Advanced Fiber-reinforced Composites Research Articles

Injection Pultrusion of Glass-Reinforced Epoxy: Cure Kinetics, Rheology, and Force Analysis.

Pultrusion is a highly efficient continuous process to manufacture advanced fiber-reinforced composites. The injection pultrusion variant permits a higher control of the resin flow, enabling the manufacturing of a high reinforcement volume fraction. Moreover, it reduces the emission of volatile compounds that are dangerous for the operators and for the working environment. The present study proposes an experimental analysis of injection pultrusion in three different operative conditions. In particular, the activity focused on the effects of the temperature setup on the thermochemical and rheological behaviors of the resin system and on the interaction between the processed materials and the pultrusion die wall. The setup of the parameters was selected to evidence the behavior of the viscous interaction during the thermoset transition to the solid state, which is particularly challenging due to the localization of high adhesive forces related to the sharp increase in resin viscosity. Microscope observations of the cross-sections were performed to discuss the effects of the process parameters.

Design and testing of polar-orthotropic multi-layered composites under rotational load

The growing requirements for efficient and reliable high-performance rotors have led to an increased application of advanced fiber-reinforced composites. For an efficient feasibility analysis, analytical calculation methods for composite structures can provide a first design draft of typical composite components without cumbersome finite element models to engineers having low experience with anisotropic materials. In these investigations, an analytical solution for polar-orthotropic multi-layered composite rotors under rotational load is presented by transferring the well-known formulation of the classical laminate theory given in Cartesian coordinates into a formulation given in a polar coordinate system taking into consideration of centrifugal loads. The analytical results are verified under different rotational speeds with standard finite element solutions and also with experimental results at selected positions from strain gauges and diffraction grating strain sensors. The results show the usefulness of analytical solutions for the design engineer and can be further expanded to take into consideration temperature and shrinkage effects.

Mode I fracture toughness of fiber-reinforced polymer composites: A review

Composite materials are known for their high stiffness and strength at lower weight as compared to conventional structural materials. Recently, there has been a growing interest in finding the new ways to decrease delamination failure, which is a life limiting factor of laminated composites. This review paper emphasizes on the effects of different reinforcement structures on mode I fracture toughness and possible ways to improve fracture toughness. A brief description on intrinsic and extrinsic mechanisms of crack growth has been discussed along with the earlier investigations and recent developments for mode I fracture toughness testing. Factors that affect the fracture toughness are also discussed. A brief knowledge of mode I fracture toughness of traditional and advanced fiber-reinforced composites is given, which could help researchers to understand fracture behaviors of composites and thus, it can help engineers to design composites with higher interlaminar strength.

Shaping the future design and manufacturing of a composite rotor blade for multi-megawatt wind turbines

Given that wind energy is a key solution in fighting against global warming emissions arising from human activities, the interest for developing more powerful wind turbines has become nowadays a challenging area to research and innovation for the wind-turbine industry worldwide. As the electric power supplied by the wind turbine is directly related to the size of its blades: the larger the size of blades the more energy is captured, new design and manufacturing strategies of these critical components, using advanced fibre-reinforced composites such as carbon nanotubes (CNTs) and resin transfer moulding (RTM) process, have become a major focus for manufacturers and competitors of wind turbines. In this perspective, the outcomes presented in this paper can be exploited by the modern wind turbine industry with the aim of developing reliable, efficient and cost-effective rotor blades that are capable to withstand, without adverse effects, severe static and/or dynamic loading to which they may be exposed during their lifetime. As part of long-term strategic plans, CNTs offer the required criteria to help researchers design and manufacture 200 m rotor blades for 50 MW turbines.

Deleterious localized stress fields: the effects of boundaries and stiffness tailoring in anisotropic laminated plates.

The safe design of primary load-bearing structures requires accurate prediction of stresses, especially in the vicinity of geometric discontinuities where deleterious three-dimensional stress fields can be induced. Even for thin-walled structures significant through-thickness stresses arise at edges and boundaries, and this is especially precarious for laminates of advanced fibre-reinforced composites because through-thickness stresses are the predominant drivers in delamination failure. Here, we use a higher-order equivalent single-layer model derived from the Hellinger–Reissner mixed variational principle to examine boundary layer effects in laminated plates comprising constant-stiffness and variable-stiffness laminae and deforming statically in cylindrical bending. The results show that zigzag deformations, which arise due to layerwise differences in the transverse shear moduli, drive boundary layers towards clamped edges and are therefore critically important in quantifying localized stress gradients. The relative significance of the boundary layer scales with the degree of layerwise anisotropy and the thickness to characteristic length ratio. Finally, we demonstrate that the phenomenon of alternating positive and negative transverse shearing deformation through the thickness of composite laminates, previously only observed at clamped boundaries, can also occur at other locations as a result of smoothly varying the material properties over the in-plane dimensions of the laminate.

Optimization and preparation of an allyl phenoxy-modified bismaleimide resin

A thermosetting resin system has been developed by the copolymerization of allyl phenoxy, bismaleimide (BMI), and diallyl bisphenol A and optimized using response surface methodology. An optimized modified resin system with enhanced properties was achieved based on empirical second-order models expressing the relationship between the modifier contents and the mechanical properties. Dicumyl peroxide (DCP) was selected as initiator to further improve the curing behavior and mechanical properties of the optimized resin system. The effect of initiator contents on impact, flexural strength, and heat distortion temperature was also investigated. The curing behavior, morphology, and thermal stability of the optimized resin were carefully characterized using differential scanning calorimeter, scanning electron microscope, and thermogravimetric and dynamic mechanical analyzers, respectively. For evaluating the efficiency of modified BMI resin system, laminated composites using glass fiber cloth were fabricated using a hot press and tested for mechanical properties. The results showed that the DCP reduced the curing temperature significantly, improved the curing process, and proved to be very effective in heat resistance. Meanwhile, the laminated composite with initiator showed 13–27% higher mechanical properties and 5–7% higher retention rate at high temperature when compared with the neat resin composite system. The optimized resin system with higher mechanical properties, good heat resistance, and better manufacturability can be used as matrix resin for making advanced fiber-reinforced composites.

In situ assessment of carbon nanotube flow and filtration monitoring through glass fabric using electrical resistance measurement

Filtration of nanofillers into porous fabric media is still an issue during the preparation of advanced fiber-reinforced composites. The assessment of resin/multiwall carbon nanotube (MWCNT) flow, MWCNT filtration, and the cure monitoring of glass fiber/carbon nanotube-polyester composites by means of the measurement of the electrical resistance was introduced. The vacuum-assisted resin transfer molding technique was used. The electrical resistances measured over the span of a composite were qualitatively correlated with MWCNT flow and the degree of MWCNT filtration. It was found that while the complexity of the fabrics could likely introduce preferential deposition of MWCNTs, their filtration is mainly affected by their dispersion state in the resin suspension. Relationships among critical parameters such as the lengths and diameters of MWCNTs, the inter- and intra-tow dimensions of glass fabrics, the dispersion level of MWCNTs, and the viscosity of nanocomposite samples are discussed and correlated to the filtration, cure, and flow phenomena. We showed that our method can also serve as an early warning to obviate defects in the resulting composite.

Geometrical modeling of near-net shape braided preforms

Circular braiding has been successfully adapted for producing near-net shape structures for advanced fiber-reinforced composites. Net-shape manufacturing is significantly important for fabricating complex three-dimensional (3D) preforms. Geometrical modeling of braid patterns on 3D preforms plays a key role in determining their mechanical behavior. In this research work, the geometrical models of strand trajectory on the surface of cylindrical and conical mandrels with diamond, regular and triaxial braid patterns have been developed. These geometrical models of strand profiles were then simulated using Virtual Reality Modeling Language. Subsequently, the strands on complex-shaped mandrels, including ‘bottle’ and ‘funnel’, were simulated and accordingly, the braid angles have been predicted and compared with the experimental results. A virtual experiment was also conducted to compare the trajectory of the strands having constant and varying braid angles on the surface of conical mandrels.

Stimuli-triggered self-healing functionality in advanced fibre-reinforced composites

Inspired by the sensory and autonomous healing processes of living organisms, whether from the Animalia or Plantae biological kingdoms, a microvascular network that undertakes a dual role of sensing structural damage before initiating a triggered healing response has been developed and embedded within an advanced fibre-reinforced composite [−45/90/45/0]2S laminate. In this study, a single vascule is used as a sensing pathway, which detects the introduction of ply delamination and matrix microcracking following a 10-J low-velocity impact event. Once damage connectivity between the sensing vascule and those open to the ambient environment is established, the delivery of a healing agent to the damage zone is triggered. An investigation into a commercially available epoxy healing agent (RT151) and an in-house healing resin formation (diglycidyl ether of bisphenol-A/diethylenetriamine) epoxy system has been evaluated. The pressure-assisted delivery of the liquid epoxy healing agent to the damage zone was observed to occur within 49 s across all specimens. The recovery of compression strength post impact was 91% and 94% for the RT151 and diglycidyl ether of bisphenol-A healing agents, respectively. This study provides further confirmation on how a bio-inspired vascular healing network could substantially enhance the reliability and robustness of advanced composite materials.

The design and optimisation of a rig to enable the lateral spreading of fibre bundles

Fibre-reinforced composites consist of three key components: the reinforcing fibres, the matrix and the interface between the fibre and the matrix. The efficient impregnation of the reinforcing fibre bundle by the matrix is a primary prerequisite for the production of advanced fibre-reinforced composites. This process can be significantly enhanced by spreading the filaments in the reinforcing fibre bundle. The authors previously reported on a manual technique for spreading the filaments in a bundle. This involved subjecting a fibre bundle to a series of reciprocating motions over a rod. The effect of releasing the tension on the bundle was also considered. On the basis of the observations made in the previous study, a mechanised rig was designed, manufactured and optimised to enable the lateral spreading of the filaments in a bundle of E-glass fibres. A Taguchi-based approach was used to optimise the variables on the rig such as the number and configuration of rollers, haul-off speed of the fibre bundle, pre-tension in the bundle and the rotational speed of the roller carrier hub. The maximum degree of fibre spreading achieved for a commercially available 2400 tex E-glass fibre bundle was 250%.

Circumventing the Mechanochemical Origins of Strength Loss in the Synthesis of Hierarchical Carbon Fibers

Hierarchical carbon fibers (CFs) sheathed with radial arrays of carbon nanotubes (CNTs) are promising candidates for improving the intra- and interlaminar properties of advanced fiber-reinforced composites (e.g., graphite/epoxy) and for high-surface-area electrodes for battery and supercapacitor architectures. While CVD growth of CNTs on CFs has been previously shown to improve the apparent shear strength between fibers and polymer matrices (up to 60%), this has to date been achieved only at the expense of significant reductions in tensile strength (~30-50%) and stiffness (~10-20%) of the underlying fiber. Here we demonstrate two approaches for growing aligned and unaligned CNTs on CFs that enable preservation of fiber strength and stiffness. We observe that CVD-induced reduction of fiber strength and stiffness is primarily attributable to mechanochemical reorganization of the underlying fiber when heated untensioned above ~550 °C in both hydrocarbon-containing and inert atmospheres. We show that tensioning fibers to ≥12% of tensile strength during CVD enables aligned CNT growth while simultaneously preserving fiber strength and stiffness even at growth temperatures >700 °C. We also show that CNT growth employing CO2/acetylene at 480 °C without tensioning-below the identified critical strength-loss temperature-preserves fiber strength. These results highlight previously unidentified mechanisms underlying synthesis of hierarchical CFs and demonstrate scalable, facile methods for doing so.

Damping Analysis of Fibre-Reinforced Composites and Structure

The purpose of the paper is to analyze the damping characteristics of several advanced fibre-reinforced composites, and ulteriorly optimize the damping performance of a composite structure. Numerical modelling of the damping was developed considering the finite element method based on the modal strain energy. The modal damping characteristics of the fibre-reinforced composites were evaluated, including the modal frequencies, modal shapes and modal loss factors. The damping performances of different fibre-reinforced composites were compared. Furthermore, the damping performance of a laminated composite structure was optimized by considering the effects of different lay-up sequences and layer orientations. An optimum design scheme of the composite structure was presented.

Fiber-optic sensor design for chemical process and environmental monitoring

“Curing” is a term that is used to describe the cross-linking reactions in a thermosetting resin system. Advanced fiber-reinforced composites are being used increasingly in a number of industrial sectors including aerospace, marine, sport, automotive and civil engineering. There is a general realization that the processing conditions that are used to manufacture the composite can have a major influence on its hot–wet mechanical properties. This paper is concerned with the design and demonstration of a number of sensor designs for in situ monitoring of the cross-linking reactions of a commercially available thermosetting resin system. Simple fixtures were constructed to enable a pair of cleaved optical fibers with a defined gap between the end-faces to be held in position. The resin system was introduced into this gap and the cure kinetics were followed by transmission infrared spectroscopy. A semi-empirical model was used to describe the cure process using the data obtained at different cure temperatures. The same sensor system was used to detect the ingress of moisture into the cured resin system.

Characterisation of the cross-linking process in an E-glass fibre/epoxy composite using evanescent wave spectroscopy

The term “self-sensing composites” is sometimes used to describe the case where the reinforcing glass fibres in advanced fibre-reinforced composites are used as the sensors for chemical process-monitoring (cure monitoring). This paper presents conclusive evidence to demonstrate that reinforcing E-glass fibres can be used for in situ cure monitoring. The cure behaviour of an epoxy/amine resin system was compared using evanescent wave spectroscopy via the reinforcing E-glass fibres and conventional transmission Fourier transform infrared spectroscopy. This paper also reports for the first time that evanescent wave spectroscopy via E-glass fibres can be used to detect the presence of silane coupling agents. Preliminary results indicated that the cure kinetics on the E-glass fibre surface, as observed using evanescent wave spectroscopy, were influenced by the silane coupling agent.

Process Monitoring of a Thermosetting Resin Using Optical-Fiber Sensors in a Microwave Environment

Conventional means of curing thermosetting resins used in advanced fiber-reinforced composites involves heating the sample in an oven or autoclave where heat is transferred to the sample through conduction and convection. In general, the cure schedules that are used can be time consuming and expensive. Hence, significant attempts are being made to improve the processing efficiency and productivity of this class of material. Microwave-based processing has been claimed to offer the advantage of a significantly faster processing time. In this study, two remote (noncontact) sensor systems were designed and developed to facilitate spectral-based quantitative process monitoring inside a custom-modified commercial microwave oven. The two fiber-optic sensor systems were 1) a noncontact optical-fiber reflectance probe and 2) a reusable transmission probe assembly. Spectroscopic data were obtained using the fiber-optic probes at specified microwave power levels. Since conventional metal-based temperature monitoring devices cannot be used in the microwave oven, a low-cost disposable fiber-optic probe was developed. This design was based on an optical fiber-based extrinsic Fabry-Perot interferometer (FPI). The extrinsic FPI temperature sensor demonstrated an accuracy of /spl plusmn/0.5/spl deg/C over the range from ambient to 300/spl deg/C. The temperature of the resin system was also measured simultaneously, along with the spectral data, to monitor the progress of the chemical reaction (curing).

Effect of structure on thermal behaviour of phosphorus containing addition polyimides and epoxy resins

Addition polyimides, [e.g. endo-5-norbornene-2.3-dicarboximide(nadimides), maleimides etc.] and epoxy resins have been used extensively in the past as matrix resins for advanced fiber-reinforced composites. However, inadequate flame resistance and thermal stability of these resins is one of their serious limitations. Our earlier studies have shown excellent flame resistance properties of addition polyimides based on bis(3-aminophenyl) methyl phosphine oxide (B) or tris(3-aminophenyl) phosphine oxide (T). Carbon fiber reinforced composites based on such matrix resin did not burn even in pure oxygen. In order to investigate the effect of weight percent of phosphorus on thermal behaviour and flame resistance, a series of addition polyimides and epoxy resins were prepared using B, T and amide-amines. The effect of structure on curing behaviour of resins and thermal and flame resistance of cured resins was evaluated. The results demonstrate an improvement in char yield of resins containing higher weight percent of phosphorus.

In-situ process and condition monitoring of advanced fibre-reinforced composite materials using optical fibre sensors

This paper presents a general overview of a number of optical fibre sensor systems which have been developed and used in advanced fibre-reinforced composites for in-situ process and condition monitoring. The in-situ process monitoring techniques were optical-fibre-based evanescent wave spectroscopy, transmission near-infrared spectroscopy and refractive index monitoring. The optical fibre sensors were successful in tracking the cure reaction. The condition monitoring of advanced fibre-reinforced composites was carried out using two intensity-based optical fibre sensor systems: an extrinsic multi-mode Fabry-Pérot sensor and Bragg gratings. In addition to this, the feasibility of using the reinforcing fibre as a light guide was demonstrated. These sensor systems were evaluated under quasi-static, impact and fatigue loading. The test specimens consisted of prepreg-based carbon-fibre-reinforced epoxy and glass-fibre-reinforced epoxy filament-wound tubes. Excellent correlation was obtained between surface-mounted strain gauges and the embedded optical fibre sensors. The feasibility of using these sensor systems for the detection of impact damage and stiffness reduction in the composite due to fatigue damage was successfully demonstrated.

Compressive strength of delaminated and repaired composite plates

Compressive strength is a critical concern in the design of fiber-reinforced adhesive matrix composite plates. This is mainly due to the fact that Compressive strength is generally lower than tensile strength for most advanced fiber-reinforced composites, e.g., 180 Ksi versus 200 Ksi for a graphite/epoxy composite T300/976 (HYE-1076E). In this study, the Compressive strength is used to assess the effect of delamination and repair of T300/976 graphite/epoxy composite plates. The sequence and dimension of composite test plates are [0/45/90/-45/-45/90/45/0]s and 4 × 6 in., respectively. A peel ply is used to separate the plies and to make the delamination across the test plate. The delamination is located at the center and between the 8th and 9th plies. The sizes of delamination are 1/4, 3/4, 1, 3 and 6 in. The compression test of delaminated plates is performed and the effect of delamination on strength is estimated from the Compressive strength. The adhesive injection method is the primary approach for repairing delamination without removing the delaminated materials. A two-part low viscosity adhesive (EA9394) is injected into the delaminated area and re-bonds the damaged area. The repair efficiency is assessed by compressive strength of repaired composite. The experimental results show that the compressive strength of delaminated composite plate is reduced as the delaminated area increases, and the repair efficiency of repaired plate is also reduced as the re-bonded area decreases. If the adhesive can fully fill and re-bond the delaminated plies, the repair efficiency can be as high as 98% and satisfy the repair requirement. As a result, the adhesive injection method is seen to be a useful repair method for the delaminated composites.

An intensity-based optical fibre sensor for fatigue damage detection in advanced fibre-reinforced composites

The detection of fatigue-induced damage within fibre-reinforced composite materials is of vital importance in areas where these materials are used for critical load-bearing applications. Embedded fibre-optic sensors are suitable for damage detection in composite materials because they can be easily integrated into the structure with minimal interference with the bulk properties of the composite. In this current study, a novel intensity-based optical fibre sensor was designed and evaluated under tension/tension and tension/compression fatigue loading conditions. This intensity sensor has the ability to monitor rapid strain changes without loss of the reference level. Furthermore, this sensor utilizes relatively simple and inexpensive instrumentation. The fibre-optic strain sensor was embedded within a 16-ply 0/90 carbon/epoxy laminate at the manufacturing stage. The embedded optical fibre sensor was not found to affect the fatigue life of the composite both under tension/tension and tension/compression loading. The feasibility of using the embedded sensor for fatigue damage detection was demonstrated. Excellent correlation was also obtained between the strain data from a surface mounted optical fibre sensor and a surface-mounted extensometer under fatigue loading conditions.

A nondestructive approach to correlate the interfacial defects induced during processing of MMCs and CMCs with the consolidation parameters

An ultrasonic nondestructive methodology for evaluating the consolidation and microstructure of advanced fiber-reinforced composites has been developed to aid in their design and fabrication. The use of this nondestructive evaluation (NDE) technique can enable optimization of the processing parameters to obtain complete densification around the fibers. In addition, the methodology can be used to ensure that the composite panels are devoid of any global problems such as fiber swimming, ply delamination, embedded manufacturing anomalies such as voids, etc. Such a post processing NDE is also essential before any interfacial characterization is performed. The technique described in this paper, being generic, is applicable to both metal matrix and ceramic matrix composites.