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An approach to detecting discontinuities in carbon fiber-reinforced polymers, caused by impact loading followed by compression testing, was developed. An X-ray sensor-based installation was used, while some algorithms were developed to improve the quality of the obtained low-contrast radiographic images with negligible signal-to-noise ratios. For epoxy/AF (#1) composite subjected to a "high-velocity" steel-ball impact with subsequent compression loading, it was not possible to detect discontinuities since the orientation of the extended zone of interlayer delamination was perpendicular to the irradiation axis. After drop-weight impacts with subsequent compression loading of epoxy/CF (#2) and PEEK/CF (#3) composites, the main cracks were formed in their central parts. This area was reliably detected through the improved radiographic images being more contrasted compared to that for composite #3, for which the damaged area was similar in shape but smaller. The phase variation and congruency methods were employed to highlight low-contrast objects in the radiographic images. The phase variation procedure showed higher efficiency in detecting small objects, while phase congruency is preferable for highlighting large objects. To assess the degree of image improvement, several metrics were implemented. In the analysis of the model images, the most indicative was the PSNR parameter (with a S-N ratio greater than the unit), confirming an increase in image contrast and a decrease in noise level. The NIQE and PIQE parameters enabled the correct assessment of image quality even with the S-N ratio being less than a unit. [ABSTRACT FROM AUTHOR]

The increasing need for lightweight, high strength and cost-effective materials for automotive and aerospace industries worked as the impetus for this investigation. E-glass fibers are layered with 3K-carbon fibers in proportions of 7:6 & 6:7 for fabricating hybrid fiber reinforced polymer (FRP) composites. First stage of the study involves the identification of the optimized fiber stacking pattern. Later, nanocomposites were made for further enhancement of tensile properties by infusing titanium dioxide (TiO2) nanoparticles to hybrid FRP composite ‘C2G3C3G3C2’ with optimized stacking pattern. The TiO2 nanoparticles are equally disseminated in the epoxy resin using ultrasonication with five different wt. % of 0, 1.25, 2.5, 3.75, and 5. The peak load, ultimate tensile strength (UTS), tensile modulus, and % elongation at break were examined as per ASTM D3039 tensile testing. The UTS of optimized hybrid FRP composite, carbon non-hybrid FRP composite, and nanocomposite (at 2.5 wt. % TiO2 nanoparticles) were 424.98 MPa, 429.0 MPa, and 478.51 MPa, respectively. Further, the cost of optimized hybrid FRP composite and nanocomposite (at 2.5 wt. % of TiO2 nanoparticles) is approximately 36.93% and 21.51% less than most expensive carbon non-hybrid FRP composite. The morphological analysis revealed that TiO2 added composites up to 2.5 wt. % exhibit fewer defects, indicating a better interaction between the matrix and the fibers. [ABSTRACT FROM AUTHOR]

The automation of the manufacturing processes of thermoplastic composite laminates has become dependent on open mold processes such as automated tape placement (ATP), which couples tape layering with in situ consolidation. The manufacturing parameters of ATP open mold processes, which comprise processing time, consolidation pressure and temperature, affect the bond strength between the plies and the quality of the laminates produced. Therefore, the effect of the manufacturing parameters should be characterized. This work experimentally evaluates the feasibility of fabricating thermoplastic laminates using an open mold process that reasonably models that of ATP. Glass fiber-reinforced polypropylene laminates are fabricated from unidirectional tapes under different consolidation periods, pressures, and temperatures. The bond quality in the produced laminates is assessed by measuring their interlaminar shear strength, which is measured using a short beam standardized shear test in conjunction with digital image correlation. Results show that consolidation can occur at temperatures slightly below the composite tapes' complete melting temperature, and consolidation times between 7 and 13 min can result in acceptable bond strengths. The results confirmed the feasibility of the process and highlighted its limitations. Analysis of variance and machine learning showed that the effect of process parameters on interlaminar shear strength is nonlinear. [ABSTRACT FROM AUTHOR]

This study introduces a Radial Basis Function-Genetic Algorithm-Back Propagation-Importance Sampling (RBF-GA-BP-IS) algorithm for the multi-scale reliability analysis of Fiber-Reinforced Polymer (FRP) composite structures. The proposed method integrates the computationally powerful RBF neural network with GA, BP neural network and IS to efficiently calculate inner and outer optimization problems for reliability analysis with hybrid random and interval uncertainties. The investigation profoundly delves into incorporating both random and interval parameters in the reliability appraisal of FRP constructs, ensuring fluctuating parameters within designated boundaries are meticulously accounted for, thus augmenting analytic exactness. In application, the algorithm was subjected to diverse structural evaluations, including a seven-bar planar truss, an architectural space dome truss, and an intricate nonlinear truss bridge. Results demonstrate the algorithm's exceptional performance in terms of model invocation counts and accurate failure probability estimation. Specifically, within the seven-bar planar truss evaluation, the algorithm exhibited a deviation of 0.08% from the established failure probability benchmark. [ABSTRACT FROM AUTHOR]

The packing characteristics of fiber/polymer powder in powder bed fusion additive manufacturing exhibit a high correlation with the mechanical behaviours of printed composite parts such as homogeneity and anisotropy. A discrete element model has been developed to investigate the packing characteristics of glass fiber/polyamide 12 (PA12) powder, which include fiber orientations, fiber homogeneity, and packing density. The predicted probability distributions of fiber orientations in the powder bed are comparable with those measured in glass fiber–reinforced PA12 composites printed via multi jet fusion. Three types of fibers with different length distributions are adopted to study the effects of the fiber length distribution on their packing characteristics. The simulation results reveal that a large average fiber length is beneficial to fiber alignment in the powder spreading direction but lowers the fiber homogeneity and packing density of the powder bed. Furthermore, varying the fiber length can provide an effective way to regulate fiber orientations in the powder packing process, which would help achieve satisfactory anisotropic mechanical properties for composite parts.

The stress–strain characteristics and failure behavior of composites are strain rate dependent and affected by the fiber areal density. To elucidate the combined influence of areal density and strain rate on the strength of basalt fiber‐reinforced polymer composites (BFRP), an experimental study was conducted on BFRP laminates of two different fiber areal densities, that is, 380 GSM and 200 GSM, having distinct stacking sequence under three different loading rates. Failure modes, failure strength, and Weibull parameters were used to characterize the experimental outcomes. The experiment was carried out to investigate the mechanical responses and associated failure modes at strain rates ranging from quasi‐static 0.1 mm/min to a high strain rate of 10 mm/min. It has been demonstrated that there is a substantial correlation between the fiber areal density, loading rate, and stacking order of BFRP laminates and the increase in maximum flexural strength and interlaminar shear strength. For an increase in the fiber areal density from 200 GSM to 380 GSM flexural strength is increased by 18%–30%, while ILSS strength is increased by 30%–52%. Based on the finding, the asymmetric type‐2 laminate exhibits better properties than the symmetric and asymmetric type‐1 laminates due to the presence of more (0°/90°) laminae at the tensile side of the laminate. Inferring the mechanical characteristics of composite materials and their relationship to strain rate from experimental data required a statistical technique. The statistical analysis and experimental findings demonstrate that the shape parameter and linear coefficient are not reliant on the strain rate. [ABSTRACT FROM AUTHOR]

The automation of the manufacturing processes of thermoplastic composite laminates has become dependent on open mold processes such as automated tape placement (ATP), which couples tape layering with in situ consolidation. The manufacturing parameters of ATP open mold processes, which comprise processing time, consolidation pressure and temperature, affect the bond strength between the plies and the quality of the laminates produced. Therefore, the effect of the manufacturing parameters should be characterized. This work experimentally evaluates the feasibility of fabricating thermoplastic laminates using an open mold process that reasonably models that of ATP. Glass fiber-reinforced polypropylene laminates are fabricated from unidirectional tapes under different consolidation periods, pressures, and temperatures. The bond quality in the produced laminates is assessed by measuring their interlaminar shear strength, which is measured using a short beam standardized shear test in conjunction with digital image correlation. Results show that consolidation can occur at temperatures slightly below the composite tapes’ complete melting temperature, and consolidation times between 7 and 13 min can result in acceptable bond strengths. The results confirmed the feasibility of the process and highlighted its limitations. Analysis of variance and machine learning showed that the effect of process parameters on interlaminar shear strength is nonlinear.

In this study, we present a novel approach for the production of continuous fiber-reinforced thermoplastic composites by combining injection molding and additive manufacturing. After exploring the design requirements, we manufactured inserts via continuous fiber-reinforced 3D printing, then we used them as reinforcement for injection-molded samples. Improper fiber placement can cause warpage as the continuous fibers prevent shrinking; however, warpage can be compensated with the insert geometry. The reinforcement resulted in an increase of about 30% in the properties tested. [ABSTRACT FROM AUTHOR]

We formulate a 3D multi-phase field model to simulate bulk and interfacial fracture in fiber-reinforced composites. The purpose of this study is to predict the complex crack interaction between the interlaminar and intralaminar fracture in FRPCs. For that, the discrete crack and the sharp interface are both smeared using the phase-field approach. A second-order anisotropic tensor is incorporated in the elastic as well as the fracture surface energy to promote direction-dependent crack growth. The interface constitutive behavior is represented by the uncoupled 3D traction–separation laws—in exponential and bi-linear form. The proposed model captures the complicated failure phenomena in FRPCs such as matrix damage, interfacial delamination, and their interactions for different configurations of lamina with different fiber orientations and fracture energies. This model's capability is further illustrated to study the three common modes of fracture i.e. modes I, II, and III, in 3D FRPC laminates where the crack impinges on the interface. Finally, the obtained framework is validated by two benchmark problems including the mode I and mode II delamination test, and is compared against experimental data reported in the literature. • A 3D phase-field model to predict intra and interlaminar fracture in FRPC laminates. • The model simulates different fracture modes, observing crack propagation dependency. • Validated 3D simulations with experiments for accuracy and robustness of the model. [ABSTRACT FROM AUTHOR]

This work describes a novel method of embedded damage detection within glass fiber-reinforced polymer composites. Damage detection is achieved by monitoring the spatially distributed electrical conductivity of a strain-sensitive multiwalled carbon nanotube thin film. First, thin films were spray-deposited directly upon glass fiber mats. Second, using electrical impedance tomography, the spatial conductivity distribution of the thin film was determined before and after damage-inducing events. The resolution of the sensor was determined by drilling progressively larger holes in the center of the composite specimens, and the corresponding electrical impedance tomography response was measured by recording the current-voltage data at the periphery of the monitored composite sample. In addition, the sensitivity to damage occurring at different locations in the composite was also investigated by comparing electrical impedance tomography spatial conductivity maps obtained for specimens with sets of holes drilled at different locations in the sensing area. Finally, the location and severity of damage from low-velocity impact events were detected using the electrical impedance tomography method. The work presented in this study indicates a paradigm shift in the available possibilities for structural health monitoring of fiber-reinforced polymer composites. © The Author(s) 2013.

Fiber-reinforced polymer matrix composites continue to attract scientific and industrial interest since they offer superior strength-, stiffness-, and toughness-to-weight ratios. The research herein characterizes two sets of E-Glass/Epoxy composite skins: stressed and unstressed. The stressed samples were previously installed in an underground power distribution vault and were exposed to fire while the unstressed composite skins were newly fabricated and never-deployed samples. The mechanical, morphological, and elemental composition of the samples were methodically studied using a dynamic mechanical analyzer, a scanning electron microscope (SEM), and an x-ray diffractometer, respectively. Sandwich composite panels consisting of E-glass/Epoxy skin and balsa wood core were originally received, and the balsa wood was removed before any further investigations. Skin-only specimens with dimensions of ~12.5 mm wide, ~70 mm long, and ~6 mm thick were tested in a Dynamic Mechanical Analyzer in a dual-cantilever beam configuration at 5 Hz and 10 Hz from room temperature to 210 °C. Micrographic analysis using the SEM indicated a slight change in morphology due to the fire event but confirmed the effectiveness of the fire-retardant agents in quickly suppressing the fire. Accompanying Fourier transform infrared and energy dispersive X-ray spectroscopy studies corroborated the mechanical and morphological results. Finally, X-ray diffraction showed that the fire event consumed the surface level fire-retardant and the structural attributes of the E-Glass/Epoxy remained mainly intact. The results suggest the panels can continue field deployment, even after short fire incident. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymers contribute significantly to weight-reducing components for various industrial applications. A discontinuous glass fiber-reinforced thermoset resin is considered which is produced by the sheet molding compound (SMC) process. Related to the production process, the samples considered in this work exhibit an anisotropic fiber orientation distribution which highly affects the thermomechanical properties. The thermoviscoelastic material behavior of three selected samples is characterized by means of dynamic mechanical analysis. These tests show the temperature-dependent elastic modulus and the glass transition of the composite. Measurements of the thermal expansion of the SMC composite provide data on the coefficient of thermal expansion (CTE). These experimental investigations provide data for the thermoelastic material modeling. Aiming at the prediction of the effective thermal and mechanical properties, a Hashin–Shtrikman-based homogenization method is presented. Based on an eigenstrain formulation, the effective Young's modulus and CTE are computed in two steps. Moreover, the mean-field method is given in dependence of a variable reference stiffness allowing to tailor the approach to the material system. The influence of this variable reference stiffness on the effective quantities as well as the predicted behavior is analyzed with respect to the experiments. The presented numerical results are in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polyester composites have received significant attention in a variety of applications due to their considerable potential due to such characteristics as high strength, stiffness, and modulus. However, one of the most important concerns about polymeric composites is their sensitivity to moisture attack. This work has been conducted to investigate the effects of nanoclay addition on reinforcing glass/polyester composites against water absorption and the resultant deterioration of flexural strength. Therefore, chopped strand mat and woven fiberglass polyester specimens were fabricated by using the hand lay-up technique with varying weight percentages of Cloisite 20A nanoclays (0, 1.5, and 3 wt%) and immersion in water for a time duration of 21 days. The specimens were weighed for the water absorption test. The results showed a remarkable drop in water absorption of the composite samples with the increase of nanoclay content. Moreover, although all the pure and nanocomposite specimens underwent degradation in flexural strength due to the water absorption, the strength was found to significantly increase with increasing the percentage of nanoclay at all immersion periods. The experimental results were confirmed by scanning electron microscopy (SEM). SEM images indicated that the presence of nanoclay protected the fiber/matrix interfaces. [ABSTRACT FROM AUTHOR]

Failure criteria for a crack between two dissimilar materials are presented. Test data from three different material pairs are considered. The first two consist of an interface crack between glass and epoxy and two dissimilar ceramic clays. For the third material pair, there is a delamination between two unidirectional carbon fiber polymer-reinforced plies with fibers in the 0 deg and 90 deg directions. Failure criteria for these materials have been presented previously. In this study, invariant parameters are used in the failure criteria. It is seen that the invariant length parameter has values which are unrealistically large and small. The preferred criterion is based on first principles. [ABSTRACT FROM AUTHOR]

In the present work, a unified phase field-based cohesive zone model (PF-CZM) has been utilized to predict bulk fracture and most importantly interfacial delamination in carbon fiber-reinforced polymer composites (CFRP). The formulation is based on the classical phase-field approach of energy minimization, with a rational degradation function and a polynomial-type crack geometric function. Incorporation of those provides the PF theory to integrate various traction–separation behaviors by choosing optimal constitutive functions and makes the theory capable of predicting the quasi-brittle type of fracture. Since the interfacial delamination in composites occurs either due to adhesive failure or decohesion between stacked lamina, the PF-CZM model is used to predict such failure by exploiting the cohesive zone flavor of the model. Further, the model inherently incorporates the strength of the material without any additional calibration. Various standard simulations are performed considering CFRP geometry where delaminations due to adhesive failure or interface decohesion occur in a mode-I and mode-II fashion. The model is observed to predict mode-I and mode-II separation accurately. To test the accuracy of the model an experimental validation is also performed. In addition to the delamination, open hole tension (OHT) simulations of CFRP lamina for different fiber orientations are considered to validate the anisotropic crack growth predictions, and the results are compared with experiments. [ABSTRACT FROM AUTHOR]

The present paper aims to study different laser drilling/cutting parameters including laser intensity, cutting speed, and gas pressure in order to achieve minimum surface roughness (Ra), heat-affected zone (HAZ), taper angle (TA), and maximum tensile strength (TS) of the laser-drilled glass fiber-reinforced plastic (GFRP) laminate. Full factorial design of experiment method and analysis of variance were adopted to investigate the effect of each parameter on the responses, and in addition, the surface roughness and tensile strength were compared with that of conventional drilling. The morphology of the laser-drilled holes was studied through the scanning electron microscopy (SEM). It was found that optimum laser drilling parameters can yield higher tensile strength and lower surface roughness in drilled GFRP laminate compared to the conventional drilling. Moreover, multi-response optimization was carried out on the results in order to obtain maximum tensile strength, minimum HAZ, minimum surface roughness, and minimum taper angle. The results revealed that the laser intensity, cutting speed, and assist gas pressure should be set at 2.04 W/cm2, 8 mm/s, and 4 bar, respectively, to obtain higher tensile strength and surface integrity. The results drawn from this study can be applied to improve the surface quality and mechanical properties of laser drilling of FRP materials especially in aircraft part manufacturing. [ABSTRACT FROM AUTHOR]

This study introduces a Radial Basis Function-Genetic Algorithm-Back Propagation-Importance Sampling (RBF-GA-BP-IS) algorithm for the multi-scale reliability analysis of Fiber-Reinforced Polymer (FRP) composite structures. The proposed method integrates the computationally powerful RBF neural network with GA, BP neural network and IS to efficiently calculate inner and outer optimization problems for reliability analysis with hybrid random and interval uncertainties. The investigation profoundly delves into incorporating both random and interval parameters in the reliability appraisal of FRP constructs, ensuring fluctuating parameters within designated boundaries are meticulously accounted for, thus augmenting analytic exactness. In application, the algorithm was subjected to diverse structural evaluations, including a seven-bar planar truss, an architectural space dome truss, and an intricate nonlinear truss bridge. Results demonstrate the algorithm's exceptional performance in terms of model invocation counts and accurate failure probability estimation. Specifically, within the seven-bar planar truss evaluation, the algorithm exhibited a deviation of 0.08% from the established failure probability benchmark.

Summary: A comprehensive life cycle assessment of panels for aircraft interiors was conducted, including both a conventional glass fiber‐reinforced panel and different novel sustainable panels. The conventional panel is made of a glass fiber‐reinforced thermoset composite with halogenated flame retardant, whereas the sustainable panels are made of renewable or recyclable polymers, natural fiber reinforcements, and nonhalogenated flame retardants. Four different sustainable panels were investigated: a geopolymer‐based panel; a linseed‐oil–based biopolymer panel; and two thermoplastic panels, one with polypropylene (PP) and another with polylactic acid (PLA). All of the sustainable panels were developed to fulfil fire resistance requirements and to be lighter than the conventional panels in order to reduce fuel consumption and air pollutant emissions from the aircraft. The environmental impacts associated with energy consumption and air emissions were assessed, as well as other environmental impacts resulting from the extraction and processing of materials, transportation of materials and waste, panel manufacturing, use, maintenance, and end of life (EoL). All the sustainable panels showed better environmental performance than the conventional panel. The overall impacts of the sustainable panels were offset by the environmental benefits in the use stage attributed to weight reduction. One square meter of the novel panels could save to 6,000 kilograms of carbon dioxide equivalents. The break‐even point (in months) at which the use of sustainable panels would yield an environmental benefit relative to the impacts arising in production and EoL was as follows: 1.2 for the geopolymer panel; 1.7 for the biopolymer panel; 10.4 for the PLA panel; and 54.5 for the PP panel. [ABSTRACT FROM AUTHOR]

In the present work, a phase-field fracture model is developed to create an experimentally validated, physically motivated, and computationally tractable framework to predict the orientation-dependent complex fracture response of the unidirectional fiber-reinforced polymer matrix composites (PMC). The damage mechanism in PMC is many-fold, such as matrix micro-cracking, fiber breakage or pullout, fiber debonding, and delamination. The progressive damage and failure in the composite can be attributed to the competition of all these different damage mechanisms individually or in concert, depending on fiber volume fraction and orientation of the plies. Moreover, other important factors at the constituent level, such as viscoelasticity or plasticity of the matrix, can significantly influence the constitutive behavior of the composite. In this work, a homogenized, coupled thermo-mechanical constitutive model is developed considering the viscoelasticity of the polymer matrix for the fiber-reinforced PMCs. Subsequently, a phase-field fracture model is adopted to capture the essential damage mechanisms as matrix cracking and fiber pullout/breakage for unidirectional composite lamina. The phase-field variable considers an anisotropic tensorial term in its gradient to account for the orientation-dependent fracture of fiber-reinforced PMC. The model is numerically implemented by writing an ABAQUS user-element subroutine (UEL). The model can predict the direction-dependent damage propagation and the final load-deformation response at fracture in commercially acquired unidirectional glass-fiber-reinforced epoxy composites at different fiber orientations in reasonable agreement with the experiments. Based on the decoupling of the driving energies the model can also identify the dominant damage mechanism among the two such as fiber breakage or matrix cracking, for a given fiber orientation. Simulations are also conducted to validate the model predictions for carbon-fiber reinforced epoxy lamina with available experimental data in the literature. • A phase-field fracture model is developed for unidirectional fiber composites. • The model is based on a homogenized constitutive description of the PMC. • Incorporates viscoelastic behavior of polymer matrix. • Considers the anisotropy in crack propagation influences by the fiber orientation. • Experimentally validated and computationally tractable in a finite element framework. [ABSTRACT FROM AUTHOR]

Structural health monitoring (SHM) of safety-critical composite components is essential to ensure mechanical stasis and detect local damage before it can produce global failure. SHM technologies must also adapt to ever-evolving materials and geometries, but traditional piezoresistive strain sensors lack the ability for end-user customization and modifications throughout service-life. Additive manufacturing via fused filament fabrication (FFF) provides a practical pathway to overcome such sensor shortcomings. However, the electrical conductivity of conventional polymer feedstock is not sufficient for accurate strain measurements without compromise to melt viscosity and thus printability. Here we report the development of a new 3D-printable and electrically responsive nanocomposite by melt-mixing poly(ethylene-co-methacrylic acid) (EMAA) with multi-walled carbon nanotubes (MWCNT). Bulk electrical conductivity of 43.9 S m−1 is achieved at only 5 wt% loading – higher than comparable materials – where the nano-dispersion heterogeneity of MWCNT in EMAA is linked to the favorable conductivity while retaining molten flowability. FFF is employed to print thin (150 μ m) serpentine strain sensors onto the surface of a glass fiber-reinforced polymer composite, which exhibit strong adhesion and accurate piezoresistive sensing under cyclic flexural loading. Twenty consecutive cycles with converged sensor readings (i.e., < 1% variation in measured resistance) demonstrates reliable performance across a relevant service strain range (0.4 – 0.8 %) for such fiber-composites. This rapid fabrication and transferable sensing strategy, suitable for new and existing structures, thus provides a crosscutting SHM solution. [Display omitted] • Achieved high electrical conductivity (> 40 S m−1) at low MWCNT loading (5 wt%). • Favorable conductivity reached by controlling nano-dispersion of MWCNTs in copolymer. • 3D printing onto a glass-fiber composite provides reliable and repeatable strain sensing. • Twenty flexural load-unload cycles are demonstrated with minimal sensor hysteresis. [ABSTRACT FROM AUTHOR]

A repair technique for severely damaged precast reinforced concrete (RC) bridge columns with grouted splice sleeve (GSS) connections has been developed that uses a carbon fiber-reinforced polymer (CFRP) shell and epoxy-anchored headed bars to relocate the column plastic hinge. Four original specimens were built using an accelerated bridge construction (ABC) technique with two different GSS systems and were tested to failure using cyclic quasi-static loads. One GSS system was used to connect an RC bridge pier cap to a column and the second GSS system was used to connect an RC footing to a column. Failure of the four original specimens occurred at drift ratios between 5.6 and 8.0% with longitudinal bar fracture or pullout from the GSS connections. The repair method successfully relocated the plastic hinge to the original column section adjacent to the repair and was capable of restoring the diminished load and displacement capacity. The method is a viable and cost-effective technique for rapid seismic repair of severely damaged precast bridge assemblies. [ABSTRACT FROM AUTHOR]

The incorporation of nanotube-covered fibers in continuous fiber/epoxy composites has been shown to influence the mechanical, electrical, and thermal properties of the composite. Increased interlaminar shear stress, flexural strength and modulus have been reported in such composites over composites containing bare fibers. In this study, the microstructure and interfacial shear strength (ISS) of continuous silicon carbide fiber/epoxy composites with and without nanotubes grown from the SiC fiber surface were investigated with micro-Raman spectroscopy (MRS) and microscopy. The fibers with nanotubes grown from the surface were found to have a reduced ISS compared with the bare fibers. Electron microscopy showed good wetting of epoxy in the nanotube forests, but poor attachment of the nanotube forests to the fibers. These results suggest that the mechanism leading to improvements in bulk composite properties is not due to an improvement in the fiber/matrix ISS. [ABSTRACT FROM AUTHOR]

Abstract: Power transmission poles are subjected to dynamic cantilever bending due to wind gusts and cable unilateral failure, or may also be subjected to vehicle impacts. In this paper, transient dynamic analysis of tapered fiber-reinforced polymer (FRP) composite transmission poles with circular thin-walled cross-section subjected to dynamic cable tension and vehicle impacts is investigated by combination of tapered beam finite element and precise time integration method. It is assumed that the material behavior is linearly elastic and the laminate of the cross-section of the wall is symmetric or antisymmetric angle-ply. The effect of fiber type and orientation, the pole geometry, and the concentrated mass at the pole tip are evaluated by performing the dynamic analysis of FRP poles under step, triangular and sine pulses. There is a good agreement between the results of the present method and those obtained from the poles modeled by ANSYS commercial finite element software and existed literatures. Also, there is a significant shorter run-time in the present method. It is concluded that beyond 10 layers for the laminate with constant thickness of the wall, the pole tip deflection does not change. [Copyright &y& Elsevier]

Abstract: An innovative use of fiber-reinforced polymer (FRP) composite materials, to control the manifestation of local buckling in a flanged steel section, is proposed. In this method, the high stiffness and linear behavior of FRP materials are utilized to provide “bracing” against web or flange local buckling in a manner that strategically leverages the unique mechanical properties of each material in an efficient application domain. The experimental research reported is aimed at demonstrating the feasibility of using small quantities of FRP to provide cross-sectional stability through the bonding of FRP strips to flange elements of the cross-section, thereby increasing the critical load of the member; constraining plastic flow in the cross-sectional flange elements; and facilitating the manifestation of a well-formed and stable hysteretic response of the member under cyclic loading. The member becomes, in effect, an FRP stabilised steel section. [Copyright &y& Elsevier]

Flexural testing provides a rapid and straightforward assessment of fiber-reinforced composites' performance. In many high-strength composites, flexural strength is higher than compressive strength. A finite-element model was developed to better understand this improvement in load-bearing capability and to predict the flexural strength of three different carbon-fiber-reinforced polymer composite systems. The model is validated against publicly available experimental data and verified using theory. Different failure criteria are evaluated with respect to their ability to predict the strength of composites under flexural loading. The Tsai–Wu criterion best explains the experimental data. An expansion in compressive stress limit for all three systems was observed and is explained by the compression from the loading roller and Poisson's effects. [ABSTRACT FROM AUTHOR]

Micro/meso-mechanical models for unidirectional and angle-ply laminates under an off-axis tensile loading are presented. Three-dimensional unit/ multi-cell representations of laminates with periodical fiber arrays are established. The matrix is described by a nonlinear viscoelastic model, and the fiber is described by an elastic one. Matrix cracking is modeled by the smeared crack method, which permits a crack description in terms of stress strain relations and stiffness reduction in particular orientations. The modeling of material nonlinearity, damage initiation and growth, and the multiaxial loading with appropriate periodic boundary conditions are successfully incorporated in a FEM analysis. As examples of this unified analysis procedure, numerical results for unidirectional, angle-ply, and cross-ply laminates are presented. Using the same set of material constants of the constituents, different response characteristics of the three types of laminates are predicted, and the results are found to be in good agreement with the test data. [ABSTRACT FROM AUTHOR]

Fiber-Reinforced Polymer (FRP) composites are considered to be a potentially promising material for use as pile foundations in harsh environments such as industrial or matine conditions. As with piles made of other materials, FRP piles may buckle under extreme loading situations during their installation by driving orjacking into soils or when they are subjected to permanent superstructure loads. Since FRP composites generally have anisotropic properties, relatively low moduli, and relatively high elastic to shear modulus ratios compared to steel, for example, the shear deformation effect plays a more important role in dictating the buckling loads of FRP piles. In this paper, the Timoshenko beam theory is adopted to derive solutions which include the shear deformation effect for buckling loads of vertically loaded transversely isotropic FRP piles with five different boundary conditions. Parametric studies show that the buckling loads for the cases where the shear defonnation effect is considered are always lower than when the shear deformation effect is ignored. [ABSTRACT FROM PUBLISHER]

• A methodology was proposed to study the energy absorption of hybridized CFRPs. • The residual velocities of projectile in hybrid laminates were predicted using ANN. • The effect of laminate stacking sequence was investigated on the energy absorption. • The absorbed energy of hybridized CFRP composite laminates was optimized. In this paper, the energy absorption capability of hybridized carbon fiber-reinforced polymer (CFRP) composites with Kevlar and glass layers subjected to high-velocity impact loading was studied. The developed methodology was based on meso‑macro scale finite element modeling, experimental testing and utilizing a predictive algorithm relying on artificial neural network. The induced damage mechanisms were considered in the numerical model by employing appropriate failure criteria for the yarns and matrix of the composite laminates and the results were validated against experimental results. An artificial neural network-based algorithm was used to optimize the energy absorption capability of the hybridized CFRP composites. It was found out that a considerably higher improvement in the energy absorption can be achieved by considering an appropriate laminate layup without a considerable increase in the laminate mass. The optimum hybridization of CFRP laminates resulted in 135% improvement in the absorbed energy with only 9% increase in the mass of the laminate. This study can provide a reliable and cost-effective method for designing and analyzing hybrid composite laminates under high velocity-impact loading. [ABSTRACT FROM AUTHOR]

The construction industry has seen a proliferation of continuous and discontinuous fiber-reinforced polymer composites due to their high strength-to-weight and stiffness-to-weight ratios while being resilience to harsh environmental and operating conditions. This paper reports a comparative experimental investigation of field-deployed E-glass/Epoxy composite panels with those freshly manufactured to elucidate the effect of deployment conditions on the performance of these panels. Samples extracted from underground vault after power-line explosion due to arcing fault were examined using optical and atomic force microscopes to assess the extent of the damage. The samples were also characterized using a dynamic mechanical analyzer to explain the change in the properties post-explosion quantitatively. Microscopy investigations show no significant change in morphology or topography of the panels as well as the absence of failure modes such as delamination, fiber-failure or core-failure. Nonetheless, surface contaminations due to service conditions were found to exaggerate the surface burn marks despite the fire retardation properties of the composite panels. The dynamic mechanical properties of the panels were found to change as a function of temperature and loading frequency slightly. A shift in transition temperatures (T β and T g) was found to be within a few degrees Celsius. The storage modulus was reduced by 21%, while the overall complex modulus remained relatively constant when comparing fresh and field-deployed samples. A frequency and temperature dependent model was used to fit the dynamic data and found to provide insights into the microstructure evolution of the polymer matrix. [ABSTRACT FROM AUTHOR]

In this paper, exploratory studies on the application of fibre-reinforced polymer composites in the construction of gearbox housing elements that are used for transport means, as an alternative to traditionally used materials, were undertaken. Composite materials with three types of reinforcement were used: a glass-chopped strand mat, glass fabric, and carbon fabric. The manufactured elements were subjected to weight assessment and vibroactivity tests, including the recording of vibration and noise. The obtained results were compared to the values recorded for housings made of steel. It has been found that composite housings, while maintaining geometric similarity, are characterised by at least 60% lower weight compared to steel housings. It has been shown that in the frequency range below 1 kHz, composite housings are characterised by the presence of resonant frequencies with higher amplitudes than steel housings. At higher frequency ranges—above 1 kHz—composite housings had a lower vibroactivity level than steel housing. They allowed a significant reduction in the level of vibration and noise in this frequency range. The results obtained indicate that composite gearbox housings can be a good alternative to steel-based solutions. [ABSTRACT FROM AUTHOR]

To investigate the mechanical degradation of the shear properties of glass fiber-reinforced polymer (GFRP) laminates in bridge decks under hygrothermal aging effects, short-beam shear tests were performed following the ASTM test standard (ASTM D790-10A). Based on the coupled hygro-mechanical finite element (FE) analysis method, an inverse parameter identification approach based on short-beam shear tests was developed and then employed to determine the environment-dependent interlaminar shear modulus of GFRP laminates. Subsequently, the shear strength and modulus of dry (0% Mt/M∞), moisture unsaturated (30% Mt/M∞ and 50% Mt/M∞), and moisture saturated (100% Mt/M∞) specimens at test temperatures of both 20 °C and 40 °C were compared. One cycle of the moisture absorption–desorption process was also investigated to address how the moisture-induced residual damage degrades the shear properties of GFRP laminates. The results revealed that the shear strength and modulus of moisture-saturated GFRP laminates decreased significantly, and the elevated testing temperature (40 °C) aggravated moisture-induced mechanical degradation. Moreover, an unrecoverable loss of shear properties for the GFRP laminates enduring one cycle of the moisture absorption–desorption process was evident. [ABSTRACT FROM AUTHOR]