structure can fully exploit the intrinsic strength of the fiber reinforced composite. The microstructure and organizations of fibers of fabricated composite structures were examined using scanning electron microscope. The crushing response of the truss cores was also investigated and the corresponding failure modes were studied and complemented with an analytic model of the core crushing response. Our results show that the fabricated low-density truss cores have superior compressive strength and thus, could be used in development of novel lightweight multifunctional structures.

Predicting low-velocity impact damage on a stiffened composite panel

An intralaminar damage model, based on a continuum damage mechanics approach, is presented to model the damage mechanisms occurring in carbon fibre composite structures incorporating fibre tensile and compressive breakage, matrix tensile and compressive fracture, and shear failure. The damage model, together with interface elements for capturing interlaminar failure, is implemented in a finite element package and used in a detailed finite element model to simulate the response of a stiffened composite panel to low-velocity impact. Contact algorithms and friction between delaminated plies were included, to better simulate the impact event. Analyses were executed on a high performance computer (HPC) cluster to reduce the actual time required for this detailed numerical analysis. Numerical results relating to the various observed interlaminar damage mechanisms, delamination initiation and propagation, as well as the model’s ability to capture post-impact permanent indentation in the panel are discussed. Very good agreement was achieved with experimentally obtained data of energy absorbed and impactor force versus time. The extent of damage predicted around the impact site also corresponded well with the damage detected by non destructive evaluation of the tested panel.

Investigation, manufacture, and testing of damage-resistant airframe structures using low-cost carbon fibre composite materials and manufacturing technology

A BAE systems/UK Government funded project, Flaviir, is investigating the design and manufacture of low-cost and damage-resistant carbon fibre composite airframe structure for large-scale unmanned aircraft vehicles (UAVs). The article discusses requirements and design concepts for low-cost wing structure. Techniques for manufacturing cost reduction and damage resistance enhancement are investigated through the design and test of spar to skin joints. The use of vacuum infusion processing with slotted-type joints and stitching of the detail parts by tufting were demonstrated to greatly increase the strength of spar to skin joints and to reduce the manufacturing cost. The results will be used for the design and manufacture of a large wing box demonstrator test structure, to be manufactured in 2009.

Numerical Investigation of Dynamically Loaded Bolted Joints in Carbon Fibre Composite Structures

Numerical Investigation of Dynamically Loaded Bolted Joints in Carbon Fibre Composite Structures

Experimental Investigation of Dynamically Loaded Bolted Joints in Carbon Fibre Composite Structures

This paper presents quasi-static and dynamic modelling of bolted composite structures using the explicit finite element code PAM-CRASH. User controlled point link (PLINK) elements were investigated for modelling the bolted composite joints used in the structures. Simulation results were compared with quasi-static and dynamic structural testing reported previously. Two loading configurations were considered. It was shown that the PLINK element modelling approach agreed well with the experimental results for both loading configurations and for one case offered significant improvements over other simplified bolt modelling methods. A stacked shell modelling approach was used to model the interlaminar delamination damage present in the ball-loaded impact mode. The overall response of the structure was significantly improved by the addition of these energy absorbing interfaces.

Celebrating innovators in engineering

PurposeIn looking to ensure that engineers receives the recognition they deserve, the Royal Academy of Engineering, which provides a voice for Britain's engineering community, annually awards a number of medals to the very best of British engineering talent.Design/methodology/approachThis briefing is prepared by an independent writer who adds their own impartial comments and places the articles in context.FindingsFive of the UK's finest engineers have won the Royal Academy of Engineering's prestigious Silver Medal for their outstanding personal contribution to British engineering. These contributions include the successful development and commercialization of Snake Arm Robots; research into “high volume” carbon fiber composite structures; helping to revolutionize the computing world through virtualization, and major developments in membrane technology, which could significantly improve the energy efficiency of industrial processes. In addition, biomedical engineer Dr Silvia Schievano has won the 2009 Sir George MacFarlane Medal from the Academy, which has been awarded for her pioneering work in engineering patient‐specific heart valves.Originality/valueThe briefing saves busy executives and researchers hours of reading time by selecting only the very best, most pertinent information and presenting it in a condensed and easy‐to‐digest format.

Effect of thermal aging and fatigue on failure resistance of aerospace composite materials

Exposing a laminate structure to thermal cycles and to temperatures close to the curing temperature, followed by mechanical fatigue, cause interlaminar and intralaminar cracks leading to degradation of the mechanical properties. The effect of thermal and mechanical fatigue, as well as thermal aging, on carbon fiber composite laminate structures is under study in the present paper.

Automated analysis and advanced defect characterisation from ultrasonic scans of composites

With the rapidly escalating usage of composite materials, not only in military aircraft but in civil airliners as well, production NDT throughput is already stretched to its limit internationally. NDT data analysis is set to become the bottleneck preventing the required rise in production rates of composite civil aircraft in the next few years. Thus there is an urgent requirement for rapid, automated analysis of up to a Terabyte of data per airliner, escalating to over 200 Terabytes per year - worldwide. The primary aim of automated analysis is to release operators from the time-consuming analysis of all scans and focus operator attention on non-compliant structures. A secondary aim is to provide three-dimensional quantitative information that lightens the operator's decision-making burden. Through advanced characterisation methods, NDT also has the potential to provide crucial feedback to control the composite production process, increase production yield and decrease costs. Current analysis methods for ultrasonic scans produce through-thickness average parameters, which provide little useful information to assist the stress analysis for defects, or the production process. Three-dimensional characterisation of defects can increase yield by informing the concession/disposition process for defects. For future process control, information is required about the 3D distribution of material properties in the structures on the production line, providing comprehensive long-term trend analysis. As well as demonstrating a new rapid automated analysis method for large-area ultrasonic scans, this paper proposes new ultrasonic methods for generating quantitative 3D profiles of porosity, resin layer thickness, ply spacing and fibre orientation. From these it is possible to determine ply stacking sequence and measure in-plane fibre waviness and out-of-plane fibre wrinkling. Armed with these new tools there is the potential for solving the NDT data analysis bottleneck for composite aircraft. Examples are given of the use of these tools on carbon-fibre composite structures. Stacking sequence has been determined in structures up to 18 mm thick, and out-of-plane wrinkling measured in 35 mm-thick structures, although penetration depends on the measurement parameter, material quality and the ply spacing. The tools are software based and are applied through post-processing of full-waveform data acquired using one of several suitable ultrasonic acquisition systems. These include commercially available phased array systems.

Development of an automated preforming technology for resin infusion processing of aircraft components

A novel approach for the affordable manufacture of carbon fibre composite aircraft secondary structures through vacuum infusion moulding is presented. The requirements for a low-cost preforming process for optimized carbon fibre composite component processing by vacuum infusion techniques are discussed. A novel technique termed high-speed tape laying is described. The process involves the laying of unidirectional fibre tapes, stabilized with a powdered resin binder using an automated gantry and a tape deposition head. Each stage of the automated process for manufacture of preforms for a complete integrated moulding for a demonstrator aircraft aileron is described. The resin transfer moulding (RTM) type process for the aileron demonstrator is described, which is cost modelled compared with conventional RTM technology. This shows a cost reduction of 20 per cent.

The ATLAS semiconductor tracker (SCT)

The ATLAS detector (CERN/LHCC/94-43 (1994)) is designed to study a wide range of physics at the CERN Large Hadron Collider (LHC) at luminosities up to 10 34 cm −2 s −1 with a bunch-crossing rate of 40 MHz. The Semiconductor Tracker (SCT) forms a key component of the Inner Detector (vol. 1, ATLAS TDR 4, CERN/LHCC 97–16 (1997); vol. 2, ATLAS TDR 5, CERN/LHCC 97-17 (1997)) which is situated inside a 2 T solenoid field. The ATLAS Semiconductor Tracker (SCT) utilises 4088 silicon modules with binary readout mounted on carbon fibre composite structures arranged in the forms of barrels in the central region and discs in the forward region. The construction of the SCT is now well advanced. The design of the SCT modules, services and support structures will be briefly outlined. A description of the various stages in the construction process will be presented with examples of the performance achieved and the main difficulties encountered. Finally, the current status of the construction is reviewed.

Neural network method based on a new damage signature for structural health monitoring

Adopting wide-band Lamb wave based active monitoring technology, this study focuses on a neural network method based on a new damage signature for on-line damage detection applied to thin-walled composite structures. Honeycomb sandwich and carbon fiber composite structures are studied. Two kinds of damage are considered: delamination and impact damage. A new damage signature is introduced to determine the presence and extent of damage in composites, while eliminating the influence of different distances between the actuator and sensor. Neural network method is researched to take advantage of this new damage signature combined with several other signatures to decide the damage mode. Kohonen neural network is developed. The proposed method is shown to be effective, reliable, and straightforward for the specimens considered in the present study, which are composed of different materials and suffer various levels of damage.

The calibration and environmental testing of the engineering module of GLAST CsI calorimeter

GLAST is the next generation space-based gamma ray telescope for the energy range 30 MeV-300 GeV, to be launched by NASA in 2007. For photon energy measurements it will use a CsI crystal calorimeter made of 16 identical modules. The engineering module (EM) calorimeter is the first full scale prototype built with the same technology as flight modules to verify the design and technological choices before starting calorimeter production. The module contains 96 CsI crystals, supported by a carbon fiber composite structure and read out with silicon PIN photodiodes from both ends. In this paper, we report the results of EM calibration using cosmic ray muons and charge injection during environmental tests. The EM showed stable functioning in the required temperature range from -30 C to +50 C during six months of continuous testing, including seven thermal vacuum cycles between -30 C and +50 C and vibration testing with amplitudes significantly higher than expected during launch. None of 96 crystals experienced mechanical or optical degradation after the tests. The longitudinal position measurement is accomplished using light asymmetry from two ends of each crystal, providing /spl sim/6--12 mm position resolution (2-4% of crystal length) for cosmic ray muons. The production of flight modules will start in late 2003.

Design and testing of the Minotaur advanced grid-stiffened fairing

A composite grid-stiffened structure concept was selected for the payload fairing of the Minotaur launch vehicle. Compared to sandwich structures, this concept has an advantage of smaller manufacturing costs and lighter weight. To reduce weight the skin pockets are allowed to buckle visibly up to about 0.5 cm peak displacement. Various failure modes were examined for the composite grid-stiffened structure. The controlling criterion for this design was a joint failure in tension between the ribs and skin of the structure. The identification of this failure mechanism and the assessment of bounding strains required to control it required extensive test and analysis effort. Increasing skin thickness to control skin buckling resulted in reduced strains between the skin and ribs. Following the identification of the relevant failure criteria, a final design for the fairing was generated. The resulting 6 m tall fairing was constructed of a tow-placed carbon fiber composite grid structure that was over-wrapped to create a laminated skin. Upon completion of curing and machining, the fairing was cut in half to create the classic “clam-shell” fairing. Static qualification testing demonstrated the structural integrity of the fairing, thereby proving the design and manufacturing process. Loads were applied incrementally in a static loading scenario. The applied load envelope exceeded worst-case dynamic flight conditions with an added safety factor of 25%. At peak load the fairing maintained structural integrity while remaining within the required displacement envelope for payload safety. Data were collected during the test from a variety of sensors including traditional displacement transducers and strain gages. In addition, full field displacement was monitored at critically loaded fairing sections by means of digital photogrammetry. This paper summarizes the test results, presents the overall performance of the fairing under the test loads, correlates test response and analysis, and identifies lessons learned. Work continues at the Air Force Research Laboratory (AFRL) and Boeing to identify means of further controlling tensile failure of the un-reinforced polymer bonded joint between the ribs and skin. Stiffening of skin adjacent to the joints and introduction of lightweight foam jackets at the interior of the fairing both show promise of delaying joint failure to higher loads.

Active Monitoring for On-Line Damage Detection in Composite Structures

This study focuses on an active monitoring method for damage detection applied to composite structures. Honeycomb sandwich and carbon fiber composite structures are studied. Two kinds of damage are considered: delamination and impact damage. Wavelet analysis methods are adopted to postprocess the raw monitored signal. A new damage signature is introduced to determine the presence and extent of damage in composites, while eliminating the influence of different distances between the active actuator and active monitoring elements. The proposed method is shown to be effective, reliable, and straightforward for the specimens considered in the present study, which are composed of different materials and suffer various levels of damage. An online real-time active monitoring system for damage detection is described that is based on this research.

Aspects of Flow Transition Detection When Flight Testing Engine Nacelles

Some lessons learned as regards flow transition detection in the course of flight-testing carbon fiber composite conventional and laminar flow nacelles are presented. The key objective of the flight trials was to examine the influences of environmental phenomena and of the engine nacelle structure regarding the interpretation of the nacelle infrared images and of the nacelle surface temperature measurements, both with reference to determining the location of the flow transition front on the nacelles. As regards the application of the infrared thermography, the study has yielded the result that the quality of the infrared images is influenced by heat radiation to space and from Earth, as well as by reflection of sunlight from the airframe. For good quality infrared images, only carbon fiber composites should be used as structural material for the nacelle fan cowls. In addition, the mechanical design of the fan cowls should not contain any stiffening frames in the substructure, but the should be designed as a uniform honeycomb skin structure. Concerning the surface temperature measurements, for good-quality results,a uniform honeycomb carbon fiber composite skin structure is just as essential as for the infrared imaging system.

Optical fibreacoustic emission sensor for damage detection in carbon fibre composite structures

In this note we describe a proof-of-principleacoustic emission based damage detection experiment. A largecarbon fibre composite structure was made to resemble anaerospace component. This structure was dynamically loadedafter damage. Optical fibre sensors detected acoustic emissionevents during the load cycles. These trials demonstrate thatacoustic emission is suitable for damage detection in carbonfibre composite structures and that optical fibre sensors aresensitive enough for the task.

Novel quantitative non-destructive testing method for composite structures

A novel QNDT (quantitative non-destructive testing) method is developed that is combined with a phase-shifting shearing speckle and thermograph, and, it aims at the detection of faults such as cracks, voids, delamination and weak areas. The technique is immune to ambient noise and is suitable for measuring the in situ environment. Some different depth defects that would produce deformation differing from other positions could be found with shearing speckle when the sample is loaded, however, a thermograph based on the thermal resistance effect of a defect can detect only those varisized defects embedded deeply in the composite structure by measuring the surface temperature distribution. The resolution is examined for artificial delaminated defects in carbon-fiber composite structures using a phase-shifting shearing speckle and thermograph. The experimental results have demonstrated that the technique is effective for revealing defects in composite structures.

Repairing composite aircraft structures — RAF experience of peacetime and battle damage techniques

AbstractThe RAF has been undertaking peacetime repairs on composite rotor blades, fibreglass radomes and panels on a variety of combat aircraft types for many years. However, the Service's main experience in the repair of aircraft structures manufactured from carbon fibre composite (CFC) material has been related to the Harrier II. In any future conflict involving air power, battle damaged aircraft, including the Harrier II, will have to be repaired rapidly to meet operational requirements. The repair techniques used may have to differ from those undertaken in peacetime but they must still aim to restore the full static strength of the aircraft within the time constraints imposed by the operational situation. Therefore, aircraft battle damage repair (ABDR) is a carefully planned alternative to conventional repair techniques. This paper describes the main CFC structures of the Harrier II and outlines the variety of RAF peace and wartime structural repair procedures applied to the aircraft. Examples are provided of various simple and complex repairs. Greater emphasis on adhesively bonded repairs is called for if Harrier II experience is to aid in the logistic support of Eurofighter 2000 (EF2000) in peace and war.

Real-time inspection of carbonfiber composite structures under thermal and mechanical load by acoustic emission : 49774 Block, J. Developments in the Science and Technology of Composite Materials, 4th European Conference, Stuttgart (Germany), 25–28 Sep. 1990. pp. 697–702. Edited by J. Fuller, G. Gruninger, K. Schulte, A.R. Bunsell, A. Massiah. Elsevier Applied Science (1990)

Real-time inspection of carbonfiber composite structures under thermal and mechanical load by acoustic emission : 49774 Block, J. Developments in the Science and Technology of Composite Materials, 4th European Conference, Stuttgart (Germany), 25–28 Sep. 1990. pp. 697–702. Edited by J. Fuller, G. Gruninger, K. Schulte, A.R. Bunsell, A. Massiah. Elsevier Applied Science (1990)