Composites Part B Engineering

Published by Elsevier
Print ISSN: 1359-8368
Publications
A new type of biocomposite of nano aptite (NAP)/poly(1,4-phenylene sulfide)–poly (2,4-phenylene sulfide acid) (PPS–PPSA) copolymer (NAP/PPS–PPSA) was prepared by polycondensation in 1-methyl-2-pyrrolidone (NMP). The NAP particles with a size of about 40–60 nm in diameter and 60–80 nm in length uniformly distributed in the composite. The presence of 2,4-phenylene sulfide acid in copolymer increased the copolymer affinity to NAP. Duo to some strong combinations of calcium ion (Ca2+), carboxyl (–COO−) and phosphate radicals ion () in the composite, interfacial chemical binding exists between copolymer and NAP. The NAP/PPS–PPSA biocomposite not only had good homogeneity but also had high NAP content of 60%, which was a potential bioactive material to be used as load-bearing implants or fixation in bone repair.
 
Drop test experiments have been performed with a FRP sandwich panel instrumented with a network of 16 fibre optic (FO) Bragg strain sensors, together with conventional electrical strain gauges for control and verification. The drop tests simulate slamming loads on the wet deck of a surface effect ship (SES). The objectives were to show the possibility of using a network of FO sensors to monitor strain during a slamming impact, and to test out a technique for signal processing. The strain measurements provided both peak strain data and served as a base for frequency analysis. The results showed that the FO strain sensors performed satisfactorily and were in general agreement with the conventional strain gauges used. The FO interrogation system was, however, not designed with sufficiently large dynamic range for the most extreme drop sequences. The peak strain in the panel was found to increase almost proportionally with the drop velocity, or drop height, and the wet fundamental frequency increased with increasing drop angle. Furthermore, the frequency decreased with increasing drop velocity.
 
The effect of strain rate on the compressive and shear behavior of carbon/epoxy composite materials was investigated. Strain rate behavior of composites with fiber waviness was also studied. Falling weight impact system and servohydraulic testing machine were used for dynamic characterisation of composite materials in compression at strain rates up to several hundred per second. Strain rates below 10 s−1 were generated using a hydraulic testing machine. Strain rates above 10 s−1 were generated using the drop tower apparatus developed. Seventy-two-ply unidirectional carbon/epoxy laminates (IM6G/3501-6) loaded in the longitudinal and transverse directions and [(08/908)2/0̄8]s crossply laminates were characterised. Off-axis (30 and 45°) compression tests of the same unidirectional material were also conducted to obtain the in-plane shear stress–strain behavior. The 90° properties, which are governed by the matrix, show an increase in modulus and strength over the static values but no significant change in ultimate strain. The shear stress–strain behavior, which is also matrix-dominated, shows high nonlinearity with a plateau region at a stress level that increases significantly with increasing strain rate. The 0° and crossply laminates show higher strength and strain values as the strain rate increases, whereas the modulus increases only slightly over the static value. The increase in strength and ultimate strain observed may be related to the shear behavior of the composite and the change in failure modes. In all cases the dynamic stress–strain curves stiffen as the strain rate increases. The stiffening is lowest in the longitudinal case and highest in the transverse and shear cases. Unidirectional and crossply specimens with fiber waviness were fabricated and tested. It is shown that, with severe fiber waviness, strong nonlinearity occurs in the stress–strain curves due to fiber waviness with significant stiffening as the strain rate increases.
 
The analytical and numerical foundations are laid out for the formulation of the boundary element method (BEM) for plane piezoelectric solids. We extend a physical interpretation of Somigliana's identity to piezoelectricity and give a direct formulation of the BEM in terms of the continuous distributions of point forces/charges and displacement/electric potential discontinuities in the infinite piezoelectric domain. We adopt Stroh's complex variable formalism for piezoelectricity to derive the point force/charge and the displacement/electric potential discontinuity, their dipoles and continuous distributions systematically. The duality relations between the force/charge and the displacement/electric potential solutions, embedded in the Stroh formalism, are exploited as the foundations for the analytic and the numerical approaches to the piezoelectric boundary value problems in two dimensions. These approaches enable us to solve important problems of piezoelectricity with arbitrary geometry and composition.
 
The delamination process in thin composite plates subjected to low-velocity impact is simulated using a specially developed 2D cohesive/volumetric finite element scheme. Cohesive elements are introduced along the boundaries of the inner layers and inside the transverse plies to simulate the spontaneous initiation and propagation of transverse matrix cracks and delamination fronts. The analysis is performed within the framework of the finite deformation theory of elasticity to account for the nonlinear stiffening of the thin composite plate and the large rotations which accompany the fracture process. The simulation is dynamic and uses an explicit time stepping scheme. Comparison with existing experiments performed on graphite/epoxy laminates indicates that the cohesive/volumetric finite element scheme is able to capture the complex mechanisms leading to the delamination, including the initial micro-cracking of the matrix, the appearance of critical transverse matrix cracks and the rapid propagation of delamination cracks initiated at the intersections between the critical matrix cracks and the adjacent plies.
 
Numerical solutions obtained by the meshless local Petrov-Galerkin (MLPG) method are presented for 2D functionally graded solids, which is subjected to either mechanical or thermal loads. The MLPG method is a truly meshless approach, as it does not need any background mesh for integration in the weak form. In this MLPG analysis, the penalty method is used to efficiently enforce the essential boundary conditions, and the test function is chosen to equal the weight function of the moving least squares approximation. Two types of material gradations are considered; one is based on the continuum model in which material properties are assumed to be analytical functions (e.g. exponential or power law variation of material properties) and the other is based on the micromechanics model in which the effective material properties are determined by either the Mori–Tanaka or self-consistent scheme. Examples are given for different types of 2D structural components made of the functionally graded materials, namely, the link bar, circular cylinder and simply supported beam. Results obtained from the MLPG method are validated by available analytical and numerical solutions. Different profiles of non-homogeneity of material constituents are also investigated to assess the response of 2D functionally graded solids.
 
This paper proposes a new analytical solution to predict the elastic 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. Four sets of experimental data pertinent to four kinds of 2D orthogonal PWF composites have been implemented to validate the new model. The calculations from the new model are also compared with those by using four models in the earlier literature. It is shown that the experimental results correlate well with predictions from the new model.
 
{3,2} and {1,2} theory correction factors
A higher-order bending theory is derived for laminated composite and sandwich beams thus extending the recent {1,2}-order theory to include third-order axial effects without introducing additional kinematic variables. The present theory is of order {3,2} and includes both transverse shear and transverse normal deformations. A closed-form solution to the cylindrical bending problem is derived and compared with the corresponding exact elasticity solution. The numerical comparisons are focused on the most challenging material systems and beam aspect ratios which include moderate-to-thick unsymmetric composite and sandwich laminates. Advantages and limitations of the theory are discussed.
 
An area of interest in the automated manufacture of composite components is the prediction in real-time of the deformed shape of a textile reinforcement in 3D space during robotic handling operations. The deformed shape can be used to guide robotic end-effectors to ensure accurate fabric placement and avoid collisions. In this paper, a nonlinear mathematical model using large deflection plate and shell theories is presented. The model is able to predict the 3D deformed shape of limp sheet materials being picked-up by multi robotic grippers for three boundary conditions. The main factors affecting the deformation behaviour of the sheet during the operation are identified and analysed, and the contributions of different energies during deformation are presented in detail. Good agreement is obtained when comparing the solutions of the model with FE simulation results. This study demonstrates the possibility of developing a modelling capability for material on-line response in automatic flexible material handling.
 
A meshless collocation (MC) and an element-free Galerkin (EFG) method, using the differential reproducing kernel (DRK) interpolation, are developed for the approximate three-dimensional (3D) analysis of simply supported, multilayered composite and functionally graded material (FGM) circular hollow cylinders under mechanical loads. The strong and weak formulations of this 3D elasticity problem are derived on the basis of the Reissner mixed variational theorem (RMVT). The former consists of the Euler–Lagrange equations of this problem and its associated boundary conditions, while the latter represents a weighted residual integral in which the differentiation is equally distributed among the primary field variables and their variations. An earlier proposed DRK interpolation is used to construct the primary field variables where the Kronecker delta properties are satisfied, and the boundary and continuity conditions related to the primary variables themselves can be directly applied. The system equations of both the RMVT-based MC and EFG methods are obtained using these strong and weak formulations, respectively, in combination with the DRK interpolation. In the illustrative examples, the accuracy and convergence rate of the present MC and EFG methods are examined, and some guidance for using these methods is suggested.
 
By using an adjacent-layer interlocking method on a weaving machine, multi-layer preform structures are developed. The on-loom interlocking method eliminates the yarn breakage resulting from needle penetration which is the case for off-loom interlocking of fabric layers. The concept of this three-dimensional (3D) fabric design is to bind each pair of adjacent layers at one connecting point in every other plain-weave repeat within each layer. The mechanical properties of the resulting composites are investigated by means of impact, short-beam shear and the long-beam flexural testing. The failure mechanisms found in 3D on-loom interlocked composites include fiber breakage, fiber debonding and fiber pull-out.
 
A qualitative analysis of experimental results from small caliber ballistic impact and dynamic indentation on a 3D glass fiber reinforced composite are presented. Microscopic analysis of the damaged specimens revealed that the current 3D weaving scheme creates inherently two weak planes which act as potential sites for delamination in the above experiments. It is concluded that while the z-yarns may be effective in limiting the delamination damage at low loads and at low rates of impact, at high loads and high loading rates delamination continues to be the dominant failure mode in 3D woven composites. It is shown that dynamic indentation can be used to capture the progression of damage during impact of 3D woven composites.
 
The creep properties of AS-4/PEEK [ ± 45]4s laminated composites have been investigated. The temperature dependence of the viscoelastic behavior of the matrix composite and the bulk resin was studied by dynamic mechanical analysis. The tensile properties were measured at elevated temperature. The tensile failure morphology at elevated temperature was investigated by scanning electron microscopy. The accelerated characterization of the creep response due to the effect of temperature and stress was studied. Short-term creep tests were performed at various stress levels and at elevated temperatures. The creep strain data were fitted by using the Findley equation. It was found that results predicted by the Findley equation showed good agreement with the experimental data. Coupling with the Findley equation, a master curve was constructed by using the time-temperature-stress superposition principle (TTSSP) with a reference condition at 85°C and 94.38 MPa. From the short-term creep test obtained after 600 min, a smooth master curve was obtained by the proposed procedure of accelerated characterization and a creep strain of 106.29 min can be predicted.
 
The primary aim of this paper is to present results describing in detail the behaviour of ±45° E-glass/MY750 (GRP) tubes, of various wall thicknesses, subjected to equal biaxial tension–compression loading, generated under combined internal pressure and axial compression. The role played by the non-linear lamina shear has also been assessed by comparing various shear stress–strain curves for embedded laminae (extracted from tests on ±45° tubes subjected to circumferential: axial stress ratios SR=1:0, 1:−1 and 2.3:−1) with that of an ‘isolated’ lamina (measured from torsion of 90° tubes). Extracted shear failure strains, for embedded laminae, were more than four fold larger than those measured at ultimate failure for an ‘isolated’ lamina. Soft characteristics were observed in the embedded lamina and these were believed to be due to interaction between early matrix damage initiation (and propagation) and shear. Factors affecting the behaviour of the tubes, such as bulging, scissoring, thermal stresses and stress variation through the thickness are discussed.
 
Notch fatigue strengthening under different cyclic stress levels and elapsed number of cycles has been studied in [0/90]4S AS4/PEEK laminates. Quick and extensive 0° fiber splitting and the corresponding 90° fiber shear off were found to be the underlying causes of stress concentration alleviation. This effectively raised the residual strength of the notched laminates and increased their fatigue lives to beyond one million cycles. On the other hand, re-consolidation of fatigued specimens removed most of the internal damages and in the meantime reverses the above strengthening. Detailed study of the residual strength changes and damage development history using re-consolidation lent support to the above deductions on the notch fatigue strengthening phenomenon in [0/90]4S AS/PEEK laminate.
 
The purpose of this study is to demonstrate that the measurement of elastic constants, especially the longitudinal Young's modulus, by frequency measuring, can be used as a non-destructive method to evaluate the damage in a Ti-6A1-4V/SiC metal matrix composite. Therefore, the composite was thermally cycled between 200 and 700°C, which was found to induce a decrease in the longitudinal Young's modulus by 20%. By comparison with mechanically tested samples, this decrease is shown to correspond to a point where the failure stress drops dramatically. Thus, this non-destructive method has proved successful to follow and evaluate the damage evolution. It could be applied to concrete situations such as evaluating the damage in composite components.
 
It is an important issue not only for military purposes but also from commercial point of view, whether or not the reflective wave from an incident electromagnetic wave can be nullified. In this research, by blending conductive carbon black with the binder matrix of glass/epoxy composite, a radar absorbing structure (RAS) which can support loads and absorb the electromagnetic wave has been designed. The RAS was specially designed so as to exhibit the optimum absorptivity for X-band of (8.2–12 GHz) frequency ranges, centered at 10 GHz. Its absorbing characteristics were also investigated. Unlike other existing measurements of reflection loss using a waveguide, the reflection loss within an anechoic chamber simulating the radar cross section method of measurement was used with a plane-wave shaped incident wave. An optimized multi-layer design of RAS using the adopted material system is described in this research.
 
The energy absorption characteristics of E-glass/Polypropylene (PP) isogrid composite panels under quasi-static transverse load conditions have been investigated. Experimental tests and finite element simulations were performed for isogrid composite panels in three-point bending boundary condition. Results of tests and simulations show that loading the panels on rib side results in greater specific energy absorption along with larger displacements compared to skin side loading which is more abrupt. These results can be used to advantage in weight-critical automotive side impact crashworthy applications. Vibration response measurements which were used as a non-destructive tool for evaluating the quality of as-manufactured isogrid plates and monitoring induced damage in one of the plates are also presented.
 
This experimental study investigated the combined effects of liquid (water, Skydrol, fuel, and dichloromethane) absorption, impact damage and drilling on aramid and carbon fibre/epoxy composites. The static and fatigue behaviour of the composite samples was determined after the treatments. The response to impacts was analysed, and elastic and absorbed energy were measured. The mechanism of moisture diffusion into the composites was studied and a method for the accelerated ageing of the composites applied. Penetrant radio-opaque dye and stereo-radiography were used to determine the onset and growth of damage during fatigue life and the decay of mechanical characteristics. Optical microscopy was used to investigate the microscopic mechanisms of absorption and damage, in order to propose interpretative models.
 
The failure of polymer matrix composites upon exposure to the environment has been assessed in the present study. In order to investigate the combined action of temperature, humidity and UV radiation on polymers and composites, an environmental ageing chamber has been constructed and tested. The accelerated environmental ageing was based on two kinds of alternating cycles, which provided humidity, temperature and ultraviolet radiation. The materials examined were isophthalic polyester and isophthalic polyester reinforced with a glass fiber random mat onto which glass fibers were knitted at 0°/90° (GFRPC) at a total volume fraction of approximately 20%. Dynamic mechanical analysis, for a range of temperatures and frequencies under tensile and three-point bending loadings, revealed that the aged materials gained in stiffness, whereas a small deterioration in strength was found. Scanning electron microscopy studies performed before and after environmental chamber conditioning revealed that some microcracks had occurred on the surface of the specimens. Nevertheless, the length of these microcracks was less than the critical value of 0.1 mm, required for crack propagation.
 
We have proposed the accelerated testing methodology for the long-term durability of polymer composites based on the time–temperature superposition principle (TTSP) to be held for the viscoelasticity of polymer matrix. In this paper, the prediction of long-term strength of CFRP laminates for innovative marine use under water absorption conditions were performed by our developed accelerated testing methodology. Three kinds of CFRP laminates employed were conventional plain fabric T300 carbon fibers/vinylester, flat yarn plain fabric T700 carbon fibers/vinylester and multi-axial knitted T700 carbon fibers/vinylester for innovative marine use as a lightweight CFRP ship structures. These CFRP laminates were prepared under three water absorption conditions of Dry, Wet and Wet + Dry after molding. The three-point bending constant strain rate (CSR) tests for three kinds of CFRP laminates at three conditions of water absorption were carried out at various temperatures and strain rates. As the results, the master curves of CSR strength for these CFRP laminates at three water absorption conditions are constructed by using the test data based on TTSP. Furthermore, the time–temperature–water absorption superposition principle (TTWSP) hold for the flexural CSR strengths for these CFRP laminates. Therefore, it is possible to predict the long-term strength for these CFRP laminates under an arbitrary temperature and water absorption conditions using the master curves. It is cleared from the master curves that the degradation rate of CSR strength of these CFRP laminates is determined only by increasing of time, temperature and water absorption and is independent upon the type and weave of carbon fibers.
 
An approximate superposition technique is proposed for calculating the stress fields around individual fibers in composite materials. The method uses the closed form solution for an isolated fiber to construct the local fields. The resulting local fields are validated by comparing with the results from a method based on singular integral equations. The maximum error is 4.3% when the fiber radius to the unit cell length ratio reaches 0.8, with the fiber to matrix modulus ratio up to 20. The associated volume averaged properties can be shown to be approximate upper and lower limits that have the unique feature of depending not only on the fiber volume fraction but also on the fiber packing.
 
Using a C0 eight-noded plate element developed based on an accurate higher-order theory, the nonlinear dynamics analysis of thick composite and sandwich plates are investigated. The formulation is based on a theory that accounts for the realistic variation of in-plane and transverse displacements through the thickness. It also includes the inertia terms pertaining to the higher-order terms involved in the displacement functions. The geometric nonlinearity is introduced in the formulation based on the relevant Green's strain vector for the laminate. The governing equations of motion obtained here are solved through eigenvalue solution for free vibration case whereas the direct integration technique is employed for the transient response analysis. The performance and the applicability of the proposed discrete model for the nonlinear free flexural and forced vibration responses of thick laminates are discussed among alternate models, considering multi-layered cross- and angle-ply, and sandwich plates.
 
Glass fibre-reinforced plastic beams were pre-fatigued in four-point bending up to selected portions of their fatigue life. The samples were then monotonically brought to failure while recording their acoustic emission response. The trend of the cumulative event counts was markedly affected by the specimen load history: for a given stress level, a higher activity was obtained from samples subjected to a higher number of cycles. A good empirical correlation was found between the material residual strength and the total event counts detected at maximum stress applied during pre-fatiguing cycles. Moreover, the correlation was improved when a previous model, relying on fracture mechanics concepts, was utilised. However, in both cases, a difficulty was experienced in evaluating the residual strength of the composite laminate after a small number of fatigue cycles. Under these conditions, the modulus decrease was seemingly more sensitive to the effects of fatigue than the acoustic emission response.
 
Interfacial evaluation and the durability of alkaline and silane treated Jute fibers/polypropylene (PP) composites were investigated by micromechanical test combined with the wettability and nondestructive acoustic emission (AE). After boiling water test, tensile strength and interfacial shear strength (IFSS) between Jute fibers and PP matrix decreased due to the deterioration of swelled fibrils by water infiltration and microfailure. The IFSS decrement of the untreated and treated Jute fibers/PP composites was different from each other, respectively. IFSS between silane treated Jute fiber and PP matrix was higher than the untreated or even alkaline treated cases. From the dynamic contact angle results, micromechanical IFSS was not always consistent with thermodynamic work of adhesion, Wa in the interface. Since hemicellulose and lignin could be removed from Jute fiber after boiling water test, Jute fiber surface became more hydrophilic and surface roughness increased. With water present, the work of adhesion did not only decrease but they were negative, which indicates the instability of the interfacial system. Microfailure pattern of boiled Jute fiber was obviously different from the untreated case based on monitored AE parameters. AE energy increased for the alkaline and silane treated Jute fibers/PP composites, whereas AE energy for all three cases decreased distinctly after boiling water test.
 
Sandwich composite static and fatigue testing results indicated the predominant failure to be the core damage followed by interfacial debonding, resin cracking and fiber rupture. Under static testing, crack was observed to initiate in the core and ensue planar propagation near the interface with the facesheets; whereas, onset of crack initiation in the facesheets served as a precursor to the catastrophic failure. Multiple failure initiation and propagation sites in the core and intermittent interfacial debonding were consistently observed under fatigue. An acoustic emission based stiffness reduction model is presented that seems to accurately identify the extent of damage in sandwich composites subjected to fatigue loading conditions.
 
Acousto-ultrasonics (AU) is a nondestructive evaluation (NDE) technique that utilizes two ultrasonic transducers to interrogate the condition of a test specimen. The sending transducer introduces an ultrasonic pulse at a point on the surface of the specimen while a receiving transducer detects the signal after it has passed through the material. The aim of the method is to correlate certain empirical parameters of the detected waveform to characteristics of the material between the two transducers. The waveform parameter of interest is the attenuation due to internal damping for which information is being garnered from the frequency domain. Here, the three parameters utilized to indirectly quantify the attenuation are the ultrasonic decay rate, the mean square value of the power spectrum, and the centroid of the power spectrum. The sensitivity for each of these AU parameters was assessed with respect to the damage state of two types of SiC/SiC ceramic matrix composites (CMC). The two composite systems both had Hi-Nicalon™ fibers with a carbon interface but had different matrix compositions that led to considerable differences in damage accumulation. Load/unload/reload tensile tests were performed and in situ AU measurements were made over the entire stress range. After analyzing the AU parameters, the overall sensitivity of the AU technique to material change or damage was quantified and shown to correlate well with the observed damage mechanisms for both material systems. In addition, the AU response was shown to be dependent on the stress state of the composites. This stress dependent behavior was observed while unloading the specimens from the maximum stress, thereby, maintaining a constant damage state.
 
The concept of functionally graded materials (FGMs) was proposed in 1984 by materials scientists in the Sendai area as a means of preparing thermal barrier materials. Continuous changes in the composition, microstructure, porosity, etc. of these materials results in gradients in such properties as mechanical strength and thermal conductivity. In 1987, a national project was initiated entitled ‘Research on the Basic Technology for the Development of Functionally Gradient Materials for Relaxation of Thermal Stress’. In 1992 when the project was finished, samples of 300 mm square shell and 50 mm diameter hemispherical bowls for SiC-C FGM nose cones were prepared. The concept of FGMs is of interest not only in the practical design of super refractory materials, but also in the development of various functional materials. In 1993, the second national project was initiated for the research and development of FGMs as functional materials; ‘Research on Energy Conversion Materials with Functionally Gradient Structure’. This program aims to apply functionally graded structure technology to the improvement of energy conversion efficiency. The project will continue until the fiscal year 1997.
 
In this paper, the modeling, numerical simulation and experimental validation of the deformation of a composite cantilever beam actuated by shape memory alloy (SMA) wires are presented and discussed. The structural model incorporates a number of non-classical features such as laminated construction and anisotropy of constituent material layers, transverse shear deformability, distortion of the normals, and fulfillment of interfacial shear traction continuity requirement. Suitable for use in standard finite element codes, a numerical procedure is developed for solving the geometric non-linearity of the host structure and the hysteretic non-linearity of SMA wires, which is based upon the updated Lagrangian formulation. The application concerns an elastomeric beam with embedded and pre-stressed SMA wires at an offset from the neutral axis, which act as large bending actuators resulting from the thermally induced reversible transformation strains. The experiments and numerical simulation demonstrate the good predictive capability of the model proposed and the powerful role played by SMAs as large bending actuators.
 
The advanced active twist rotor (AATR) blade incorporating single crystal macro fiber composite (single crystal MFC) actuators is designed and the aeroelastic analysis is performed. The AATR blade is designed based on an existing passive blade and the NASA/ARMY/MIT active twist rotor (ATR) prototype blade. The AATR blade is designed to satisfy all the requirements and the properties of the AATR blade are compared with those of the ATR prototype blade. The aeroelastic analysis of the designed AATR blade for the hover condition is conducted. In order to predict the vibration reduction capability, the twist actuation frequency response of the AATR blade is investigated by the numerical simulation. As a result, although lower input voltage is used, the AATR blade can achieve higher twist actuation that is sufficient to alleviate the helicopter vibration as compared with the ATR blades using the AFC and the standard MFC.
 
Piezoelectric actuators are usually mounted to the top and bottom surfaces of plates and may induce in-plane extension, bending and localized shear deformations at the structural element. The in-plane stresses may have a significant influence on the mechanical behavior of thin plates as initial and/or residual stresses affect the flexural stiffness and in turn the dynamic and stability characteristics of plates. In this work, the effect of the in-plane piezoelectric induced stresses on the natural frequencies of composite plates is numerically and experimentally investigated. A finite element formulation is presented for the analysis of laminated plates with an arbitrary number of piezoelectric actuators and/or sensors. Von Kàrmàn non-linear strain–displacement relations are used and ideal linear behavior is assumed for the piezoelectric actuation. The problem is decomposed into an in-plane problem where the strain field induced by the piezoelectric actuators is computed. The natural frequencies and vibration modes are then computed taking the stress stiffening effects of these piezoelectric stresses into account. A number of different configurations are numerically and experimentally analyzed to verify the proposed theory. The configurations use eight PZT actuators bonded to three layer glass fiber/epoxy plates. The plates are square and clamped along two opposing edges and free along the other two. Good agreement is obtained between the predicted and measured natural frequencies.
 
In order to evaluate closed loop performances of the composite plates with distributed piezoelectric actuators, it is essential to obtain more exact system parameters such as natural modes, damping ratios, and modal actuation forces. In this paper a refined analysis of composite plates with distributed piezoelectric actuators for vibration control has been performed. The in-plane displacements through the thickness have been modeled using the layerwise theory. This layerwise model can describe more refined strain distributions and has the capability of more realistic modeling of boundary conditions. The finite element method based on the developed mechanics has been formulated. The constitutive equations for piezoelectric materials have been used to determine piezoelectric actuation forces, and the modal strain energy method has been applied to analyze the damping capacity of the structures. Through the comparison of present results with those available, the accuracy of the present method was verified. The closed loop performances have been evaluated using the simple control algorithms. Through the comparison of present results with those based on shear deformation plate theory, it is concluded that the developed model can describe more realistic smart composite plates with distributed piezoelectric actuators.
 
A refined higher order laminate theory is developed to analyze smart materials, surface bonded or embedded, in composite laminates. The analysis uses a refined displacement field which accounts for transverse shear stresses through the thickness. All boundary conditions are satisfied at the free surfaces. Non-linearities are introduced through the strain dependent piezoelectric coupling coefficients and the assumed strain distribution through the thickness. The analysis is implemented using the finite element method. The procedure is computationally efficient and allows for a detailed investigation of both the local and global effects due to the presence of actuators. The finite element model is shown to agree well with published experimental results. Numerical examples are presented for composite laminates of various thicknesses and the results are compared with those obtained using classical laminate theory. The refined theory captures important higher order effects which are not modeled by the classical laminate theory, resulting in significant deviations.
 
The non-linear behavior of slightly crooked slender composite beams with piezoelectric actuators is addressed. Von Kármán non-linear strain–displacement relations and linear constitutive relations for both the piezoelectric and composite materials are used. The piezoelectric control of crooked beams subjected to axial compression renders its equilibrium path as close as possible to that of the ideal perfect beam. A modal analysis demonstrates that, through the application of suitable voltages to the actuators, the elimination of certain buckling mode contributions to the beam response is feasible and highly desirable. The equilibrium path of imperfect structures is shown to be dramatically changed via piezoelectric control; this has potential applications in the post-buckling of structures with negative slope of the secondary equilibrium path.
 
A concept for an automatic thermal regulation system using the changes in surface properties associated with the oxidation of silver and the reduction of silver oxide at a critical temperature has been evaluated. The change in thermal transfer properties between silver and silver oxide are favorable for thermal regulation if the incident radiation is in the infrared range. However, while the reduction of silver oxide occurs rapidly, the oxidation of silver occurs slowly, so a silver/silver-oxide composite coating will not likely produce reversible adaptive thermal regulation. However, thermal regulation at higher temperatures, where the oxidation and reduction reactions are more rapid, may be possible using other materials. The criteria for selection of potential metal/metal-oxide composite coatings for adaptive thermal regulation are discussed.
 
In resin transfer molding (RTM), the plates employed often consist of reinforcements with a variable number of plies and stacking sequences. A correct simulation of this process requires taking into account all these parameters. The present study relates to the simulation of the effect of thickness variation in the isothermal filling of the molds in RTM process. The approach is based on the control volume finite element method (CV/FEM) and volume of fluid (VOF) on an adaptive fixed mesh, where the numerical algorithm predicts the displacement of the flow front. For a discretization of the calculation domain, we developed a mesh generator to optimize and refine the mesh for the finite element analysis and to assure the accuracy of the simulation. The mesh generator allows to discretize the field in unstructured triangular elements with possibility of local refinement and inclusion of inserts. The developed numerical code aims at optimizing the molding parameters like the positioning of the points of injection and the vents and determining the optimal pressures of injection to minimize the cycle times. This simulation provided useful information for mold filling and can be used to design an optimum mold and make the RTM process more efficient.
 
This paper investigates the influences of additive contents and the additive ratio to dopants on the electrical characteristics of ZnO-based varistors. Bismuth and antimony are chosen as the additives while cobalt and manganese are selected as the dopants in this study. Our previous works discussed the influences of the initial additive content on the electrical characteristics of ZnO-based varistors without considering the dopant content and the weight loss during processing and sintering. Therefore, in this study, we fabricated varistors with same initial formula after sintering for 1, 3 and 5 h, respectively. The sintering temperatures were 950 and 1100 °C. After sintering, the additive content and dopant content of the varistors were measured using an inductively coupled plasma-atomic emission spectrometer (ICP). The experimental results showed that when the additive-content varies from 1.44 to 3.59 at % and the dopant/additive ratio changes from 0.16 to 0.69, the nonlinear coefficient, α, reaches up to 48 and the breakdown field Ebk is to 895 V mm−1. The average grain size is 2.7 μm. The α value is higher with the higher additive-content and the lower dopant/additive ratio and vice versa. The breakdown field Ebk is increased with the additive-content increasing and sintering temperature lowering at a given dopant-content. The grain size is increased with the increase in sintering temperature while the bismuth concentration decreased. The observed effects are related to the quality of grain boundaries and the conductivity of grains.
 
There have been a number of review papers on layered silicate and carbon nanotube reinforced polymer nanocomposites, in which the fillers have high aspect ratios. Particulate–polymer nanocomposites containing fillers with small aspect ratios are also an important class of polymer composites. However, they have been apparently overlooked. Thus, in this paper, detailed discussions on the effects of particle size, particle/matrix interface adhesion and particle loading on the stiffness, strength and toughness of such particulate–polymer composites are reviewed. To develop high performance particulate composites, it is necessary to have some basic understanding of the stiffening, strengthening and toughening mechanisms of these composites. A critical evaluation of published experimental results in comparison with theoretical models is given.
 
Three-dimensional stress analysis is performed for double-lap composite-to-composite adhesive bonded joint exposed to uniaxial extension. The submodeling approach using 27-node solid element available in the recent versions of abaqus is utilized. Principal objectives are: to explore computational advantages provided by the multi-step submodeling approach and perform a comprehensive numerical study of three-dimensional (3D) stress variations in the joint structure, considering adhesive layers as 3D elastic entities. Numerical results obtained from the “global” analysis show fast displacement convergence everywhere in the joint, but do not clearly indicate if the stresses converge in the regions near the ends of the overlap. Besides the fact that a huge number of elements is required for the stress convergence study in the aforementioned regions, a serious computational obstacle have been also experienced: the element aspect ratio gets so high that the “zero or negative element volume error” is indicated, and results become unreliable. In order to overcome these problems, a multi-step submodeling approach is further explored. Its application has allowed to convincingly demonstrate that some stress components are not converging along certain lines belonging to the ends of the overlap. Additional numerical study performed with a two-dimensional plane stress formulation has shown that when taking into account a spew fillet, stress distributions near the end of the overlap change radically. It is concluded that submodeling approach provides an efficient computational tool for enhancing stress analysis in the sites of high stress gradients.
 
A design concept for engineering structures consisting of brittle FRP components and ductile adhesive joints is proposed. The elastoplastic adhesive joints provide system ductility that compensates for the material ductility that FRP composites lack. In the elastic phase, the adhesive offers sufficient stiffness to provide continuity of stiffness over the joint, thus meeting the short- and long-term serviceability requirements for the structure. In the plastic phase, the adhesive develops a uniform stress distribution along the overlap length, thereby enabling sufficient joint rotation to provide an internal force redistribution that increases structural safety and robustness. The application of the design concept to a two-span FRP beam system with an elastoplastic hinge at mid-support showed an increase in structural robustness of almost 140% compared to a continuous FRP beam. FRP structures designed according to the proposed concept exhibit much higher structural safety than brittle structures without force-redistribution capacity.
 
Adhesive bonding technique has been widely used in construction as an alternative to conventional joint techniques, particularly in retrofitting schemes using carbon fibre reinforced polymer (CFRP) plates. Spew fillets and end tapers have been suggested for reducing the stress concentration in the adhesive layer of retrofitted beams. In this paper, finite element (FE) analyses were employed to determine the effects of the spew fillet and the taper on interfacial adhesive stresses and the strains in the CFRP plate. A total of eight cases with different configurations of spew fillet and different tapers have been considered. The results largely agree with the findings on lap joints, but the effect of spew fillets is far less than that for lap joints. Moreover, the results show a combination of an inside taper in the plate and a triangular fillet give the most reduction in the maximum interfacial stresses. The results of the FE analyses also show that the effect of spew fillet size on the strain close to the plate end, which confirms previous analytical and experimental results.
 
The objective of this work is to evaluate the mechanical and physical properties of three-layer boards made with wheat straw and bonded with a tannin-based adhesive. The mechanical properties of panels were evaluated by the static bending (modulus of rupture – MOR and modulus of elasticity – MOE) and the internal bond (IB) tests. Physical properties such as water absorption (WA) and thickness swelling (TS) in water were determined. In general, all types of straw panels made produced in this work met the MOR, MOE, IB and TS requirements for general uses according to European standards. Although, wheat straw boards made with tannin-modified PF resins had slightly poorer mechanical properties compared to the boards made with pure PF, the T10% PF resin showed higher bond ability than other modified PF resins. The highest MOR, MOE, IB and the lowest WA and TS were achieved at a 12 min press time and by using T10% PF resin. An increase in the press time positively affected the physical and mechanical properties of the panels produced. With respect to the findings of this study, it may be stated that wheat straw can be used as a promising raw material for panel production with the use of a tannin-modified PF adhesive.
 
The application of a single-sided patch reinforcement to a woven E-glass fiber/epoxy composite panel with a central circular fastener hole is studied using three-dimensional finite element analysis. The width of the panel is 25.4 mm, while three hole diameters (3, 6 and 9 mm) are used in the study. The reinforcement patch is square in shape and is made of either E-glass fiber/epoxy or carbon fiber/epoxy laminae, with the patch-to-panel-thickness varying from 0.1 to 0.7. To simulate the ‘fastened’ condition, the patch-reinforced panel is bolted to a mild steel bar, which is fixed in the direction normal to the panel. One end of the panel is subject to unidirectional tensile load while the other end is under clamped boundary conditions. The through-thickness stress distributions and the failure loads of the patch-reinforced panels are evaluated by finite element analysis. Contact elements are used to account for interaction between contact surfaces. Experiments are also performed to verify the model.The relationships between the patch-to-panel-thickness and the strength of the panel and the material of patch reinforcement are considered and discussed.
 
This paper reports on a structural concept for engineering structures composed of FRP components to provide system ductility that compensates for the lack of material ductility inherent to FRP materials. The concept includes the use of redundant structural systems and ductile or flexible adhesive joints. To demonstrate the feasibility of the proposed concept, quasi-static experiments on pultruded GFRP beams were performed. The two-span beams were connected with flexible adhesive joints at the middle support. The flexible joints from highly non-linear adhesives provided a favorable redistribution of the internal and external forces in the statically indeterminate system compared to single-span and continuous beams, which were also examined. In the case of adhesive joint failure, structural collapse was prevented because of system redundancy. Due to the stiffness-governed design of the GFRP beams, the stresses in the flexible adhesive joints were small and creep deformations in the joints could be controlled.
 
An analytical model for determining the strain energy release rate due to a prescribed crack in an adhesively-bonded, single-lap composite joint with thick bondlines and subjected to axial tension is presented. An existing analytical model for determining the adhesive stresses within the joint is used as the foundation for the strain energy release rate calculation. In the stress model, the governing equations of displacements within the adherends are formulated using the first-order laminated plate theory. In order to simulate the thick bondlines, the field equations of the adhesive are formulated using the linear elastic theory to allow non-uniform stress distributions through the thickness. Based on the adhesive stress distributions, the equivalent crack tip forces are obtained and the strain energy release rate due to the crack extension is determined by using the virtual crack closure technique (VCCT). The specimen geometry of ASTM D3165 standard test is followed in the derivation. The system of second-order differential equations is solved to provide the adherend and adhesive stresses using the symbolic computational tool, Maple 7. Finite element analyses using J-integral as well as VCCT are performed to verify the developed analytical model. Finite element analyses are conducted using the commercial finite element analysis software ABAQUS™. The strain energy release rates determined using the analytical method correlate well with the results from the finite element analyses. It can be seen that the same prescribed crack has a higher strain energy release rate for the joints with thicker bondlines. This explains the reason that joints with thick bondlines tend to have a lower load carrying capacity.
 
A method to predict the strength of adhesively bonded single and double lap joints from pultruded GFRP composite adherends subjected to quasi-static axial tensile loading is presented. The method is based on a quadratic through-thickness shear–tensile interaction failure criterion. The failure criterion was deduced from measured combined through-thickness tensile and shear strength values in the outer fiber-mat layer of the adherends (locations of the ultimate failure). The experimental strength values were obtained from a new shear–tensile interaction device (STI-device), which allows the measurement of shear–tensile interaction strength values. The predicted joint strengths corresponded well to the measured joint strengths of adhesively bonded single and double lap joints with different geometrical configurations. The investigation also showed that the material strength depended on the expansion of the stressed surface. The resistance against local stress peaks was much higher than the resistance against large uniformly distributed stress blocks. The application of the ultimate failure load prediction method showed a small influence of the adhesive layer thickness on the joint strength although the influence of the fillet radius was seen to be much higher. A partial material safety factor for the joint strength of 1.34 was determined.
 
It is well known that geometric nonlinear effects have to be taken into account when the ultimate strength of single lap composite joints are studied. In the present paper we investigate for which level of loads or prescribed end displacements nonlinear effects become significant and how they appear. These aspects are studied by comparing finite element results obtained from geometric nonlinear models with the results from the linear ones. The well-known software package ANSYS is applied in the numerical analysis together with a self-implemented module in the C++ library Diffpack. Some of the results are also compared with classical analytical theories of idealized joints showing significant differences.The joints examined are made of cross-ply laminates having 0 or 90° surface layers. A combined cross-ply/steel joint and an isotropic joint made of steel are also studied. All the models except the all-steel one are assembled with adhesives, while the latter is welded.Through the investigation a considerable departure from linear behavior has been detected for a large regime of prescribed end displacements or external loads. Geometric nonlinear effects begin to develop for external loads that produces stresses which are far below ultimate strength limits and for average longitudinal strains that are less than 0.5%. It has also been detected that the distribution of materials within the joint has some influence on the nonlinear behavior. Thus, geometric nonlinear methods should always be applied when single lap (or other non-symmetric) composite joints are analyzed.
 
A critical strain failure criterion has been used to predict the strength of adhesive lap joints subject to out-of-plane loading. A general solution for predicting adhesive shear strains using either linear-elastic or elastic plastic shear stress-strain behavior was developed assuming rigid plate rotation. Tests with joints having a range of bond areas, adherend thicknesses, and adhesive thicknesses showed that the linear-elastic calculation of nominal critical shear strain provides a useful criterion for predicting out-of-plane fracture loads for these types of joints in engineering design.
 
Joint analysis using a non-linear finite element model has been performed to analyze the effects of adhesive ductility on the stiffness and strength of full-scale adhesively-bonded double-lap joints composed of brittle pultruded GFRP laminates. Experimental and numerical results of joint and specimen elongations and axial strains in the bondline compared well. Calculated stress states at failure location inside the adherends showed that plastification of ductile adhesives provide uniform load transfer leading to increased joint strength. Joint strength increases almost linearly with increasing overlap length. Flexible and stiff joints are defined depending on the ratio of adhesive-to-adherend modulus. Flexible joints exhibit lower stiffness than the adherends, while stiff joints provide continuity of structural stiffness. The strength of ductile adhesively-bonded joints was predicted by extending an existing through-thickness shear-tensile-interaction failure criterion developed for brittle joints with epoxy adhesive.
 
Quasi-static axial tension experiments were performed in a laboratory environment on adhesively-bonded double-lap joints from pultruded GFRP laminates. Full-scale specimens were investigated to prevent size effects. Ductile polyurethane and acrylic as well as brittle epoxy adhesives were applied to connect brittle GFRP adherends. The visco-elastoplastic/ductile and visco-elastic/brittle stress–strain behavior of adhesives was defined. It is shown that joint stiffness depends non-linearly on the ratio of adhesive to adherend modulus, and approaches a threshold value with increasing adhesive modulus. Ductile joints with plasticized adhesives develop uniform load transfer over the joint length with increased strength as compared to joints with brittle adhesives. In contrast to joints with brittle adhesives, the joint strength of ductile joints with plasticized adhesive increases almost proportionally with increasing overlap length. Axial strains are almost uniformly distributed across the joint width and allow for a 2D analysis.
 
Top-cited authors
David Hui
  • University of New Orleans
Kin tak Lau
  • Swinburne University of Technology
Hao Wang
  • University of Southern Queensland 
Kyong Yop Rhee
  • Kyung Hee University
Luciano Feo
  • Università degli Studi di Salerno