Anisotropic Composite Materials Research Articles

Evaluation of Multiple Reflections in the Characterization of Anisotropic Materials by Through Transmission Ultrasonic Technique

Ultrasonic through-transmission techniques have been widely used along decades for determining the elastic constants of anisotropic composite materials. This work presents a new methodology that takes into account not only the transmitted waves but also the multiple reflections inside the sample. The superposition of different reflected waves are theoretically and experimentally identified and excluded, which avoids the distortion caused by pulse superposition. Additionally, the results of the Christoffel tensor optimization are evaluated in the reflected waves, improving the results validation. Experimental results using unidirectional carbon-matrix composite samples used in the aeronautical industry are presented.

Finite Element Model of Trimming CFRP Aerospace Composites

Carbon Fiber Reinforced Plastics (CFRP) exhibit superior characteristics such as high specific strength, high specific modulus, fatigue strength and endurance. During the manufacture of components from CFRP, it is usually necessary to carry out a post-machining step. Generally, after curing process, the parts will be trimmed in order to meet the required tolerances. Due to the heterogeneous composition and anisotropy of composite materials, machining procedures can damage the material in ways that directly affect its mechanical properties. Currently a costly try and error approach is employed to determine the optimal process conditions. Thus, this study focused on numerical modelling of machining CFRP composites using cross-nick router. A set of machining tests were performed in order to validate the accuracy of the model. Good agreement between simulation and experimental results show the validity of the model in handling real-field problems. The proposed numerical modeling technique can be used as an input in the process planning and decision making levels.

Modeling and analysis of water-hammer in coaxial pipes

The fluid-structure interaction is studied for a system composed of two coaxial pipes in an annular geometry, for both homogeneous isotropic metal pipes and fiber-reinforced (anisotropic) pipes. Multiple waves, traveling at different speeds and amplitudes, result when a projectile impacts on the water fillinhe annular space between the pipes. In the case of carbon fiber-reinforced plastic thin pipes we compute the wavespeeds, the fluid pressure and mechanical strains as functions of the fiber winding angle. This generalizes the single-pipe analysis of J. H. You, and K. Inaba, Fluid-structure interaction in water-filled pipes of anisotropic composite materials, J. Fl. Str. 36 (2013). Comparison with a set of experimental measurements seems to validate our models and predictions.

Prediction of attenuated guided waves propagation in carbon fiber composites using Rayleigh damping model

In this work, a predictive model of attenuated guided wave propagation in carbon fiber–reinforced polymer using Rayleigh damping is developed. After a brief introduction, this article reviews the theory of guided waves in anisotropic composite materials. It follows with a discussion of the piezoelectric wafer active sensors, which are lightweight and inexpensive transducers for structural health monitoring applications. Experiments were performed on a carbon fiber–reinforced polymer panel to measure the dispersion curves and the piezoelectric wafer active sensors tuning curves. Lamb wave damping coefficient was modeled using the multi-physics finite element method and compared with experimental results. A discussion about the capability to simulate, with multi-physics finite element method commercial software, guided wave in composite material using the Rayleigh damping is developed. This article ends with conclusion, and suggestions for further work are also presented.

A new viscoelastic model based on generalized method of cells for fiber-reinforced composites

A semi-analytical model is constructed to investigate the relaxation properties of polymer composites consisting of linearly viscoelastic matrices and transversely isotropic elastic fibers. A representative unit cell is subjected to some prescribed loadings to value the time-dependent properties of composite materials. The model is derived to describe the anisotropic viscoelastic properties of composite materials as functions of matrix and fiber properties. The present numerical algorithm is used to predict the viscoelastic properties of a graphite/epoxy composite and the results are compared with the finite element analysis of the micromechanical model. Very good agreement between numerical algorithm and finite element analysis results is illustrated for the viscoelastic properties of anisotropic composite materials.

Lifetime estimation of glass reinforced epoxy pipes in acidic and alkaline environment using accelerated test methodology

Accelerated ageing test methodology is valid and widely accepted procedure for estimating the lifetime of isotropic homogeneous polymeric materials. However for non-isotropic and heterogeneous polymeric compounds such as glass reinforced epoxy (GRE) pipes, accelerated ageing test methodology has not been much investigated. Various standards such as ASTM D 3681, ASTM D 5365 are being used to estimate the lifetime of GRE pipes using regression analysis which is time consuming, and requires large number of test specimens and fixtures specific to pipe dimensions. Accelerated ageing test methodology can be a viable method for estimating the lifetime. The research on accelerated test methodology as a vital tool to determine the lifetime of GRE pipes has been limited. The major concern for using accelerated ageing test methodology is primarily due to the degradation kinetics of the anisotropic composite materials which may not be governed by the Arrhenius principle. The present study on the estimation of lifetime of GRE pipes in extreme acidic and alkaline medium reveals that degradation of the composite pipe follows the Arrhenius principle and the degradation mechanism can be described by first order reaction kinetics. The degradation rate, the temperature dependence of the degradation rate and lifetime of the glass reinforced epoxy pipes in extreme acidic and alkaline medium as found in the current study are presented here, along with the morphological study of aged and un-aged GRE pipes.

Blue shift of spontaneous emission in hyperbolic metamaterial.

Spontaneous emission is one of the most fundamental quantum phenomena in optics. Following the seminal work of Purcell and in agreement with the Fermi's Golden Rule, its rate can be controlled with the photonic density of states (PDOS). In recent years, this effect has been demonstrated in metamaterials with hyperbolic dispersion – highly anisotropic composite materials, which have a broad-band singularity of the density of photonic states. At this time, we show that hyperbolic metamaterials can control spontaneous emission spectra as well. Experimentally, DCM laser dye has been embedded into lamellar metal/dielectric metamaterial. The observed 18 nm blue shift of emission is explained by strong dispersion of the density of photonic states. On the other hand, practically no spectral shift has been observed in the excitation spectra of the same dye. This suggests that the effect of PDOS on spontaneous emission is very different from its effect on excitation and absorption.

Mechanical characterization of CFRP composites by ultrasonic immersion tests: Experimental and numerical approaches

We study an innovative experimental approach for the characterization of the elastic response of anisotropic composite materials by ultrasonic immersion tests. In particular, the class of anisotropy and the elastic moduli can be determined starting from measurements of the velocities of ultrasonic waves propagating in suitable directions. To this aim, we have designed and developed a goniometric ultrasonic test bench and a software for the management of the test and the processing of the acquired data. By employing this experimental device, we determine in a non-destructive way the five elastic moduli of a transversely isotropic unidirectional CFRP composite. The experimental analyses are supported by numerical simulations, which are useful for a deeper insight of the propagation phenomena and for enhancing the experimental strategies to be adopted.

Improved understanding of the dynamic response in anisotropic directional composite materials through the combination of experiments and modeling

Recently there has been renewed interest in the dynamic response of composite materials; specifically low density epoxy matrix binders strengthened with continuous reinforcing fibers. This is in part due to the widespread use of carbon fiber composites in military, commercial, industrial, and aerospace applications. The design community requires better understanding of these materials in order to make full use of their unique properties. Planar impact testing was performed resulting in pressures up to 15 GPa on a unidirectional carbon fiber - epoxy composite, engineered to have high uniformity and low porosity. Results illustrate the anisotropic nature of the response under shock loading. Along the fiber direction, a two-wave structure similar to typical elastic-plastic response is observed, however, when shocked transverse to the fibers, only a single bulk shock wave is detected. At higher pressures, the epoxy matrix dissociates resulting in a loss of anisotropy. Greater understanding of the mechanisms responsible for the observed response has been achieved through numerical modeling of the system at the micromechanical level using the CTH hydrocode. From the simulation results it is evident that the observed two-wave structure in the longitudinal fiber direction is the result of a fast moving elastic precursor wave traveling in the carbon fibers ahead of the bulk response in the epoxy resin. Similarly, in the transverse direction, results show a collapse of the resin component consistent with the experimental observation of a single shock wave traveling at speeds associated with bulk carbon. Experimental and simulation results will be discussed and used to show where additional mechanisms, not fully described by the currently used models, are present.

On the Deflexion of Anisotropic Structural Composite Aerodynamic Components

This paper presents closed form solutions to the classical beam elasticity differential equation in order to effectively model the displacement of standard aerodynamic geometries used throughout a number of industries. The models assume that the components are constructed from in-plane generally anisotropic (though shown to be quasi-isotropic) composite materials. Exact solutions for the displacement and strains for elliptical and FX66-S-196 and NACA 63-621 aerofoil approximations thin wall composite material shell structures, with and without a stiffening rib (shear-web), are presented for the first time. Each of the models developed is rigorously validated via numerical (Runge-Kutta) solutions of an identical differential equation used to derive the analytical models presented. The resulting calculated displacement and material strain fields are shown to be in excellent agreement with simulations using the ANSYS and CATIA commercial finite element (FE) codes as well as experimental data evident in the literature. One major implication of the theoretical treatment is that these solutions can now be used in design codes to limit the required displacement and strains in similar components used in the aerospace and most notably renewable energy sectors.

Modeling and Dynamical Behavior of Rotating Composite Shafts with SMA Wires

A dynamical model is developed for the rotating composite shaft with shape-memory alloy (SMA) wires embedded in. The rotating shaft is represented as a thin-walled composite of circular cross-section with SMA wires embedded parallel to shaft’s longitudinal axis. A thermomechanical constitutive equation of SMA proposed by Brinson is employed and the recovery stress of the constrained SMA wires is derived. The equations of motion are derived based on the variational-asymptotical method (VAM) and Hamilton’s principle. The partial differential equations of motion are reduced to the ordinary differential equations of motion by using the Galerkin method. The model incorporates the transverse shear, rotary inertia, and anisotropy of composite material. Numerical results of natural frequencies and critical speeds are obtained. It is shown that the natural frequencies of the nonrotating shaft and the critical rotating speed increase as SMA wire fraction and initial strain increase and the increase in natural frequencies becomes more significant as SMA wire fraction increases. The initial strain of SMA wires appears to have marginal effect on dynamical behaviors of the shaft. The actuation performance of SMA wires is found to be closely related to the ply-angle.

Finite element modeling of porous thermoelastic composites with account for their microstructure

The paper discusses approaches to the determination of the effective moduli of porous anisotropic thermoelastic composite materials based on the effective moduli method, modeling of the representative volumes with account for microstructure and finite element technologies of solving static thermoelastic problems for heterogeneous bodies. The paper formulates a set of boundary-value problems of thermoelasticity with boundary conditions of the first kind that enables computing all effective stiffness moduli, thermal conductivity moduli and coefficients of thermal stresses for porous anisotropic material. The technology of the finite element method is described for the numerical solution of boundary-value thermoelastic problems in the representative volume of a porous material. A range of the simulation methods for the inner structure of the representative volumes in cubic finite element lattice is represented by the way of giving properties of the porous material to a certain number of finite elements. The methods considered are the simple random method, the method that supports the connectivity of the skeleton for the porosity up to 90%, the Witten-Sander method that enables us to obtain a cluster from the pores of fractal type, and the growing from the plane method that forms several clusters from the pores in the vicinity of one of the edges of the cubic lattice. The example considered is the model of porous silicon that is taken as a material of cubic symmetry. The computation results allow analyzing the influence of various structures of the representative volumes on the effective moduli. The comparison of the computed effective characteristics of porous silicon with a range of known analytical and numerical results has been carried out. The results of the test computations have demonstrated that the effective material properties of porous thermoelastic composites can strongly depend on the models of their representative volumes.

Non-damage-related influences on Lamb wave–based structural health monitoring of carbon fiber–reinforced plastic structures

Structural health monitoring with actively excited Lamb waves is a promising technology for health monitoring of aerospace carbon fiber–reinforced plastic structures with reasonable effort, as it has the potential to cover large areas with few sensors and a high sensitivity to damage. The high signal complexity inherent to wave propagation in complex, finite, and anisotropic structures, especially in combination with the sensitivity toward environmental and operational loads, presents the main challenge on the road toward the utilization of this potential. In this study, the effects that real structural features and damage, in combination with environmental conditioning, have on Lamb wave propagation and measurement are investigated. For this, the composite-specific behavior is discussed. Based on the local temporal coherence method, changes of sensor responses containing reflections and interaction with stiffness discontinuities, both unrelated and related to damage, are identified in anisotropic composite materials. The amount of signal changes unrelated to damage, as well as their high dependency on the specific conditions of the measurements, is an indicator for the complex issues faced by compensation. While damage of a sufficient size will be detectable even in the most complex, finite, and anisotropic structures, the establishment of a sufficiently reliable damage detection with an acceptable low detection threshold will require even more careful consideration of the monitored structure and its environmental and operational loads than for metallic structures.

Ultrasonic assembly of anisotropic short fibre reinforced composites

We report the successful manufacture of short fibre reinforced polymer composites via the process of ultrasonic assembly. An ultrasonic device is developed allowing the manufacture of thin layers of anisotropic composite material. Strands of unidirectional reinforcement are, in response to the acoustic radiation force, shown to form inside various matrix media. The technique proves suitable for both photo-initiator and temperature controlled polymerisation mechanisms. A series of glass fibre reinforced composite samples constructed in this way are subjected to tensile loading and the stress–strain response is characterised. Structural anisotropy is clearly demonstrated, together with a 43% difference in failure stress between principal directions. The average stiffnesses of samples strained along the direction of fibre reinforcement and transversely across it were 17.66±0.63MPa and 16.36±0.48MPa, respectively.

Computational structural tailoring of continuous fibre reinforced polymer matrix composites by hybridisation of principal stress and thickness optimisation

Anisotropic composite material properties require special designing capabilities to fully exploit the remarkable properties in longitudinal fibre direction for stiffness and strength improvement. Therefore an optimisation scheme with hierarchical structuring of an optimality criterion based orientation optimisation and a gradient based technique for thickness optimisation is presented. The procedure is applied on two different space scales. Orientation optimisation based on principal stress directions is applied on a local laminate position. There fibre directions are affixed throughout the laminate until convergence is reached. On a local ply position thicknesses are determined in an iterative procedure. It is switched between both algorithms until global convergence is achieved. The application of algorithms on two different space scales leads to a robust engineering approach. The ability of the presented algorithm is tested with a numerical example and a structural optimisation of laminated composites is shown.

Evaluation system for effective thermophysical properties of anisotropic composite material

A simultaneous evaluation system is presented and developed for the temperature dependent effective thermophysical properties (ETPs) of anisotropic material over a wide temperature range up to high temperature. Another feature of the present system is the intelligent performance, providing the evaluated values of ETPs and relevant analysis, using friendly operation interface, visualizing data, and processing the wide range of data types, i.e. from the experiment measurement to the proper data from the literature sources. Verification of the present system is conducted comparing the evaluated with the referenced thermophysical properties. One application is carried out and the result shows the potential of the present system in finding the acceptable values of ETPs of anisotropic material over a wide range up to high temperature.

Inspection of anisotropic composites using ultrasonic phased arrays

An approach, referred to as Velocity Variation Compensation (VVC), used to improve the inspection sensitivity of composites utilizing an ultrasonic phased array technique is described. This approach uses an ultrasonic immersion through-transmission technique to measure the velocity variation within an anisotropic composite material as the incident angle varies. Delay laws for phased array inspection were then modified using the measured velocity, and applied to a 5 MHz 32 elements phased array probe. Both methods were performed on the same sample, and the results showed that the detectability of defects was significantly increased.

Multi-scale design of composite materials and structures for maximum natural frequencies

This paper introduces a hierarchical concurrent design approach to maximizing the natural frequency of a structure. Multiple material phases are considered in the topology optimization performed on both the macro and micro scales. A general problem for composite structure and material design is formulated that contains the cellular design problem as a special case. The design of the macro structure and material micro structure is coupled. The designed material properties are applied to the analysis of the macro structure, while the macro structure displacement field is considered in the sensitivity analysis on the micro scale. The material edistribution is controlled by an optimality criterion for frequency maximization. Convergent and mesh-independent bi-directional evolutionary structural optimization (BESO) algorithms are employed to obtain the final optimal solution. Several numerical examples of composite structures and materials are presented to demonstrate the capability and effectiveness of the proposed approach. Results include various orthotropic or anisotropic composite materials, as well as vibration-resisting layouts of the macro structure. In-depth discussions are also given on the effects of the base material phases and the assignment of the volume fractions on each scale.

Inspectability of interfaces between composite and metallic layers using ultrasonic interface waves

The interface between an anisotropic composite material and a metallic material is inspected non-destructively for disbonded regions using ultrasonic guided waves. The material properties of the composite and metal have been tailored to demonstrate their effect on inspectability. The material properties have been designed to be either favorable or unfavorable to the existence of propagating Stoneley waves. Stoneley waves can exist because the layer thicknesses are large enough compared to the wavelength to be considered half-spaces. The existence of Stoneley waves between generally anisotropic materials depends on the elastic constants and densities in a complicated way. The range of material properties that allow Stoneley waves is small; however, when the vertically polarized shear wave speeds are similar in the two materials, the existence of Stoneley waves is generally possible. If the conditions do not strictly allow Stoneley waves, other interface waves can still exist such as leaky waves. Disbonds are inserted into the materials before bonding and are inspected using interface waves. Sensitivity to disbonds is determined and thus inspectability is demonstrated for cases that are favorable and unfavorable to Stoneley waves. Both numerical and physical experimental results are shown.

Axisymmetric Vibrations of Stepped Cylindrical Shells Made of Composite Materials. Part 21

Axisymmetric vibrations of stepped cylindrical shells are studied. The shells are made of elastic unidirectionally reinforced composite materials and have a piecewise constant (stepped) thickness. It is assumed that, at the reentrant corners of steps, stable circumferential cracks of constant depth are located. The method of a “massless rotational spring” developed in earlier papers for investigation of homogeneous shells made of isotropic materials is extended to the case of anisotropic composite materials. Analytical expressions are derived for shells with an arbitrary number of steps, and numerical results are presented for shells with one and two steps.