Search

Structural power composites, a class of multifunctional materials, may facilitate lightweighting and accelerate widespread electrification of sustainable transportation. In the example considered in this paper, structural power composite fuselage components could provide power to open aircraft doors in an emergency and thus reduce or eliminate the mass and volume needed for supercapacitors currently mounted on the doors. To demonstrate this concept, an 80 cm long multifunctional composite C-section beam was designed and manufactured, which powered the opening and closing of a desktop-scale composite aircraft door. Twelve structural supercapacitor cells were made, each 30 cm × 15 cm × 0.5 mm, and two stacks of four cells were integrated into the web of the beam by interleaving and encasing them with low-temperature-cure woven carbon fibre/epoxy prepreg. This article culminates by considering the engineering challenges that need to be addressed to realise structural power composite components, particularly in an aerospace context. [Display omitted] [ABSTRACT FROM AUTHOR]

Strain-rate effects in a unidirectional non-crimp fabric carbon/epoxy composite are addressed. To allow for kink-band formation including strain-rate effects and damage in such composites, the paper advances a recent model focused on compression loading at small off-axis angles. The model is based on computational homogenization with a subscale represented by matrix and fibre constituents at finite deformation. The fibre constituent is assumed to be elastic transversely isotropic and the matrix is viscoelastic–viscoplastic with damage degradation. Novel model improvements of special importance to small off-axis loading relate to the isostress formulation of the homogenized response in transverse shear. In this context, an enhanced homogenized elastic response is proposed based on Halpin–Tsai corrections to account for the nonuniform stress distribution on the microscale. The model captures the strongly rate sensitive kink-band formation due to localized matrix shearing and fibre rotation, confirming the experimentally observed increase in compressive strength for high strain rates. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an experimental characterisation of the mechanical performance and behaviour of through-thickness reinforced composite laminates. To achieve this, composite blocks with individual reinforcing pins were manufactured, quality assessed and tested. Individual specimens were inspected using X-ray Computed Tomography and only the specimens with acceptable quality pin insertions were tested experimentally under a range of mode mixities. Two stacking sequences, uni-directional (UD) and quasi-isotropic (QI) were investigated. It was found that the pins inside the UD samples experienced significantly larger pin/matrix bond strength than those in the QI laminates. The resulting experimental data indicates that a non-UD laminate type may experience pin pull-out and thus increased energy absorption for a wider range of mode mixities than a UD laminate type. Energy plots show a clear transition from a pull-out to a pin rupture region for both laminate types. Specimens that experienced pin rupture during low mode mixity tests exhibited similar failure energies to those loaded in pure mode II. [Copyright &y& Elsevier]

Abstract: The paper presents the results of an examination of fibrous composites of polylactide and polyvinylidene fluoride. Fibres composed of biodegradable polylactide matrix (PLA) reinforced with fibrous structures of polyvinylidene fluoride (PVDF), a polymer exhibiting good chemical resistance and mechanical strength, were obtained using the classical method of producing fibres from a melt. The composite has an interesting internal morphology suggesting potentially promising applications. The fibres were examined using DSC and TGA thermal methods, WAXS and FTIR spectroscopy. Their morphology was investigated using optical microscopy and SEM electron microscopy. [Copyright &y& Elsevier]

Abstract: This paper presents a modified failure envelope to predict final failure mode and strength of multi-bolt composite joints based on the conventional failure envelope method, which was presented by Hart-Smith from abundant strength tests of open hole laminates, double-lap single and multi-bolt joints with quasi-isotropic or near-quasi-isotropic lay-ups. In contrast to the bearing “cutoff” of conventional failure envelope method, the modified one takes into account the effect of bypass load on the bearing failure and a new polyline consisted of two oblique lines is proposed. It is able to be established by an additional compressive strength test of laminates and semi-analytical or numerical analysis of fastener-hole laminates and open-hole laminates, combined with the point stress criterion. A flowchart of integration structure failure analysis process with the modified failure envelope method is provided. Series of tests were carried out to provide basic parameters and finite element analysis was conducted for the modified failure envelope. The method proposed was applied to two-bolt and four-bolt carbon-to-carbon double-lap joints with near-quasi-isotropic lay-ups. The predictions of the method proposed and the conventional one were compared with the tests data. The results indicate that the method proposed can obtain effective prediction of failure modes, and more accurate ultimate failure loads in double-lap composite bolted joints. [Copyright &y& Elsevier]

Abstract: Fibre reinforced composites have excellent specific properties and are widely sought after by engineers seeking to reduce mass. However, end of life disposal is a significant problem and so research into more sustainable natural fibre composites is extremely topical. This paper examines the applicability of natural fibre composites for high performance structural applications. Woven flax and regenerated cellulose (Cordenka) textiles were pre-impregnated with commercially available epoxy resins and consolidated into test laminates in an autoclave to determine their static (compressive, tensile, flexural) and dynamic (energy absorption) properties. The range of compressive strengths was 77.5–299.6MPa. Tensile strengths ranged from 63 to 92.6MPa and interlaminar shear strength (ILSS) from 10.7 to 23.3MPa. Specific energy absorption (SEA) varied between 21.2–34.2kJ/kg. Biotex flax combined with MTM49 resin matched the SEA of T300 carbon fibre using the same resin system and layup. This work has demonstrated that natural fibre composites have significant scope for use in structural applications but additional work is required on fibre to matrix bonding in order to maximise their properties whilst remaining an environmentally credible option. [Copyright &y& Elsevier]

Abstract: This paper experimentally and theoretically investigates the influence of creep of the concrete and the adhesive on the edge debonding failure load of reinforced concrete beams strengthened with externally bonded composite materials. The theoretical study includes a stress analysis phase and a fracture analysis phase. In the stress analysis phase, the strengthened beam is modeled as a layered structure and creep of the different materials involved, as well as cracking and material nonlinearity of the concrete are considered using rheological generalized Maxwell models. The theoretical model also accounts for the development of shear and vertical normal stresses through the thickness of the adhesive layer. The failure criterion for edge debonding is based on a fracture mechanics model that uses the concept of the energy release rate through the J-integral and accounts for the creep deformations. An experimental study is conducted, which includes testing of three strengthened beams and one control under short-term loading for evaluation of their failure loads, and long-term testing of another set of beams under different levels of sustained loads for a period of one year. After the sustained loading period, the beams are further loaded to failure without releasing the existing load, in order to estimate the influence of creep deformations on the residual load carrying capacity. In both sets of tests, the failure mode is characterized by interfacial edge debonding. The experimental and theoretical results show that, depending on the material properties, creep can increase or decrease the maximum load that leads to edge debonding failures in FRP strengthened members. [Copyright &y& Elsevier]

Abstract: Materials with negative Poisson’s ratio (NPR) effects have been studied for decades. However, the studies have mainly focused on 2D periodic structures which only have NPR effects in certain in-plane directions. In this paper, a novel concept is proposed to develop composite structures with isotropic NPR effects using NPR random inclusions. The study starts from a finite element analysis of deformation mechanisms of two 2D representative cells which are embedded with a re-entrant square and a re-entrant triangle, respectively. Based on the analysis results, the re-entrant triangles are selected as random inclusions into a matrix to form 2D composite structures. Four such composite structures are built with different numbers of inclusions through a parametric model, and their NPR effects and mechanical behaviors are analyzed using the finite element method. The results show that the isotropic NPR effects of composites can be obtained with high random re-entrant inclusions. Thus, the novel concept proposed is numerically proved by this study. [Copyright &y& Elsevier]

Abstract: This paper compares the mechanical properties of virgin and recycled woven carbon fibre prepreg and goes on to assess the potential for recycled carbon fibre reinforced plastic (rCFRP) to be used in high performance energy absorption structures. Three sets of material were examined: fresh containing virgin fibres and resin, aged which was an out of life but otherwise identical roll and recycled which contained recycled fibre and new resin. The compressive strength and modulus of rCFRP were approximately 94% of the values for fresh material. This correlated directly with the results from impact testing where rCFRP conical impact structures were found to have a specific energy absorption of 32.7kJ/kg versus 34.8kJ/kg for fresh material. The tensile and flexural strength of rCFRP were 65% of the value for fresh material. Tensile and flexural moduli of rCFRP were within 90% of fresh material and ILSS of rCFRP was 75% that of fresh. Overall rCFRP has been proved to remain a highly satisfactory engineering material. This is a significant finding as it proves that carbon fibre can be recycled and reused in high performance applications. This has significant implications for use of carbon fibre in the automotive industry where End of Life Vehicle (ELV) legislation requires 85% of materials to be recyclable. This work opens the door to significantly expanded use of carbon fibre in the automotive industry. Of equal significance is the finding that the mechanical properties of fresh and aged carbon fibre were all within ±6% of one another. This highlights an opportunity to understand resin system life in more detail in order to prevent the creation of composite waste at source. [Copyright &y& Elsevier]

Abstract: The paper considers a Direct Method for the evaluation of the maximum load corresponding to pre-assigned limits on the non-linear behaviour of the matrix and fibres in a laminate structure. This is achieved by combining a consistent micro–macro model for linear behaviour with an extension of the Linear Matching Method (LMM), previously extensively applied to Direct Methods in plasticity. The method is developed with assumptions that allow the methodology to be displayed in its simplest form. Applications to examples of laminate elements and a laminate plate containing a hole are described, assuming a matrix with a limit on ductility. [Copyright &y& Elsevier]

Abstract: This review paper gives an overview of test methods for multiaxial and out-of-plane strength of composite laminates, with special consideration of non-crimp fabrics (NCF) and other textile systems. Tubular and cruciform specimens can provide arbitrary in-plane loading, while off-axis and angle-ply specimens provide specific biaxial loadings. Tensile and compressive out-of-plane strength may be determined by axial loading of specimens with a waisted gauge section, while bending of curved specimens allow determination of the out-of-plane tensile strength. Tests suited for out-of-plane shear strength include the short beam shear test, the inclined double notch test and the inclined waisted specimen. Testing of arbitrary tri-axial stress states using tubular or cruciform specimens with superimposed through-the-thickness loading is highly complex and significant problems have been reported in achieving the intended stress states and failure modes. Specific tri-axial stress states can be obtained by uniaxial loading of specimens with constrained expansion, as in the die channel test. [Copyright &y& Elsevier]

Abstract: In this paper, one structurally integrated array that is conformed around cylindrical bodies has been designed and fabricated for a part of the aircraft outer skin. The effects of the mechanical topology are considered in the antenna design, confirming the optimal integration without compromising either the mechanical or the electrical performance. The measured results show that the radiation pattern is strongly dependent on the radius of curvature. The results of a study of the response and failure characteristics subjected to low-speed impact damage are presented; they indicate that the value of the contact force decreases as the radius of curvature decreases. As the impact energy was increased, the specimens experienced significant damage consisting of a dent localized in the region of impact. These results suggest that the radius of curvature is an important electrical and mechanical parameter in the design of conformal arrays. [Copyright &y& Elsevier]

Abstract: Fiber-reinforced composites are usually designed using constant fiber orientation in each ply. In certain cases, however, a varying fiber angle might be favorable for structural performance. This possibility can be fully utilized using tow placement technology. Because of the fiber angle variation, tow-placed courses may overlap and ply thickness will build-up on the surface. This thickness buildup affects manufacturing time, structural response, and surface quality of the finished product. This paper will present a method for designing composite plies with varying fiber angles with composite plates or panels. The thickness build-up within a ply is predicted as function of ply angle variation using a streamline analogy. It is found that the thickness build-up is not unique and depends on the chosen start locations of fiber courses. Optimal fiber courses are formulated in terms of minimizing the maximum ply thickness, maximizing surface smoothness or combining these objectives with and without periodic boundary conditions. [Copyright &y& Elsevier]

Abstract: Little attention has been paid to joining unidirectionally-reinforced high strength natural fibre composites in the manufacture of engineered structures. Therefore the main objective of the paper is to investigate the effect of joint geometry on the strength of natural fibre composite joints. Epoxy-bonded single lap shear joints (SLJs) between henequen and sisal fibre composite elements were manufactured and tested in tension to assess the shear strength of the structural bonds. The performance of co-cured joints, termed “intermingled fibre joints” (IFJs) and “laminated fibre joints” (LFJs) was also evaluated. These IFJ and LFJ configurations possess much higher lap shear strengths than the single lap shear joints and the failure modes of the three joint configurations are compared. SLJ and LFJ joints have been modelled using finite element analysis, allowing interpretation of the experimental observations. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental investigation of the mechanical response and the industrial manufacturability of CFRP–titanium hybrid laminates using the example of a spacecraft payload adaptor. The local hybridization with metal within a bolted joint region of composite laminates is proven to be an effective method of increasing the mechanical joint efficiency of highly loaded bolted joints. High-strength titanium foils are locally embedded into the composite laminate by means of ply substitution techniques, thus avoiding any local laminate thickening and providing for a local laminate with high bearing and shear capabilities. An extensive sample and component test program has been performed evaluating the impact of different design parameters and load conditions. The verification of the hybrid technique’s processability, inspectability and compatibility with a standard industrial fibre placement process has been successfully demonstrated through the manufacturing of a spacecraft payload adaptor featuring diverse applications of the hybridization technique. [Copyright &y& Elsevier]

Abstract: A stochastic model of fragmentation of brittle constituents, which has been pioneered for ceramic matrix composites, is revisited. It consists in a stochastic description of the brittle fracture of successive fragments having a decreasing size. Fracture of a fragment is caused by its most severe flaw. The approach is based on the low extremes of fragment flaw strength distributions. The paper is aimed at assessing the approach and its potential application to various composite systems containing brittle constituents, such as polymer matrix composites or multi-layers. Expressions for fragments and fibers failures involve various random variables. The influence of the random variables on tensile stress–strain behaviour predictions was investigated. Fragment strength–size relationships were established using tensile tests performed on C/SiC minicomposites in the chamber of a scanning electron microscopy (SEM). Experimental data and predictions validated the approach. Important implications for the prediction of multiple cracking and resulting stress–strain behaviour are discussed. Then simple analytical expressions were derived. [Copyright &y& Elsevier]

Abstract: This paper is the follow-up on the previous work by the authors on the experimental evaluation of the impact damage resistance of laminates with dispersed stacking sequences. The current work focuses on the evaluation of the impact performance of the tested laminates by innovative numerical methods. Constitutive models which take into account the physical progressive failure behaviour of fibres, matrix, and interfaces between plies were implemented in an explicit finite element method and used in the simulation of low-velocity impact events on composite laminates. The computational effort resulted in reliable predictions of the impact dynamics, impact footprint, locus and size of delaminations, matrix cracks and fibre damage, as well as the amount of energy dissipated through delaminations, intraply damage and friction. The accuracy achieved with this method increases the reliability of numerical methods in the simulation of impact loads enabling the reduction of the time and costs associated with mechanical testing. [Copyright &y& Elsevier]

Abstract: The stacking sequence design of composite laminates is often limited to combinations of 0°, 90°, and ±45° fibre angle plies. Furthermore, in order to comply to certain stiffness requirements, clustering of plies becomes unavoidable. Although such laminates might have the desired stiffness properties, they may show poor impact and/or compression-after-impact behaviour. A method to redesign the traditional stacking sequences such that the alternative laminates have improved damage resistance whilst keeping similar in-plane and bending stiffness properties as their original traditional stacking sequences is proposed. This method makes use of optimisation tools based on genetic algorithms. In the alternative laminates, the difference between fibre angles of two consecutive plies is maximised and allowed to vary in the 0–90° fibre angle range at intervals of 5°. Manufacturing of such laminates is practical nowadays as the industry is changing its production techniques into accurate automated fibre-placement and tape-laying technologies. A two-step approach is proposed for the design of laminates. In the first step, the optimal laminate is designed in the traditional fashion to cope with the expected quasi-static loads on the structure. The second step consists of redesigning this laminate to better withstand impact loads by dispersing its stacking sequence while keeping similar stiffness properties as in the first step. A traditional laminate and two dispersed stacking sequence alternative layups were tested under low-velocity impact and compression-after-impact loads in order to compare their impact resistance and damage tolerance characteristics. The evaluation of these laminates will also be carried out by the innovative numerical tools proposed in the follow-up of the present paper. [Copyright &y& Elsevier]

Abstract: A novel corrugated composite core, referred to as a hierarchical corrugation, has been developed and tested experimentally. Hierarchical corrugations exhibit a range of different failure modes depending on the geometrical properties and the material properties of the structures. In order to understand the different failure modes the analytical strength model, developed in part 1 of this paper, was used to make collapse mechanism maps for the different corrugation configurations. If designed correctly, the hierarchical structures can have more than 7 times higher weight specific strength compared to its monolithic counter part. The difference in strength arises mainly from the increase in buckling resistance of the sandwich core members compared to the monolithic version. The highest difference in strength is seen for core configurations with low overall density. As the density of the core increases, the monolithic core members get stockier and more resistant to buckling and thus the benefits of the hierarchical structure reduces. [Copyright &y& Elsevier]

Abstract: This paper presents the results of current research on the fatigue life prediction of carbon/epoxy laminate composites involving twelve balanced woven bidirectional layers of carbon fibres and epoxy resin manufactured by a vacuum moulding method. The plates were produced with 3mm thickness and 0.66 fibre weight fraction. The dog bone shape specimens were cut from these plates with the load line aligned with one of the fibre directions. The fatigue tests were performed using load control with a frequency of 10Hz and at room temperature. The fatigue behaviour was studied for different stress ratios and for variable amplitude block loadings. The damage process was monitored in terms of the stiffness loss. The fatigue life of specimens submitted to block loading tests was modelled using Palmgren–Miner’s law and taking in to account the stress ratio effect. The estimated and experimental fatigue lives were compared and good agreement was observed. [Copyright &y& Elsevier]

Abstract: In this paper, analytical elastic constitutive relations and failure criterion of planar lattice composites are established based on the assessment of static equilibrium and deformation relations of the representative unit cell. Finite element simulations are carried out to validate the analytical solutions. Furthermore, optimizations are performed to minimize the weight of a cantilever sandwich beam for a given flexibility for four different categories of cellular core materials, including metal foam, metal hexagonal honeycomb, metal lattice and lattice composite. It is found that the out-of-plane constraint plays an important role in the in-plane strength of lattice materials made of unidirectional fiber reinforced composites. The present analytical approach is capable of identifying this effect. The sandwich beam of laminate lattice composite possesses the best bending performance among all the sandwich beams considered, which suggests its superiority for application as a bending bearing structure. [Copyright &y& Elsevier]

Abstract: Composite structures can lower the weight of an airliner significantly. The increased production cost, however, requires the application of cost-effective design strategies in which cost, weight and the desired laminate quality are taken into account. This paper proposes an optimization framework for composite aircraft structures that minimizes the direct operating cost on a part level. In addition to previous models, a non-destructive testing model is implemented that calculates design allowables of a laminate based on the ultrasonic scan parameters. In a case study, the effect of the laminate quality on the direct operating cost is discussed. It is investigated how the flaw size and therefore the scan pitch of a composite laminate can influence the optimal solution in terms of cost and weight; thus, the manufacturing cost, the non-destructive testing cost and the weight of a component can be balanced by optimizing the laminate quality in an early design phase. [Copyright &y& Elsevier]