Effect of excessive bending on residual tensile strength of hybrid composite rods

Excessive bending has been identified as a concern for the hybrid composite core that is currently being used as the structural member for the Aluminum Conductor Composite Core Trapezoidal Wire (ACCC/TW™) transmission line. In this work the flexure strength of the ACCC core was measured in a series of four point bend tests while monitoring acoustic emissions. To quantify the stress state within the rods and to evaluate its flexure strength, an analytic solution for the bending stress was derived and numerically verified using the finite element method. In the second part of the study several specimens that had been subjected to excessive bending were subsequently tested for their residual tensile strength. It was found that wrapping the ACCC core around a 1 m mandrel, which is a common loading condition in practice, will not generate significant structural damage in the composite core. It was determined that the diameter of the mandrel that would cause failure of the composite core is 467 mm. From this work it was found that excessive bending, up to 90% of the flexural strength of the ACCC core, had no detrimental effect on the residual tensile strength of the hybrid composite. It was observed that the majority of the micro-structural damage that was accrued during the excessive bending of the cores presented itself in the form of matrix damage without any significant fiber kinking.     

Design optimization and fabrication of a hybrid composite flywheel rotor

This paper discusses three different rim design cases of a hybrid composite flywheel rotor using strength ratio optimization. The rotor is composed of four hybrid composite rims. These rims are made from carbon–glass/epoxy with varying volume fractions of hoop wound reinforcements. Optimization is performed to reduce the maximum strength ratio during two rotor states: stationary and the maximum allowable rotational speed. The input specifications for optimization are: maximum useable energy (35 kW h), rotational speed (15,000 rpm), height, and inner radius. In the first case, the rims are wound simultaneously by continuous winding. However, in the second case, the rims are wound separately, and interferences are incorporated for their assembly by press fit. In the third case, a hybrid version of the first two cases is used, whereby two pairs of rims are wound at the same time, and in a secondary operation, the first pair is press fitted to the second pair. Each case has different fabrication costs and different strength ratios. The third case rotor has been successfully manufactured by filament winding with in situ curing, followed by press fit assembly of machined rims.     

Compressive response of Z-pinned woven glass fiber textile composite laminates: Experiments

In the first of this two part sequel, experimental results pertaining to the compressive response and failure of Z-pinned S-Glass fiber, plain-weave laminated composites are presented. These experiments are motivated by a need to understand the effect of Z-pinning on the strength and stiffness of these composites. A series of experiments are performed based upon density of the Z-pins and the diameter of the Z-pins. It is concluded that the damage zone around a Z-pin plays an important role in influencing the stiffness and strength of the Z-pin composite. In part 2 of this sequel, a 3D finite element (FE) based numerical model (based upon the composite microstructure acquired from scanning electron micrograph-SEM images) are used to capture details of the observed failure mechanisms and to provide predictions of the stiffness and strength of the composite.     

Exact three-dimensional piezothermoelasticity solution for dynamics of rectangular cross-ply hybrid plates featuring interlaminar bonding imperfections

An exact three-dimensional (3D) piezothermoelasticity solution is presented for static, free vibration and steady state harmonic response of simply supported cross-ply piezoelectric (hybrid) laminated rectangular plates with interlaminar bonding imperfections. The bonding imperfection is modeled by considering the jump in the displacements, electric potential and temperature across the non-rigid interface proportional, respectively, to the associated tractions, transverse electric displacement and heat flux. The solution includes the case when electric potentials are prescribed at the interfaces for effective actuation. Numerical results are presented for hybrid composite and sandwich plates with varying imperfection compliance. The effect of location of imperfect bonding on the response is investigated for mechanical, electric potential and thermal load cases. The effect of weak bonding at elastic–piezoelectric interface on the actuation authority of the piezoelectric layer is also investigated. These results would serve as benchmark for assessing 2D plate theories incorporating interlaminar bonding imperfections.     

Numerical and experimental study on morphing bi-stable composite laminates actuated by a heating method

This paper presents a new heating actuation method to induce the snap-through phenomenon of bi-stable laminates by manipulating the residual stress in laminates. The mechanism of the heating actuation method is analyzed, and the snap-through process is simulated by the finite element method. The heating actuation experiments on two types of laminates with different stacking sequences are performed. Good agreement is obtained between the experimental data and the finite element analysis (FEA). Subsequently, the heating actuation method is applied to several different bi-stable laminates and investigated by FEA. The FEA results show that this heating actuation method is an effective way to induce the snap-through phenomenon of bi-stable laminates of different thicknesses, sizes and shapes.     

A multi-objective optimization approach for design of blast-resistant composite laminates using carbon nanotubes

A reliable process for the design of blast-resistance composite laminates is needed. We consider here the use of carbon nanotubes (CNTs) to enhance the mechanical properties of composite interface layers. The use of CNTs not only enhances the strength of the interface but also significantly alters stress propagation in composite laminates. A simplified wave propagation simulation is developed and the optimal CNT content in the interface layer is determined using multi-objective optimization paradigms. The optimization process targets minimizing the ratio of the stress developed in the layers to the strength of that layer for all the composite laminate layers. Two optimization methods are employed to identify the optimal CNT content. A case study demonstrating the design of five-layer composite laminate subjected to a blast event is used to demonstrate the concept. It is shown that the addition of 2% and 4% CNTs by weight to the epoxy interfaces results in significant enhancement of the composite ability to resist blast.     

Enhanced strength analysis method for composite open-hole plates ensuring design office requirements

The use of unidirectional carbon fibre-reinforced composites in the design of primary structures, such as the centre wing box, has spread increasingly over the past few years. However, composite structures can be weakened by the introduction of geometrical singularities, such as holes or notches. The semi-empirical aspect of the current open-hole failure approaches requires the allowables to be systematically fitted against specific test results. This point constitutes a strong limitation for optimum design. A simplified strength analysis method for perforated plates is presented, ensuring design office requirements in terms of precision and computational time. The predictions of the proposed approach are compared successfully with a large experimental database, with different configurations of perforations, different stacking sequences and in different Carbon/Epoxy materials.     

Reliability model of drilled composite materials

Recent studies by the authors have investigated the influence of the quality of the drilling on the fatigue behaviour of composites. From microscopical observations, a three-dimensional criterion using the Hashin’s theory has been established. It predicted well which component (fiber or matrix) has been damaged first. Firstly, a Monte Carlo technique considering the angles uncertainty of the different layers of a laminated composite was used to predict the statistical distributions of its mechanical properties. Secondly, a statistical distribution has been affected to each variable of the Hashin’s criterion in order to obtain a probability of failure rather than a deterministic value (less or equal to 1). Thus, it is possible to affect at any value of the criterion a probability.     

Ply-interleaving technique for joining hybrid carbon/glass fibre composite materials

Efficiently joining materials with dissimilar mechanical and thermal properties is fundamental to the development of strong and lightweight load-bearing hybrid structures particularly for aerospace applications. This paper presents a ply-interleaving technique for joining dissimilar composite materials. The load-carrying capacity of such a joint depends strongly on several design parameters such as the distance between ply terminations, the spatial distribution of ply terminations, and the stiffness and coefficients of thermal expansion of the composites. The effects of these factors on the strength of quasi-isotropic hybrid carbon/glass fibre composite are investigated using combined experimental, analytical and computational methods. Through fractographic analyses significant insights are gained into the failure mechanism of the hybrid joints, which are then used to aid the development of predictive models using analytical and high fidelity computational methods. To characterise the interaction between transverse matrix cracking and delamination, continuum damage mechanics model and cohesive zone model are employed. The predictions are found to correlate well with experimental data. These modelling tools pave the way for optimising hybrid joint concepts, which will enable the structural integration of dielectric windows required for multifunctional load-bearing antenna aircraft structures.     

Coupled visco-mechanical and diffusion void growth modelling during composite curing

Most critical processing step during long fiber reinforced epoxy matrix composite laminate manufacturing is the polymerization stage. If not optimized, it gives birth to defects in the bulk material, such as voids. These defects are considered as possible sources of damage in the composite parts. The aim of this work is to model the evolution of void growth in thermoset composite laminates after ply collation (autoclave processes) or resin impregnation (RTM, LCM process). A coupled mechanical and diffusion model is presented to better predict the final void size at the end of polymerization. Amongst the parameter investigated, onset of pressure application and diffusive species concentration where found to have a major effect on void size evolution during curing process.     

Deformation micromechanics of a model cellulose/glass fibre hybrid composite

Interfacial stress transfer in a model hybrid composite has been investigated. An Sm³⁺ doped glass fibre and a high-modulus regenerated cellulose fibre were embedded in close proximity to each other in an epoxy resin matrix dumbbell-shaped model composite. This model composite was then deformed until the glass fibre fragmented. Shifts of the absolute positions of a Raman band from the cellulose fibre, located at 1095 cm⁻¹, and a luminescence band from a doped glass fibre, located at 648 nm, were recorded simultaneously. A calibration of these shifts, for both fibres deformed in air, was used to determine the point-to-point distribution of strain in the fibres around the breaks in the glass fibre. Each break that occurred in the glass fibre during fragmentation was shown to generate a local stress concentration in the cellulose fibre, which was quantified using Raman spectroscopy. Using theoretical model fits to the data it is shown that the interfacial shear stress between both fibres and the resin can be determined. A stress concentration factor (SCF) was also determined for the regenerated cellulose fibre, showing how the presence of debonding reduces this factor. This study offers a new approach for following the micromechanics of the interfaces within hybrid composite materials, in particular where plant fibres are used to replace glass fibres.     

Minimization of thermal expansion of symmetric, balanced, angle ply laminates by optimization of fiber path configurations

Optimal fiber path configurations that minimize the sum of the coefficients of thermal expansion (CTE) values along the principal material directions for a class of laminates are presented. Previous studies suggest that balanced, symmetric, angle ply laminates exhibit negative CTE values along the principal directions. Using the sum of the CTE values along the principal material directions as an effective measure of the coefficient of thermal expansion (CTE_eff), we have shown and provided a proof that the smallest value of CTE_eff is rendered by straight fiber path configurations. The laminates considered are sufficiently thin so as to neglect the thermal stresses induced through the thickness of the laminate. It is found that the minimal CTE_eff values occur for [+45/−45]_ns lay-ups. This result is supported by numerical studies that consider curvilinear fiber paths. The possibility of obtaining zero CTE values along both principal material directions and the conditions that render this situation are also examined.     

Mineralized structures in nature: Examples and inspirations for the design of new composite materials and biomaterials

Through the natural evolutionary process, organisms have been improving amazing mineralized materials for a series of functions using a relatively few constituent elements. Biomineralization has been widely studied in the last years. It is important to understand how minerals are produced by organisms and also their structure and the corresponding relationship with the properties and function. Moreover, one can look at minerals as a tool that could be used to develop high performance materials, through design inspiration and to find novel processing routes functioning at mild conditions of temperature, pressure and solvent type. As important as the molecular constituents are structural factors, which include the existence of different levels of organization and controlled orientation. Moreover, the way how the hierarchical levels are linked and interfacial features plays also a major role in the final behavior of the biogenic composite. The main aim of this work is to review the latest contributions that have been reported on composite materials produced in nature, and to relate their structures at different length scales to their main functions and properties. There is also an interest in developing new biomimetic procedures that could induce the production of calcium phosphate coatings, similar to bone apatite in substrates for biomedical applications, namely in orthopedic implants and scaffolds for tissue engineering and regenerative medicine; this topic will be also addressed. Finally, we also review the latest proposed approaches to develop novel synthetic materials and coatings inspired from natural-based nanocomposites.     

Impact of electrochemical cycling on the tensile properties of carbon fibres for structural lithium-ion composite batteries

Carbon fibres are particularly well suited for use in a multifunctional lightweight design of a structural composite material able to store energy as a lithium-ion battery. The fibres will in this case act as both a high performance structural reinforcement and one of the battery electrodes. However, the electrochemical cycling consists of insertions and extractions of lithium ions in the microstructure of carbon fibres and its impact on the mechanical performance is unknown. This study investigates the changes in the tensile properties of carbon fibres after they have been subjected to a number of electrochemical cycles. Consistent carbon fibre specimens were manufactured with polyacrylonitrile-based carbon fibres. Sized T800H and desized IMS65 were selected for their mechanical properties and electrochemical capacities. At the first lithiation the ultimate tensile strength of the fibres was reduced of about 20% but after the first delithiation some strength was recovered. The losses and recoveries of strength remained unchanged with the number of cycles as long as the cell capacity remained reversible. Losses in the cell capacity after 1000 cycles were measured together with smaller losses in the tensile strength of the lithiated fibres. These results show that electrochemical cycling does not degrade the tensile properties which seem to depend on the amount of lithium ions inserted and extracted. Both fibre grades exhibited the same trends of results. The tensile stiffness was not affected by the cycling. Field emission scanning electron microscope images taken after electrochemical cycling did not show any obvious damage of the outer surface of the fibres.     

Modeling the residual strength of carbon fiber reinforced composites subjected to cyclic loading

Fatigue and residual strength data available in literature were modeled with a modified two-parameter wear-out model based on strength degradation. The model explicitly accounts for the maximum applied stress and the stress ratio and requires a limited number of experimental data to predict with accuracy the fatigue life of a series of polymer-based composites. In this paper a substantial modification of the model is proposed in order to enhance its capability in predicting the residual strength kinetics with emphasis to the “sudden drop” of strength before catastrophic failure. It is argued that the strength degradation kinetics under given loading conditions can be obtained from the statistical distribution of cycles to failure under the same loading conditions. From the new approach no new parameters are introduced, limiting to a minimum the experimental data needed to predict the residual strength. The strength degradation law reliability is verified on three different materials data sets appeared in literature. The results indicate that both the fatigue life and the residual strength are related to the statistical distribution of the static strength.     

Mechanical properties of hybrid composites using finite element method based micromechanics

A micromechanical analysis of the representative volume element of a unidirectional hybrid composite is performed using finite element method. The fibers are assumed to be circular and packed in a hexagonal array. The effects of volume fractions of the two different fibers used and also their relative locations within the unit cell are studied. Analytical results are obtained for all the elastic constants. Modified Halpin–Tsai equations are proposed for predicting the transverse and shear moduli of hybrid composites. Variability in mechanical properties due to different locations of the two fibers for the same volume fractions was studied. It is found that the variability in elastic constants and longitudinal strength properties was negligible. However, there was significant variability in the transverse strength properties. The results for hybrid composites are compared with single fiber composites.     

Interlaminar and intralaminar reinforcement of composite laminates with aligned carbon nanotubes

Three-dimensional reinforcement of woven advanced polymer–matrix composites using aligned carbon nanotubes (CNTs) is explored experimentally and theoretically. Radially-aligned CNTs grown in situ on the surface of fibers in a woven cloth provide significant three-dimensional reinforcement, as measured by Mode I interlaminar fracture testing and tension-bearing experiments. Aligned CNTs bridge the ply interfaces giving enhancement in both initiation and steady-state toughness, improving the already tough system by 76% in steady state (more than 1.5 kJ/m² increase). CNT pull-out on the crack faces is the observed toughening mechanism, and an analytical model is correlated to the experimental fracture data. In the plane of the laminate, aligned CNTs enhance the tension-bearing response with increases of: 19% in bearing stiffness, 9% in critical strength, and 5% in ultimate strength accompanied by a clear change in failure mode from shear-out failure (matrix dominated) without CNTs to tensile fracture (fiber dominated) with CNTs.     

Interlaminar stresses in composite laminates: Thermoelastic deformation

An approximate elasticity solution for prediction of the displacement, stress and strain fields within the m-layer, symmetric and balanced angle-ply composite laminate of finite-width and subjected to uniform axial extension was developed earlier [4]. In the present paper, the authors have extended that solution to treat thermal stresses and deformations induced by a uniform change in laminate temperature. The results have revealed not only the complex fields within the laminate, but also inter-relationships between the lamina axial and shearing coefficients of thermal expansion and the effective laminate coefficients of thermal expansion. Further, the solution is shown to recover laminated plate theory predictions for thermally induced fields at interior regions of the laminate, thereby confirming the boundary layer nature of the interlaminar phenomena for the thermoelastic case. Finally, the results exhibit the anticipated response in congruence with the mechanical solution of Ref. [4] and the thermoelastic results satisfy the conditions of self-equilibration necessary for the finite-width laminate subjected to free thermal deformation. Integration of the stress σ_x over the laminate cross-section in the y–z plane is shown to converge to zero as the number of Fourier terms is increased. While the exact solution for mechanical loading is known to exhibit singular behavior, non-convergence of the interlaminar shearing strain is also seen to occur at the intersection of the free edge and planes between lamina of +θ and −θ orientation under thermal loading. The analytical results show excellent agreement with the finite-element predictions for the same boundary-value problem.     

Plasma electrolytic oxidation of aluminium networks to form a metal-cored ceramic composite hybrid material

A commercially-available low density aluminium network material (Duocel™) has been processed by plasma electrolytic oxidation to produce a ceramic hybrid material comprising an assembly of ceramic struts with metallic cores. The architecture and microstructure of this material were studied using X-ray tomography, scanning electron microscopy and densitometry. Conversion fractions were determined from mass gains and by image analysis of cross-sections, and the ceramic density was evaluated by hydrostatic weighing. Tensile and compressive testing of the hybrid material was used to study the toughness, as a function of the conversion fraction. Such material retains some of the beneficial mechanical properties of a metal (ductility and toughness), while also exhibiting a low overall density and a high specific surface area of ceramic. It can thus be considered as a highly permeable ceramic scaffold, with a relatively high toughness.     

Damage sensing of adhesively-bonded hybrid composite/steel joints using carbon nanotubes

Carbon nanotube networks have been used previously for in situ sensing of matrix damage in fiber-reinforced composites. In this research, the ability of carbon nanotube networks to sense and distinguish different types of damage in adhesively-bonded hybrid composite-to-metal joints is evaluated. Toward this end, conductive networks of carbon nanotubes are introduced to the composite substrate as well as the epoxy adhesive. By altering the geometry and chemically treating the steel substrate surface, different failure mechanisms of the single-lap shear joints are achieved. It is demonstrated that these failure mechanisms each possess a distinct resistance response, therefore proving the ability to not only sense failure in situ, but also to distinguish the extent and nature of damage which occurs.