Composite Structures

This paper presents a study of the impact resistance of honeycomb structure with the purpose to mitigate impact forces. The objective is to aid in the choice of optimal parameters to minimize the thickness of the honeycomb structure while providing adequate protection to prevent injury due to head impact. Studies are presented using explicit finite element analysis representing the case of an unprotected drop of a rigid impactor onto a simulated floor consisting of vinyl composition tile and concrete. Analysis of honeycomb material to reduce resulting accelerations is also presented where parameters such as honeycomb material modulus, wall thickness, cell geometry and structure depth are compared to the unprotected case. A simplified analysis technique using a genetic algorithm is presented to demonstrate the use of this method to select a minimum honeycomb depth to achieve a desired acceleration level at a given level of input energy. It is important to select a minimum material depth in that smaller dimensions lead toward more aesthetic design that increase the likelihood of that the device is used.

A {1,2}-order theory for laminated composite and sandwich plates is extended to include thermoelastic effects. The theory incorporates all three-dimensional strains and stresses. Mixed-field assumptions are introduced which include linear in-plane displacements, parabolic transverse displacement and shear strains, and a cubic distribution of the transverse normal stress. Least squares strain compatibility conditions and exact traction boundary conditions are enforced to yield higher polynomial degree distributions for the transverse shear strains and transverse normal stress through the plate thickness. The principle of virtual work is used to derive a 10th-order system of equilibrium equations and associated Poisson boundary conditions. The predictive capability of the theory is demonstrated using a closed-form analytic solution for a simply-supported rectangular plate subjected to a linearly varying temperature field across the thickness. Several thin and moderately thick laminated composite and sandwich plates are analyzed. Numerical comparisons are made with corresponding solutions of the first-order shear deformation theory and three-dimensional elasticity theory. These results, which closely approximate the three-dimensional elasticity solutions, demonstrate that through-the-thickness deformations even in relatively thin and, especially in thick, composite and sandwich laminates can be significant under severe thermal gradients. The {1,2}-order kinematic assumptions insure an overall accurate theory that is in general superior and, in some cases, equivalent to the first-order theory.

Laminated composite shells are frequently used in various engineering applications in the aerospace, mechanical, marine, and automotive industries. This article follows a previous book and review articles published by the leading author (Qatu, 2004, 2002, 1989, 1992, 1999 [1], [2], [3], [4] and [5]). It reviews most of the research done in recent years (2000–2009) on the dynamic behavior (including vibration) of composite shells. This review is conducted with emphasis on the type of testing or analysis performed (free vibration, impact, transient, shock, etc.), complicating effects in material (damping, piezoelectric, etc.) and structure (stiffened shells, etc.), and the various shell geometries that are subjected to dynamic research (cylindrical, conical, spherical and others). A general discussion of the various theories (classical, shear deformation, 3D, non-linear etc.) is also given. The main aim of this review article is to collate the research performed in the area of dynamic analyses of composite shells during the last 10 years, thereby giving a broad perspective of the state of art in this field. This review article contains close to 200 references.

Many military and commercial aging aircrafts flying beyond their design life may experience severe crack and corrosion damage, and thus lead to catastrophic failures. In this paper, the design, fabrication and analysis of adhesively bonded thick composite patch repair of circular corrosion grind-out and a crack propagating on the periphery of the corrosion grind-out on thick 2024 T3 clad aluminum aircraft panel is presented. Thick orthogonal composite patch configurations of 7–25 plies were designed separately for crack and corrosion grind-out using CRAS. Using the principles of superimposition a single patch was designed to repair both the crack and corrosion grind-out. Finite element analysis (FEA) was performed on the test specimen subjected to uniaxial tensile loading. Stress distribution and displacements were obtained and analyzed. Dog-bone shaped tensile test panels were fabricated with damage and repaired with boron/epoxy patch of 11 plies. The patched and unpatched panels were subjected to tensile tests. The experimental and the FEA results show that the maximum skin stress decreases significantly and shifted away from damaged area after the application of composite patch. The load carrying capacity of patched specimen significantly increased over that for unpatched specimen.

The behavior of different E-glass/epoxy laminated composite plates has been experimentally studied under impact of aluminum projectile at low velocities (0.53–3.1 m/s). The results were obtained using a drop weight impact machine and presented for three different cross-ply laminates [02/906/02], [03/904/03] and [04/902/04]. The time history of the impact process such as the acceleration impactor, the projectile displacement and the target circular plate deflection due to an impact force acting at the center has been measured. The effects of the projectile velocities and lamination sequences on composite plates behavior have been discussed. Applications of the theoretical model based on Hertzian contact law were used as an efficient guide for the experimental results validation.

The influence of two-dimensional finite element modeling assumptions on the debonding prediction for skin-stiffener specimens was investigated. Geometrically nonlinear finite element analyses using two-dimensional plane-stress and plane-strain elements as well as three different generalized plane-strain type approaches were performed. The computed skin and flange strains, transverse tensile stresses and energy release rates were compared to results obtained from three-dimensional simulations. The study showed that for strains and energy release rate computations the generalized plane-strain assumptions yielded results closest to the full three-dimensional analysis. For computed transverse tensile stresses the plane-stress assumption gave the best agreement. Based on this study it is recommended that results from plane-stress and plane-strain models be used as upper and lower bounds. The results from generalized plane-strain models fall between the results obtained from plane-stress and plane-strain models. Two-dimensional models may also be used to qualitatively evaluate the stress distribution in a ply and the variation of energy release rates and mixed mode ratios with delamination length. For more accurate predictions, however, a three-dimensional analysis is required.

A two-dimensional (2D) higher-order deformation theory is presented for vibration and buckling problems of circular cylindrical shells made of functionally graded materials (FGMs). The modulus of elasticity of functionally graded (FG) shells is assumed to vary according to a power law distribution in terms of the volume fractions of the constituents. By using the method of power series expansion of continuous displacement components, a set of fundamental governing equations which can take into account the effects of both transverse shear and normal deformations, and rotatory inertia is derived through Hamilton’s principle. Several sets of truncated Mth order approximate theories are applied to solve the eigenvalue problems of simply supported FG circular cylindrical shells. In order to assure the accuracy of the present theory, convergence properties of the fundamental natural frequency for the fundamental mode r=s=1 are examined in detail. A comparison of the present natural frequencies of isotropic and FG shells is also made with previously published results. Critical buckling stresses of simply supported FG circular cylindrical shells subjected to axial stress are also obtained and a relation between the buckling stress and natural frequency is presented. The internal and external works are calculated and compared to prove the numerical accuracy of solutions. Modal transverse shear and normal stresses are calculated by integrating the three-dimensional (3D) equations of motion in the thickness direction satisfying the stress boundary conditions at the outer and inner surfaces. The 2D higher-order deformation theory has an advantage in the analysis of vibration and buckling problems of FG circular cylindrical shells.

This paper seeks to address a novel model for predicting the compression modulus of a 2D plain weave fabric (PWF) composite accounting for the interaction of orthogonal interlacing strands. The two orthogonal yarns in a micromechanical unit cell are idealized as curved beams with a path depicted by using sinusoidal shape functions. The modulus is derived by means of a strain energy approach founded on micromechanics. In order to validate the model, experiments have been performed to investigate the mechanical properties of two-dimensional (2D) orthogonal EW220/5284 PWF composites fabricated by resin transfer moulding (RTM). In-plane tensile, compressive and shear moduli have been measured. It is shown that the experimental results correlate well with predictions from the new model.

In this study, a parallelogram spring model, has been proposed to investigate the effects of fabric parameters (such as braid angle, fiber volume fraction and the aspect ratio of height to length in a unit cell) on the elastic moduli of braided composite plates. To consider the crimp arrangement of the fiber tows and the interaction among the fiber tows in the unit cell, this model first treats each fiber tow as a curve composed of a series of springs with only axial rigidity, and thus discretizes the unit cell of braided composites into a finite element mesh with both spring and solid elements. By assigning proper displacement boundary conditions onto the finite element model of the unit cell, the algebraic static equilibrium equations obtained by the finite element method are then solved. Applying a homogenization procedure, six mean stresses and six mean strains are then calculated for each independent boundary condition. This approach finally solves the constitutive equations for all the elastic moduli (E11, E22, E33, G12, G23, G13, …) of braided composite plates. A fairly good agreement is found between the present computed results and the theoretical and/or experimental data from the literatures.

Dimensional control of composite components is critical for cost effective manufacturing of large composite aerospace structures. This paper presents an engineering approach to the prediction of process-induced deformations of three-dimensional (3D) autoclaved composite components. A 6-step method that uses a two-dimensional (2D) special purpose finite element (FE) based process simulation code and a standard 3D structural FE code is presented. The approach avoids the need to develop a full 3D process model, significantly reducing the computational effort yet retaining much of the detail required for accurate analysis. The methodology is presented together with numerical examples and two case studies demonstrating the validity, utility, and limitations of the approach.

Composite materials offer numerous advantages over conventional engineering metals. Over the years, the use of composite materials has increased significantly. Braiding is a promising and already very commonly used method to form continuous fiber reinforced composite materials. Braided structures are used in a broad range of applications including, but not limited to, medical, aerospace, and automotive. This paper reviews studies published in the field of 2D braiding in order to outline advantages and disadvantages of the process, common preform impregnation techniques, and common stiffness critical applications. Furthermore, elastic property prediction models published in the field are presented for the purpose of stiffness critical designs and applications.

In this paper, we show how the published literature reveals that the approximate two-dimensional solution for the stress intensity factor associated with cracked panel repaired using an externally bonded composite repair is inconsistent with experimental data, and that for short to mid-size cracks the fibre bridging effect is often a second-order effect. The result of this finding is that prediction of the effect of a composite repair on the structural integrity of cracked components repaired by an externally bonded composite repair is dramatically simplified. We also show why structures repaired using Glare patches have a fatigue performance that is superior to structures repaired using boron epoxy or carbon fibre patches.

A full-scale 34 m composite wind turbine blade was tested to failure under flap-wise loading. Local displacement measurement equipment was developed and displacements were recorded throughout the loading history.Ovalization of the load carrying box girder was measured in the full-scale test and simulated in non-linear FE-calculations. The non-linear Brazier effect is characterized by a crushing pressure which causes the ovalization. To capture this effect, non-linear FE-analyses at different scales were employed. A global non-linear FE-model of the entire blade was prepared and the boundaries to a more detailed sub-model were extracted. The FE-model was calibrated based on full-scale test measurements.Local displacement measurements helped identify the location of failure initiation which lead to catastrophic failure. Comparisons between measurements and FE-simulations showed that delamination of the outer skin was the initial failure mechanism followed by delamination buckling which then led to collapse.

Three-point bending tests in the elastic domain were carried out on unidirectional laminates made of T800H/3900-2 laminae, holding the fibre direction coincident with the longitudinal axis of the beam. The slenderness ratio was varied in the range 10–65, in order to highlight the contribution of the shear deformation to the structural response. Local rigidity tests were also performed, to ascertain the influence of the indentation at the loading points on the rigidities measured. From the results obtained, the effect of the local deformation on the apparent flexural rigidity was the higher, as the slenderness ratio was reduced, with errors as large as 18% for the shorter spans adopted.Using Timoshenko’s beam theory and finite element analyses, a prediction of the rigidity in flexure was attempted. The theoretical results and the experimental data showed an excellent correlation, provided an unexpectedly low value of the through-thickness shear modulus was assumed in the calculations. Possible reasons for this occurrence were pointed out, involving the role played by the epoxy/polyamide interleaves in the behaviour of the material, as well as the actual shear correction factor to be used to take into account the shear effect.

An analytical approach using successive finite element analysis technique was conducted to characterize the fatigue crack growth behavior of pre-cracked aluminum plates reinforced with composite patches. For single-sided repairs, due to the asymmetry and the presence of out-of-plane bending, crack front shape would become skewed curvilinear started from a uniform through-crack profile, as observed from previous studies. As the stress intensity factor (SIF) calculated at the crack tip is much influenced by crack front shape, it is necessary to predict the actual crack front shape evolution and take it into account for the accurate analysis of fatigue behavior. Present procedure performed a three-dimensional geometrically nonlinear finite element analysis to determine the SIF distribution at a set of points along the crack front, and then estimated the crack growth increments at these points by invoking a fatigue crack growth rate relationship (power-law relationship). A new crack front was then established for the next step by using a relevant remeshing scheme. Through conducting this procedure successively, the crack path of the patched plate as well as the fatigue life was evaluated with sufficient accuracy. The analytical predictions of both the crack front shape evolution and the fatigue life were in good agreement with the experimental observations.

This work presents the use of carbon nanotube (CNT) skeletons and the resin infusion process as a path towards the production of polymer composites with high and well dispersed nanotube content. A general purpose low viscosity epoxy resin was used as matrix in the reported process assessment. Thin CNT papers, called skeletons, were initially produced to obtain CNT networks. The impregnation was made by infiltrating the non-diluted resin through the carbon nanotube structure. The results show the proposed processing approach as one capable of producing well dispersed nanocomposites with high CNT loading (more than 15 wt% CNT by composite weight), which are important for developing high performance structures based on carbon nanotubes with good thermal and electrical conductivity. The absolute mechanical performance was lower than expected, and discussed in light of manufacturing problems detected by microscopy observations under scanning electron microscopy (SEM).

Three-dimensional finite element simulations were conducted for analyzing low velocity impact behavior of sandwich beams with a functionally graded (FG) core. After validating the finite element model using available analytical data, the effects of projectile initial velocity and kinetic energy, as well as the beam’s dimensions on the impact behavior and indentation and displacement history were studied. It was concluded from these observations that for sandwich beams having functionally graded cores, the maximum contact force increases and the maximum strain decreases compared to those of sandwich beams with a homogenous core.

The analysis of 3D braided composites is more difficult due to its complex microstructure. A new type of finite element method is developed to predict the effective moduli and the local stress within 3D braided composites under the 3D mechanical loading. To verify the present method, the material properties of undamaged 3D braided composites predicted in this paper are compared with the previous work. To demonstrate this method, some examples are analyzed.

The application of a shell/3D modeling technique for the simulation of skin/stringer debond in a specimen subjected to tension and three-point bending was studied. The global structure was modeled with shell elements. A local three-dimensional model, extending to about three specimen thicknesses on either side of the delamination front was used to model the details of the damaged section. Computed total strain energy release rates and mixed-mode ratios obtained from shell/3D simulations were in good agreement with results obtained from full solid models. The good correlation of the results demonstrated the effectiveness of the shell/3D modeling technique for the investigation of skin/stiffener separation due to delamination in the adherents.

Sandwich shells faced with fiber reinforced plastic laminated sheets are commonly applied to space structures. To accurately describe the structures’ behavior and connect sandwich shell with 3D brick elements, a kind of 32-node and three-layer shell element with relative degree-of-freedom is presented based on 3D FEM. Also, to eliminate the discontinuity of the interlaminlar stress and satisfy the load boundary conditions, a post-processing method for exactly computing transverse shear and normal stresses in laminated shells of revolution is presented. 3D brick elements combined with 32-node and three-layer shell elements are applied for modeling complex structures such as laminated sheets or sandwich with variable thickness, metallic frames or reinforcements. Three numerical examples indicate that the results obtained by the present method is satisfying.

The asymptotic homogenization method is used to develop a comprehensive micromechanical model pertaining to three-dimensional composite structures with an embedded periodic grid of generally orthotropic reinforcements. The model developed transforms the original boundary-value problem into a simpler one characterized by some effective elastic coefficients. These effective coefficients are shown to depend only on the geometric and material parameters of the unit cell and are free from the periodicity complications that characterize their original material counterparts. As a consequence they can be used to study a wide variety of boundary-value problems associated with the composite of a given microstructure. The developed model is applied to different examples of orthotropic composite structures with cubic, conical and diagonal reinforcement orientations. It is shown in these examples that the model allows for complete flexibility in designing a grid-reinforced composite structure with desirable elastic coefficients to conform to any engineering application by changing some material and/or geometric parameter of interest. It is also shown in this work that in the limiting particular case of 2D grid-reinforced structure with isotropic reinforcements our results converge to the earlier published results.

This study gives a detailed analysis of load distributions around fibre breaks in a composite and the mechanisms involved in load transfer. In contrast to other studies reported in the literature the analysis considers different configurations of composite damage from the failure of a few fibres to the failure of many. The model considers the elastic case with and without debonding at the broken fibre/matrix interface. In this way, the usual limitations of the finite element approach are overcome so as to take into account the numbers and interactions of broken fibres whilst maintaining an evaluation of the various fields involved, in particular the stress fields associated with fibre failures.

The state-space method is adapted to obtain three dimensional exact solutions for the static and damped dynamic behaviors of simply supported general laminates. The state-space method is written in a general form that permits to handle both cross-ply and antisymmetric angle-ply laminates. This general form also permits to obtain exact solutions for general laminates, albeit with some constraints. For the general case and for the static behavior, either an additive term is added to the load to simulate simply supported boundary conditions, or the plate bends in a particular way. For the dynamic behavior, the general case leads to pairs of natural frequencies for each order, with associated mode shapes. Finite element simulations have been performed to validate most of the results presented in this study. As the boundary conditions needed for the general case are not so straightforward, a specific discussion has been added. It is shown that these boundary conditions also work for the two aforementioned laminate classes. The damped harmonic response of a non symmetrical isotropic sandwich is studied for different frequencies around the fundamental frequency. The static and undamped dynamic behaviors of the [-15/15], [0/30/0] and [-10/0/40] laminates are studied for various length-to-thickness ratios.

On the basis of the three-dimensional theory of elasticity, a semi-analytical method, which combines the state space approach with the technique of differential quadrature, is developed for free vibration of a cross-ply laminated composite rectangular plate. The plate is assumed to be simply supported at one pair of opposite edges such that trigonometric functions expansion can be used to satisfy the boundary conditions precisely at these two edges. The technique of differential quadrature is then incorporated into the state equation, which enables one to obtain a general solution for orthotropic laminates with arbitrary boundary conditions at the other pair of opposite edges. The method is verified by comparing the present results with the exact elasticity solution for a simply supported plate. Further numerical calculations are carried out and effects of some parameters on natural frequency are discussed.

In this paper, the bending fatigue behaviour and damage development during fatigue of polyurethane (PUR)-epoxy 3D woven sandwich composites is investigated. 3D sandwich fabrics are produced by a velvet weaving technique. It allows relatively easy production of delamination-resistant sandwich panels, compared to more traditional sandwich structures with honeycomb or foam cores. In this paper, the 3D sandwich fabrics made of glass fibre are impregnated with epoxy resin while the empty core can be foamed up with PUR to improve shear resistance of the panels. Core properties and static bending strength of the panels were evaluated. Three-point bending (3PB) fatigue tests were performed on four different materials (two unfoamed epoxy panels and two PUR foamed epoxy panels) with varying static properties. After fatigue testing, the Wöhler curves for each panel were determined. The relation between the observed damage development, the property degradation during bending fatigue and the static properties of the panels was investigated.

The main objective of this paper is to investigate the behavior of [0/±45/90]s woven FRP composites under tension, bending, and combined bending/tension loading conditions. First, the mechanical properties of the composite were determined experimentally using the ASTM testing standards. Bending properties were determined using 3-point and 4-point bending tests. The results showed that the woven composites performed better under bending loading than under tension loading. Finally, special test fixtures were designed to facilitate the study of the effect of the combined bending/tension loading. The bending moments were applied using offset shims of various thicknesses placed between the plane of the specimen and the loading axis. At the beginning, the load–strain diagrams at the specimen center showed the domination of bending strains, tension on one surface and compression on the other. With the advance of the loading process, the tension strain dominated and the strain on both sides were almost equal. The failure under combined bending/tension loading was due to the high stresses near the fixture. However, in pure bending, the material failed at the center because of the excessive delamination on the compressive side.

A comprehensive numerical investigation was carried out to evaluate the response and energy absorbing capacity of hybrid composite tubes made of unidirectional pultruded tube over wrapped with ±45° braided fiber-reinforced plastic (FRP). The numerical simulation characterized the crushing behaviors of these tubes subject to both quasi-static compression and axial dynamic impact loadings. Two types of braided FRP, glass and carbon fibers, were considered. Parametric studies were also conducted to examine the influence of the tube’s length, thickness and type of braid, as well as the loading conditions on the crushing behavior of the tubes.It was observed that although the pultruded tube, which had the highest stiffness among the tubes considered, produced the highest magnitude of energy absorption capacity, it exhibited a non-symmetric failure mode followed by a longitudinal splitting. On the other hand, the hybrid tubes exhibited a more desirable, accordion type failure mode, therefore they are considered to be more suitable candidates as energy absorbing structural members in service conditions.

In this study, the fatigue behavior of (±55°)3 filament wound composite pipes with a surface crack under alternating internal pressure has been investigated. Glass reinforced plastic (GRP) pipes were made of E-glass/epoxy and tested in an open-ended condition. For this study, a PLC controlled hydraulic test stand has been established. Test specimens have antisymmetric six layers which have ±55° winding angles. Fatigue tests of the pipes with a surface crack which have notch aspect ratio a/c = 0.2 and notch-to-thickness ratios a/t = 0.25, 0.38 and 0.50 in the axial direction have been carried out in accordance with ASTM D-2992. This standard allows a frequency of 25 cycles per minute and an R = 0.05 stress ratio. Tests have been performed at three different maximum stress levels, which were 50%, 40% and 30% of the ultimate hoop stress. Final failure of the GRP pipes has been examined and fatigue test results are presented by means of (S–N) curves and delamination damage zone area–cycle (A–N) curves.

In the present study, investigations on the ballistic impact behaviour of two-dimensional woven fabric composites has been presented. Ballistic impact behaviour of plain weave E-glass/epoxy and twill weave T300 carbon/epoxy composites has been compared. The analytical method presented is based on our earlier work. Different damage and energy absorbing mechanisms during ballistic impact have been identified. These are: cone formation on the back face of the target, tensile failure of primary yarns, deformation of secondary yarns, delamination, matrix cracking, shear plugging and friction during penetration. Analytical formulation has been presented for each energy absorbing mechanism. Energy absorbed during each time interval and the corresponding reduction in velocity of the projectile has been determined. The solution is based on the target material properties at high strain rate and the geometry and the projectile parameters. Using the analytical formulation, ballistic limit, contact duration at ballistic limit, surface radius of the cone formed and the radius of the damaged zone have been predicted for typical woven fabric composites.

A validated simulation methodology has been developed to support the bird-strike certification of the carbon fibre epoxy composite, moveable trailing edge (MTE) of the Boeing 787 Dreamliner. The explicit finite element software PAM-CRASH™ was selected to perform the simulations utilising the advanced composite material, fastener and smooth particle hydrodynamic bird models available in the code. The modelling procedures were validated firstly through comparison with existing test data and secondly through the testing and analysis of representative structures. Subsequent use of the validated modelling procedures during the analysis of the MTE facilitated the evaluation of numerous bird-strike scenarios, leading to improved design efficiency and safety, while significantly reducing certification costs.

A computationally efficient numerical method is described for predicting the details of damage growth in cross-plied and angle-plied fiber composite laminates. Separate two-dimensional plane stress meshes are used to represent each layer. Element boundaries are aligned along primary crack paths (parallel to fibers). Element corners in separate layers which share the same planar coordinates are attached to a single through-thickness node. Cracking of a layer parallel to fibers is modeled by combinations of releasing element nodal connections in the layer, creating new nodes, and/or modifying element elastic properties, depending on whether or not the layer is attached to or delaminated from one or both of its neighbors. Delaminations are modeled by releasing nodal connections between layers and creating new nodes. The essential three-dimensional character of the process, which is necessary to incorporate the effects of stacking sequence, is maintained by casting mixed-mode delamination fracture mechanics in terms of interlaminar nodal forces, and including adjacent layer constraint on the fracture mechanics of cracks in layers. Choice of element size is important to the method and has resulted in simplifications to crack propagation and fiber fracture prediction methodologies. Results to-date of predicted static damage growth in notched cross-plied carbon composites are described.

A [90/0]s orthotropic composite laminate with part-through matrix cracks is considered. Stress intensity factors are determined for the cracks using a linear-elastic analysis. These matrix cracks run along the fiber direction of the individual plies. The crack-geometry considered here is one where the matrix cracks in adjacent plies form a cross-like pattern in the plan view of the laminate. The plies are assumed bonded by thin resin-rich adhesive layers. These adhesive layers are modeled as distributed shear springs. Each ply of the laminate is modeled as a thin elastic orthotropic layer under plane stress. The laminate is subject to both tensile and shear loading. The mathematical model for the stresses and displacements in the layers reduces to a pair of Fredholm integral equations which are solved numerically. The stress intensity factors show a strong dependence on crack-sizes and nature of loading. In particular, the magnitudes of the stress intensity factors for the matrix crack in the 0° layer are increased significantly by the crack in the adjacent 90° layer.

The paper focuses on the behaviour of 0/90 plates subjected to thermal stress and free from mechanical loads. Curvatures arising from the temperature differential are regarded as functions of the relative thickness of the 90° ply, e90: it is found that curvatures are maximum for a certain value of e90, the position and the absolute value of such maximum depend on the elastic and thermoelastic properties of the ply and their invariant quantities. More precisely, the position of the maximum is affected by the elastic properties only, the magnitude of the maximum is affected very little by the elastic properties and is driven by the thermoelastic properties, i.e., the coefficients of thermal expansion of the ply. This paper represents a first effort towards the complete identification of laminates which maximise the out-of-plane deflections, i.e., curvatures, under hygro-thermal loads: the problem is posed in a general fashion, foreseeing further developments.

In previous research, a series of a thickness-tapered cruciform specimen configurations have been used to determine the biaxial (two-dimensional, in-plane) and triaxial (three-dimensional) strength of several carbon/epoxy and glass/vinyl-ester laminate configurations. Refinements to the cruciform geometry have been shown capable of producing acceptable results for cross-ply laminate configurations. However, the presence of a biaxial strengthening effect in quasi-isotropic, [(0N/90N/ ± 45N)M]S, laminates have brought into question whether the cruciform geometry could be used to successfully generate two-dimensional strength envelopes. In the present study, a two-dimensional failure envelope for a IM7/977-2 carbon/epoxy laminate was developed at the Air Force Research Laboratory, Space Vehicles Directorate, using a triaxial test facility. The electromechanical test frame is capable of generating any combination of tensile or compressive stresses in σ1:σ2:σ3 stress space and can evaluate the uniaxial (one-dimensional, in-plane), biaxial or triaxial response of composite materials. Results are promising as they indicated that failure in the majority of the IM7/977-2 specimens occurred in the gage section. This leads the authors to believe that maximum biaxial stress states were correctly generated within the test specimen. In addition to the experimental data presented, multi-continuum theory (MCT) was used to predict and analyze the onset of damage and ultimate failure of a biaxially loaded IM7/977-2 laminate. Multi-continuum theory is a micromechanics based theory and associated numerical algorithm for extracting, virtually without a time penalty, the stress and strain fields for a composites’ constituents during a routine structural finite element analysis. Damage in a composite material typically begins at the constituent level and may, in fact, be limited to only one constituent in some situations. An accurate prediction of constituent failure at sampling points throughout the laminate provides a genesis for progressively analyzing damage propagation in a composite specimen allowing identification of intermediate damage modes. A constituent-based, quadratic, stress-interactive, failure criterion was used to take advantage of the micro-scale information provided by MCT. There was reasonable correlation between analytically and experimentally developed IM7/977-2 2D failure envelope which leads us to believe that the thickness-tapered cruciform specimen can be used to determine the biaxial strength of quasi-isotropic laminates.

The scope of this paper was to establish a correlation between the damage occurring in a composite as a consequence of low-velocity impact and the energy dissipated during the impact phenomenon. To this aim, instrumented impact tests were carried out on glass fabric/epoxy laminates of three different thicknesses, using different energy levels. The irreversibly absorbed energy was obtained from the force–displacement curves provided by the impact machine. To assess damage progression as a function of impact energy, ply-by-ply delamination and fibre breakages revealed by destructive tests were measured. A previous model, based on energy balance considerations, was applied to interpret the experimental results, together with an original method of data reduction, allowing for the isolation of the maximum energy portion due to indentation and vibrational effects. From the results obtained, the contribution of fibre breakage and matrix damage to the irreversibly absorbed energy is comparable at low impact energies; with increasing initial energy levels, delamination becomes predominant in determining energy dissipation. However, the critical energy-release rate required to propagate delamination, as calculated from impact data, is considerably higher than the typical values deriving from Mode II delamination tests performed under laboratory conditions.

Moving from a validated finite element model of composite cylindrical absorbers, this work aims to optimise the shape of conical absorbers with elliptical cross-sections considering simultaneously different impact conditions. Since the use of non-linear finite element analyses to directly evaluate objectives and constraints during the optimisations would be unaffordable from a computational standpoint, a global approximation strategy is used. The crash capabilities of the absorbers are approximated with a system of Radial Basis Functions built by means of a minimum number of finite element analyses. The response surfaces are coupled with Genetic Algorithms to perform both constrained single- and multi-objective optimisations. The results prove that moderate eccentricity and conicity lead to high efficiency structures characterised by stable crush fronts and good absorption capabilities with also associated mass reduction up to the 7% considering vertical impacts and at least of the 20% considering 20° impacts with respect to ideal cylinders.

The behaviour of core and core-less composite elliptical thin walled tubes subjected to quasi-static axial crushing is examined experimentally. The core-less tubes have two different arrangement systems (single and double). For the core tubes, natural cellular fibre was used as filler. Experiments showed that the crushing behaviours of the core-less-tubes were found to be sensitive to their ellipticity ratio. Catastrophic failure mode is eradicated by both arrangement system and the existence of the core, while the elastic energy is significantly suppressed by the existence of the core. The presence of core have shown significant enhancement in tubes damage tolerance and energy absorption per unit volume compared to the core-less tubes.

In this paper, an innovative lightweight composite energy-absorbing keel beam system has been developed to be retrofitted in aircraft and helicopter in order to improve their crashworthiness performance. The developed system consists of everting stringer and keel beam. The sub-floor stringers were designed as everting stringer to guide and control the failure mechanisms at pre-crush and post-crush failure stages of composite keel beam webs and core. Polyurethane foam was employed to fill the core of the beam to eliminate any hypothesis of global buckling. Quasi-static axial crushing behaviour of the composite keel beam is investigated experimentally. The results showed that the crushing behaviour of the developed system is found to be sensitive to the change in keel beam web thickness. Laminate sequence has a significant influence on the failure mode types, average crush loads and energy absorption capability of composite keel beam. The desired energy absorbing mechanism revealed that the innovated system can be used for aircraft and helicopter and meet the requirements, together with substantial weight saving.

The quasi-static crushing response of carbon epoxy composite hat-shaped crush elements is described herein. A steeple-type triggering mechanism was used to ensure the specimens exhibited a continuous stable crushing mode of failure. The explicit finite element software PAM-CRASH was used to predict the crushing failure of these energy absorbing elements. A four-layer, stacked-shell model of the composite hat-shaped element, after calibration against experimental test data, was found to be capable of closely approximating the failure modes and provide agreement with the load vs. displacement behaviour observed during the experiments. The predicted steady state load and specific energy absorption were respectively within 1.5 and 0.2% of the experimental average. With further validation, the developed stacked-shell methodology could help provide a predictive tool to characterise the energy absorption of open section crush elements and significantly reduce the cost associated with an extensive experimental material characterisation test program.

The fiber-reinforced composite materials have been advanced to provide excellent mechanical and electromagnetic properties. The radar absorbing structure (RAS) is such an example that satisfies both radar absorbing property and structural characteristics. The absorbing efficiency of RAS can be obtained from selected materials having special absorptive properties and structural characteristics such as multi-layer and stacking sequence.In this research, to develop a RAS, three-phase composites consisted of {glass fiber}/{epoxy}/{nano size carbon materials} were fabricated, and their radar absorbing efficiency was measured on the X-band frequency range (8–12 GHz). Although some of GFR (Glass Fiber–Reinforced)-nano composites showed outstanding absorbing efficiency, during their manufacturing process, undesired thermal deformation (so called spring-back) was produced. The main cause of spring-back is thought to be temperature drop from the cure temperature to the room temperature. In order to reduce spring-back, two types of hybrid composite shells were fabricated with {carbon/epoxy} and {glass/epoxy} composites. Their spring-back was measured by experiment and predicted by finite element analysis (ANSYS). To fabricate desired final geometry, a spring-back compensated mold was designed and manufactured. Using the mold, hybrid composite shells with good dimensional tolerance were fabricated.

Since the EM properties of fiber reinforced polymeric composites can be tailored effectively by adjusting its composition, they are plausible materials for fabricating the radar absorbing structure (RAS) of desired performance. In this study, the composite RAS which has superior load bearing capacity and EM absorption characteristics has been developed by blending the conductive carbon black with the binder matrix of the E-glass/polyester composite, and its EM absorption characteristics has been measured by the free space method in the X-band frequency range (8.2–12.4 GHz). The composite RAS was designed so as to have the optimal performance for the X-band centered at 10 GHz. From the investigation, it was found that the composite RAS of 2.93 mm thickness with the conductive carbon black absorbed more than 90% of incident EM wave throughout the entire X-band frequency range.

Interlaminar fracture toughness of composite materials plays an important role in the specific energy absorption (SEA) characteristics of crushing composite materials. In this regard the effect of fibre orientation and stacking sequence on the composite crash box design is sought by studying their effects on the interlaminar fracture toughness. In order to achieve this, glass fibre/epoxy orientations of [±60]10, [02/±45]5, [0/90]10 and [0/90]5S were studied experimentally. Tensile, shear, double cantilever beam (DCB) and axial crush box specimens from different lay-ups were made and tested under quasi-static conditions to determine the mechanical properties, interlaminar fracture toughness (GIC) and SEA. It was shown that the interlaminar fracture toughness of glass fibre/epoxy affects the frond bending resistance due to the main central interwall crack in a progressive crushing failure and consequently SEA. A higher interlaminar fracture toughness between laminates will enhance the SEA in the axial crushing of the composite box.

In this paper, energy absorption capability of axial crush and bending collapse of aluminum/GFRP hybrid tubes were investigated. Glass fiber–epoxy composite prepregs were wrapped around an aluminum tube and then cured completely in the autoclave under the recommended cure cycle. Bonding process between composite and aluminum tubes was performed by excess resin extracted from the composite tube during curing process. For comparing energy absorption characteristics of the hybrid tube with those of pure aluminum and composite tubes, tests were performed using specimens made of an aluminum alloy and a composite material, respectively.Failure mechanisms of the hybrid tube under the axial compressive load and the bending load were experimentally investigated. For calculating energy absorption capability of axial crush and bending collapse behaviors of the hybrid tube, the modified plastic hinge collapse model and the modified Kecman's model for hybrid tube were suggested, respectively. Two suggested models for the hybrid tube showed a good agreement with the experimental results.

To determine certain physical properties, viz. the thickness swelling and water absorption, and mechanical properties, viz. the tensile strength and Izod impact strength, of lignocellulosic filler reinforced polyolefin bio-composites, polyolefin was used as the matrix polymer and rice-husk flour as the reinforcing filler. Wood flour was also used as a reinforcing filler, and commercial particleboard, medium-density fiberboard and solid woods (red pine and birch) were also included in this study, in order to obtain comparative water absorption behavior measurements. Test samples were prepared, in order to determine the physical and mechanical properties of the bio-composites as a function of filler loading and according to filler type as well as with respect to the thermoplastic polymer itself. The thickness swelling and water absorption of the bio-composites slightly increased as the filler loading increased, but to a negligible extent as compared with the wood-based composites (particleboard and fiberboard) and the solid woods (red pine and birch). The mechanical properties of the composites decreased as the filler loading increased, but the composites had an acceptable strength level. It was concluded that these bio-composites are suitable to be used for the interior of bathrooms, wood decks, food packaging, etc.

Composite body structures are now commonly used in road and rail vehicles, ships and submarines, aircraft and spacecraft due to their capability to effectively absorb high kinetic energy to weight ratio. One such structure designed as an energy device with pre-determined properties is a braided pultruded process (BPP) composite rod of either circular or square cross-section.This paper reports the results of an investigation on circular BPP rods and unidirectional pultruded process rods in epoxy matrix subjected to compressive loading. Test results depict BPP rods to have superior properties in comparison to the unidirectional rods in terms of energy absorption capability that is manifested through well-defined progressive crushing failure mechanisms. Generally the rods' fracture and complete failure mechanisms show distinct creation of buckling zone, followed by generation of fronds as the wedge area increases with every augmentation of applied load. Fracture morphology related to overall performance characteristics is discussed through the step-by-step analysis of microphotography. The specific energy absorption property is shown to be best achieved in carbon/carbon (C/C) BPP followed by glass/carbon (G/C) rod combination and then the glass/glass (G/G) BPP rods. The latter (G/G), although worst performer of all the rods in terms of energy characteristics, still outperforms the documented best tubes made of Kevlar fibres, steel and aluminium. On average, the carbon/carbon (C/C) BPP rod's specific energy absorption is between 35% and 55% more than the nearest comparable tubes.

In the present study, the impact energy absorption characteristics of glass fiber-reinforced hybrid composites were investigated by the instrumented Charpy impact test method with respect to the volume fraction of different materials embedded. Also, the interlaminar shear properties were measured by the short beam shear test to investigate the correlation between the interlaminar shear properties and the impact energy absorption characteristics. To predict the impact absorption characteristics of glass fiber hybrid composites, the progressive impact fracture model was proposed.

Mode-I and Mode-II interlaminar crack growth affect the failure modes of the progressive crushing of composite box structures. These failure modes which are known as lamina bending, brittle fracture, transverse shearing and local buckling contribute to specific energy absorption (SEA) of composite box. In this regard, the effect of laminate lay-up of the composite crush box was sought by studying their effects on Mode-I and Mode-II interlaminar fracture toughness. The double cantilever beam (DCB), three-point-end-notched flexure (3ENF) and axial crush box specimens were fabricated from carbon/epoxy twill–weave fabrics of [0]4, [45]4 and [0/45]2 and they were tested under quasi-static condition to determine the interlaminar fracture toughness in Mode-I (GIC), Mode-II (GIIC) and SEA of each lay-up. It was shown that interlaminar crack propagation in Mode-I and Mode-II contributes significantly on the type of the progressive crushing mode and SEA. The interfaces of 0/45 and 0/0 have higher Mode-I and Mode-II interlaminar fracture toughness and as a result the crushed box with these lay-ups showed a higher energy absorption capability in comparison with crush box lay-up of [45]4. An analytical solution was proposed to predict the mean crushing force for each failure mode. The crushing process of composite boxes was also simulated by finite element software LS-DYNA and the results were verified with the relevant experimental results.

The tensile and compressive tests of glass–epoxy composites with 1–200 s−1 strain rates which are typical strain rate range during automobile crash accidents were performed in order to measure the strength variation with respect to strain rate. The tests were performed using both a horizontal type pneumatic impact tester and a conventional dynamic universal test machine with strain-rate-increase mechanisms. Also, the impact energy absorption characteristics of glass fiber reinforced composites were estimated using the newly proposed progressive impact fracture model. From the experiments and predictions, it was found that the proposed method predicted relatively well the experimental results.

This paper describes an experimental investigation into the energy absorption properties of a foam-cored sandwich panel with integral fibre-reinforced plastic (FRP) tubes and frusta. The panels were tested under quasi-static flatwise compression and a number of different insert geometries were examined. By using X-ray analysis it was found that those panels with inserts which failed by stable progressive brittle fracture exhibited-the best specific energy absorptions. Inserts which failed catastrophically led to much lower values. Conical inserts were found to offer the most repeatable performance as their geometry assisted in ensuring consistency of manufacture.

Novel aluminised E-glass fibre reinforced unsaturated polyester composites, originally formulated for enhanced thermal and electrical shielding properties were evaluated in terms of their water absorption. One of the major obstacles delaying the acceptance of novel composites in engineering applications is the degradation of the polymer matrix material by moisture, which effects the physical and mechanical performance over time. The objective of this study was to characterise and quantify the degree of water absorption of novel aluminised E-glass reinforced unsaturated polyester composites. Aluminised E-glass composites were compared alongside their unmetallised E-glass counterparts. Two sets of temperature were used for this study. Results show that aluminised E-glass significantly reduces the saturation point compared to unmetallised E-glass. The differences between aluminised and unmetallised are correlated to fibre coatings. At elevated temperatures the aluminised E-glass sample is unstable and exhibits significantly higher water absorption indicating that a new failure mechanism is occurring.