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This paper investigates the effect of the z-pin pre-hole inserted (ZPI) process on the impact resistance and post-impact flexural properties of skin/stringer composite structure with different z-pin parameters. The responses of the specimens are characterized in terms of impact denting & cracking length, load-carrying capacity, energy absorption and so on. Experimental results demonstrate that the z-pin cannot inhibit the initiation of debonding cracks at the skin/stringer interface. However, the z-pin forms a bridging traction zone across the cracking and prevent it from developing, which ensures the skin/stringer structure preserve higher peak loading and impact resistance. The specimens with the highest z-pin density have better impact resistance and outstanding residual flexural post-impact properties. Moreover, z-pins play a bridging role through its own failure. One is the inter-fiber debonding and splitting or shear failure of z-pins due to "snubbing effect," the other is the sliding friction of z-pins and z-pins' elastic deformation. These results could provide some guidance for designing z-pinning connection of composite skin/stringer structure. [ABSTRACT FROM AUTHOR]

The vacuum bagging is a common but critical step of various manufacturing process chains for Carbon/Glass Fibre reinforced polymers (C/GFRP). Whilst developing a new generation of vacuum bagging, significant efforts have been made to deeply understand the relevant physics fields, process variables involved and to provide predictive approaches to support dimensioning and scaling exercises prior to implementation into real production process. Main features of the new bagging system are, (1) the reduction of material diversity through merging of various properties within one single layer and, (2) the suppression of critical folding steps by using thermoforming for pre-shaping purposes. This paper will mainly focus on considerations of heat transfer during the pre-shaping of the film, directly on the CFRP part short before the curing step. For the better risk assessment of the short time exposition of an uncured composite part to the heat flow, a problem of transient coupled field analysis involving heat transfer via conduction, convection and radiation is formulated and solved by using Finite Element Analysis in COMSOL under consideration of realistic process conditions. Further a section dedicated to the comparison of the obtained results with measurements performed on a system including an IR-emitter, the bagging material, a ground plate and in some studies as well a CFRP sample is included. During the study the influence of time, power output of the emitter, distance between emitter and surface as well as irradiation angle has been assessed including a comparison of simulated results and performed measurements. [ABSTRACT FROM AUTHOR]

In this work, the analysis of the static behavior of the composite honeycomb and anti-trichiral sandwich is studied. The objective of this paper is to estimate the effect of cellular geometrical parameters which are the ligament length and the cylindrical node radius on the static properties of the anti-trichiral structures. The core and the sandwiches were made from the same biobased material which is polylactic acid with flax fibers (Flax/PLA Material). 3D printing technology was used to manufacture the specimens. Several tensile tests were performed on the anti-trichiral core while bending tests were carried out on the sandwich structures. Several specimens with different geometric parameters of the anti-trichiral unit cell are tested to study their effect on the static properties of this material. The results show that the static properties of the anti-trichiral shaped meta-material depend on the unit cell dimensions. The Poisson's ratio varies from 1 to −1 and from 0.5 to−0.8 respectively for one-cell and two-cell cores in width. To validate the experimental results, numerical simulations were carried out. Experimental and computational results for the Poisson's ratio were quite similar. [ABSTRACT FROM AUTHOR]

This paper conducted experiments and theoretic analysis of the fracture performance of pultruded GFRP (Glass Fiber Reinforced Polymers) laminates. The mode-I fracture toughness of pultruded GFRP laminates was evaluated by the eccentrically loaded, Extended Compact Tension (ECT) specimen. The geometry of specimen and experimental procedure refer to ASTM E1922. A total of 16 specimens were tested, which included 0° and 90° roving orientation. Finite element analysis with the Virtual Crack Closure Technique (VCCT) was implemented to calibrate the experimental results. A correction function for normalized notch length was proposed for pultruded GFRP translaminar fracture toughness. Carpet plot was designed to discuss the relationship between fiber volume ratio and fracture toughness, which demonstrated that the fracture toughness strongly correlated with the fiber volume ratio. Also, the effect of fraction of 0° layers on fracture toughness and material orthotropy was studied. The relationship of material orthotropy parameter and fracture toughness was discussed. [ABSTRACT FROM AUTHOR]

The Air Force Research Laboratory led a research effort to benchmark the accuracy of static and fatigue predictions of several emerging composite progressive damage analysis techniques. The static portion of this technical effort is described in detail in a previous special issue of the Journal of Composite Materials. This paper provides the details of the fatigue experiments that were conducted to calibrate and validate the computational models. Initially, in-plane and out-of-plane S–N curves were generated through coupon tests that were performed on unidirectional laminae. The challenges experienced during fatigue testing of in-plane, matrix-dominated unidirectional coupon specimens are presented in detail. The higher fidelity test data from the fiber-dominated and out-of-plane experiments are also included in this paper. Following the calibration experiments, a series of tension–tension fatigue validation tests were conducted on open-hole coupons with three different stacking sequences. Each specimen was cycled to a pre-determined number of fatigue cycles, followed by static residual strength tests in both tension and compression. This paper provides the stress–strain responses of these validation tests as well as high-resolution X-ray computed tomography images of the subsurface damage as a function of cycles. Seven analysis teams used these test results to calibrate their models and to benchmark the accuracy of their predictions of damage and residual mechanical properties. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of different tool pin morphologies on the inter-mixing capability, microstructure, mechanical properties, corrosion, fracture, and wear behaviors of the dissimilar friction stir welded AA6061-T6 alloy--Al 20wt% Mg2Si composite. Grooved shoulder tools with varying pin profiles such as tapered cylindrical, threaded tapered cylindrical, and triangular tapered pins were used for joining the base materials. The parameter combination of 80 mm/min travel speed and 1000 r/min rotation speed (at which no visible flow-induced defect was obtained) was used for this study. Among the pin profiles, the triangular tapered pin produced significantly improved intermingling/inter-material flow, fragmentation, and dispersion of the primary Mg2Si particles in the AA6061-T6/Al-Mg2Si joint. The triangular tapered tool reduced the average grain sizes of the AA6061 alloy and the Mg2Si particles from 18.4 to 4.6 µm and from 115 to 7.5 µm, respectively. Intermetallic phases of Mg2Si, AlFe, Al3.21Si0.47, and Al0.7Fe3Si0.3 are formed in the weld nugget of all the AA6061/Al-Mg2Si joints. The tensile strengths of the joints fabricated with tapered cylindrical, threaded tapered cylindrical, and triangular tapered tools are 108, 139, and 141 MPa, respectively. Abrasive wear is promoted in the joints fabricated with triangular tapered tool due to the homogeneous dispersion and fragmentation of the inherent hard Mg2Si particles. The corrosion attack is dependent on the fragmentation level of the Mg2Si phase. Triangular tapered pin tool is recommended for dissimilar Al/composite welding due to favorable tool-induced material flow, dispersion, and fragmentation of reinforcement. [ABSTRACT FROM AUTHOR]

Ultrasonic welding is a common fusion bonding technique to join unreinforced and reinforced thermoplastics. It is expected that applying ultrasonic vibrations to thermoset prepregs can produce heat generation to promote resin flow and consolidation. This paper discusses the feasibility of using ultrasonic vibrations as a high-speed repair technique for carbon fiber/epoxy prepregs to replace the traditional vacuum-bagging scarf setup. Three material types were investigated: out-of-autoclave unidirectional and plain weave prepregs (Cycom® 5320) and a general purpose twill weave prepreg (AS4/Newport 301). Two welding modes were considered: time and travel (vibrations stop once the desired vertical displacement is reached). For each mode, vibration time, travel, force, and amplitude were investigated. Cross-sectional analysis showed that void content equal to or below the vacuum-bagged samples could be achieved with ultrasonic consolidation to meet aerospace standards (≤2%). The following ultrasonic parameters were recommended to preserve prepreg tows integrity and minimize void content: vibration time below 1.0 s, travel between 12.5% and 50% of sample's initial thickness, force equal to or below 100 N, and amplitude below 41.3 μm. Temperature values recorded during the ultrasonic process reached the manufacturer's cure temperature range (120℃ to 180℃), with a predicted maximum degree of cure of 0.24. Interlaminar shear strength values were comparable for ultrasonically consolidated and vacuum-bagged samples. Soft and hard repair patches were applied to open-hole tensile coupons, with up to 50% strength recovery for both repair methods. Overall, ultrasonic consolidation has potential as a time- and cost-efficient repair method for thermoset prepregs. [ABSTRACT FROM AUTHOR]

In this paper, a new bi-tubular corrugated composite tube, consisting of inner and outer cylindrical and conical tubes is proposed. Different models with various geometrical parameters including the radius of curvatures and their numbers are considered and studied numerically in axial and oblique crushing in order to achieve favorable crashworthiness parameters. Moreover, quasi-static compression tests have been conducted to obtain results in order to validate the finite element model. There has been a sensible agreement between the numerical results and experimental data. Finite element models are also validated using the analytical solutions for both straight and corrugated composite tubes. Regardless of the number and radius of curvatures, as the crashworthiness of bi-tubular corrugated structures both in axial and oblique crushing is investigated and compared with their single-wall and bi-tubular straight peers, a considerable improvement is achieved in all crashworthiness parameters, including desirable increase in specific energy absorption, favorable reduction in peak force, and consequently a beneficial rise in crushing force efficiency. In addition, an optimization study using a suitable multi-objective function is done to choose the best model among the existing models, in addition to finding an optimum model via genetic algorithm. In the next step, a parametric study is conducted on the best model to inspect how well it undergoes oblique crushing at different angles. Finally, this best model and two other candidates have been chosen to investigate the effect of using foams and then the energy absorption capability of the empty and foam-filled tubes has been compared. [ABSTRACT FROM AUTHOR]

The bast fibres, a subgroup of natural fibre family, have emerged as a strong competitor of widely used man-made glass fibre for use as fillers or reinforcing materials in certain types of composite materials, which do not require very high mechanical resistance. This paper investigates the manufacturing of multi-layered jute fabric-reinforced thermoplastic composite and its mechanical performance. Hessian jute fabrics in two, four and six layers without any pre-treatment were sandwiched in 0° orientation into seven layers of high-density polyethylene sheets and pressed at high temperature and pressure to form composite laminates having three different structural designs. The laminates with two, four and six layers contain approximately 6.70 wt%, 12.90 wt% and 18.50 wt% of jute fibres, respectively. Mechanical performance of the composite laminates having four and six layers of jute fabric was found to have improved significantly when compared to the pure high-density polyethylene laminates. Within a given sample thickness of 6.5 mm, the laminate with six layers of jute fabric exhibited the best mechanical performance. Optical microscopic analysis revealed that the yarn orientation of the fabrics within the composites remained stable, and there was no visible void in the laminate structure. Fracture morphology of the composite investigated by a scanning electron microscope showed good adhesion of the jute fabrics with the high-density polyethylene matrix. [ABSTRACT FROM AUTHOR]

In this paper, we set to examine the inter-laminar shear strength of a fiber-reinforced composite part manufactured via a stepped-concurrent ultraviolet curing and layering process. This process was specifically proposed for making epoxy-based thick parts, whereby a layer-by-layer, model-based, optimal layering time and ultraviolet control scheme is set up with the objective of minimizing the degree of cure deviation across the final thick part. We focus on a cationic curing process wherein additional energy savings are possible by switching off the ultraviolet source after initiating the curing reaction with the ultraviolet source at each layer addition. Since the inter-laminar sheer strength of parts made via a layering process is often a concern, we consider the application of in-situ consolidation pressure in the layering process. We then characterize the inter-laminar shear strength by manufacturing samples with application of different in-situ consolidation pressures and measuring the inter-laminar shear strength of each sample by the short-beam shear test. The results showed that the inter-laminar shear strength of composite parts fabricated with the proposed stepped-concurrent curing, and layering process increases with the applied consolidation pressure up to a point. Scanning electron microscopy of samples cured at different in-situ consolidation pressure showed that the sample with optimum consolidation pressure has relatively uniform fiber to resin distribution and hence improved inter-laminar shear strength. [ABSTRACT FROM AUTHOR]

During the last decades, environmental concerns and limited resources have set focus of research on lightweight, mechanically high-performing structures for the transportation industry, in order to reduce fuel consumptions and CO2 emissions. Friction Stir Joining (FSJ), as a variant of the Friction Stir Welding (FSW), is an innovative friction-based joining technique for metal-composite hybrid structures. Joining in the plasticized state below the melting temperature of the metal leads to a comparatively small heat-affected zone, so that only minor metallurgical changes occur. Additionally, only a short processing time and no additional weight in form of fasteners is needed. The main objective of this study is to evaluate the feasibility of metal-composite structures via FSJ, intending to enable a macro-mechanical interlocking bonding mechanism. Main focus was given to the integration of an aluminium nub inserted in a carbon fiber-reinforced polyphenylene sulfide (CF-PPS) sheet, to ensure sufficient plasticization of the aluminium part and no degradation in the polymer part. Residual stress arising from the friction stir joining process was also characterised using the Contour method. In this study, aluminium alloy 6082-T6 and CF-PPS composite sheets were used to produce long lap joints. Results have shown that the joints were created at almost constant peak temperature slightly above the melting temperature of the PPS but no physical-chemical changes were detected in the PPS. In addition, the influence of a PPS film as interlayer between the sheets was investigated in order to explore a method for preventing galvanic corrosion. Preliminary results indicate that it is not possible to integrate a metal nub to the CF-PPS without interrupting the PPS film. However, it is possible to create a nub within the PPS film. [ABSTRACT FROM AUTHOR]

In this paper, the static and fatigue behavior of flax fiber-reinforced composites with and without an interleaved natural viscoelastic layer are investigated. Viscoelastic composite plates consist of a soft natural viscoelastic layer which is confined between two identical flax fiber reinforced composites. Different stacking sequences of specimens are tested with uniaxial tensile loading until failure. The mechanical behavior and the acoustic activity of damage sources in various configurations with and without a viscoelastic layer are compared. The analysis of acoustic emission signals and the macroscopic and microscopic observations led to the identification of the main acoustic signatures of different damage modes dominant in each type of composites (with and without a viscoelastic layer). These results allow better identification of the influence of the impact of a viscoelastic layer on the mechanical behavior of different composites. In addition, static and fatigue flexural behavior of unidirectional composites with and without viscoelastic layer are characterized in 3-point bending tests. The effects of viscoelastic layer on the stiffness, hysteresis loops, and loss factor are studied for various numbers of cycles during cyclic fatigue. [ABSTRACT FROM AUTHOR]

In this paper, bacteria belonging to the species Acetobacter xylinum were used to modify the surface of natural fibers by depositing nanosized bacterial cellulose around natural fibers which enhances their adhesion to renewable polymers. Single fiber tensile test was used in order to determine their mechanical properties and surface. The practical adhesion between the modified fibers and the renewable polymers cellulose acetate butyrate is quantified using the single fiber pullout test. Simple weight gain measurements before and after the modification show that about 4 and 6% bacterial cellulose adheres to the fibers as a result of the bacterial modification procedure. Scanning electron microscopy micrographs confirm the presence of attached bacterial cellulose on the surfaces of natural fibers. [ABSTRACT FROM AUTHOR]

This paper presents an optimization of the mechanical properties of cement–Posidonia composite by means of experimental characterization. We are interested in producing composite of cement reinforced by Posidonia raw fibers. The ratio of fibers and water-to-cement mass is varied to determine its effect on the mechanical properties of the cement–Posidonia composite, particularly, its resistance to fracture. Three point bending and compression tests were carried out to study the mechanical properties of the composite. Scanning electron microscopy was used to examine the surface of the tested samples. The experimental investigation shows an improvement of flexural strength for a ratio W/C equal to 0.5 and fiber content equal to 10 vol%. [ABSTRACT FROM AUTHOR]

Tracking the variability of natural fiber-based fabrics properties, such as local areal weight, fiber volume fraction, and therefore permeability, is crucial to optimize the parts processing of the bio-composites. This paper aims at developing a cost-effective and efficient optical method in order to predict the permeability of flax fabrics used in liquid composite molding processes. This method using an LCD monitor as light source and a reflex camera as a measurement device is based on light transmission measurement through fabric thickness. The raw data given by the camera are gray scale maps, transformed into areal weight maps. FEM software based on levelset method is finally used to highlight the influence of the local variability of the fiber volume fraction, and of the related fabrics porosity and permeability on the mold filling time. The proposed method can be directly implemented on the manufacturing line of the composites. It can be used to optimize, part-to-part, the resin consumption by predicting the resin flow through perform. Interestingly, this novel optical method is auto-calibrated and does not depend on picture resolution. [ABSTRACT FROM AUTHOR]

Compared to the laminated composite made of two-dimensional preform, the three-dimensional-woven composites have been evolved as an attractive structural material. Based on the classical textile technologies such as weaving, braiding, and knitting, complex preforms have been fabricated as reinforcement for technical applications. In the recent years, many studies have been published on the development of modeling techniques to design, analyze, and understand the effect of the preform architecture on its mechanical behavior. In this paper, we propose to use these numerical approaches to evaluate the mechanical performance of multiaxis three-dimensional-woven preform. Also, comparisons of the estimated mechanical properties of this structure with that of 3D orthogonal-woven preform and that of classical laminate are conduced. [ABSTRACT FROM PUBLISHER]

Natural fibres have been widely studied as environment-friendly alternative to synthetic ones in composite manufacturing; they can be considered as reinforcement of geopolymeric foams for use as insulating materials. In this regard, the paper focuses on lightweight geopolymers reinforced with hemp fibre grids. These novel composite materials, produced in different ways, are characterized by means of wide experimental tests and their properties are compared with the ones of the plain geopolymer. Morphological analysis shows good bonding between the matrix and the hemp reinforcement; in addition, the main physical properties of the foam are not negatively affected by the presence of hemp fibres. From a mechanical point of view, the composites when subjected to quasi-static load conditions do not collapse in a brittle manner, and show improved flexural strength under dynamic load conditions. Finally, from thermal analysis, they guarantee a good thermal stability. [ABSTRACT FROM AUTHOR]

Voids are a concern in composite materials, as they may have a negative effect on the mechanical properties of the laminates. Voids may develop especially in low cost or off-optimum process conditions. In this study, samples of glass reinforced epoxy laminates with void volume fractions in the 0.5–7% range were successfully obtained by varying the vacuum in the hand layup vacuum bagging manufacturing process. Void content was experimentally characterized using four different methods: ultrasonic scanning, epoxy burn off, serial sectioning, and X-ray computed tomography. The goal of this paper was to determine how the methods compared with respect to each other at quantifying void content. The specimens were taken from nearby locations in the same panels, so a true comparison of the methods could be obtained. The results showed, for the specific material and manufacturing conditions used, that the four different techniques can quantify voids content but with a large variation in the accuracy. X-ray computed tomography was the most successful technique to characterize voids, followed by serial sectioning. Ultrasonic scanning and epoxy burn off were not recommended techniques to characterize voids for laminates manufactured with these materials and process conditions. However, epoxy burn off was a successful technique to calculate fiber and resin weight fraction. [ABSTRACT FROM AUTHOR]

This paper presents a probabilistic first ply failure analysis of composite laminates using a high-fidelity multi-scale approach called M-SaF (Micromechanics-based approach for Static Failure). To this end, square and hexagonal representative unit cells of composites are developed to calculate constituent stresses with the help of a bridging matrix between macro and micro stresses referred to as the stress amplification factor matrix. Separate failure criteria are applied to each of the constituents (fiber, matrix, and interface) in order to calculate the damage state. The successful implementation of M-SaF requires strength properties of the constituents which are the most difficult and expensive to characterize experimentally, limiting the use of M-SaF in the early design stages of a structure. This obstacle is overcome by integrating a Bayesian inference approach with M-SaF. An academic sample problem of a cantilever beam is used to first demonstrate the calibration procedure. Bayesian inference calibrates the M-SaF first ply failure model parameters as posterior distributions from the prior probability density functions drawn from lamina test data. The posterior statistics were then used to calculate probabilistic first ply failure for a range of different laminates. [ABSTRACT FROM AUTHOR]

Two new equations are developed for effective electrical conductivity of concentrated particulate composites using a differential scheme along with the solution of an infinitely dilute dispersion of particles in a continuous matrix. The proposed equations are evaluated using 16 sets of experimental data on the electrical conductivity of two-phase particulate systems. The following model developed in the paper describes the experimental data very well: (σ/ σ-m)1/3 (σd - σm)/ (σd - σ)=(1- φ/φm)-αφm) where σ, σm and σd are electrical conductivities of composite, matrix, and dispersed phase (filler) respectively, φ is volume fraction of filler, φm is the maximum packing volume fraction of filler, and α is a constant of the order of unity. In the special case of α = 1 and φm = 1, this model reduces to the well-known Bruggeman equation for the electrical conductivity of two-phase particulate systems. The predictions of the proposed model are significantly different from the predictions of the existing general effective media (GEM) model. [ABSTRACT FROM AUTHOR]

New models for the elastic properties (Young's and shear moduli, bulk modulus, and Poisson s ratio) of two-phase pore-solid composites are developed using the differential effective medium approach (DEMA). Out of the four models developed in the paper, two models predict the elastic properties to be a function of only one variable, that is, porosity (volume fraction of pores). The remaining two models include an additional parameter, namely the maximum packing volume fraction of pores. The proposed models are evaluated using published experimental data on pore-solid composites, covering a broad range of porosity. [ABSTRACT FROM AUTHOR]

In this paper, a feasible methodology is proposed for the damage-tolerant design of composite sandwich structures and the residual safety evaluation of low-velocity impacted composite sandwich structures. This methodology is based upon the implementation of an event-driven failure criterion called Damage Propagation Criterion. In this criterion, the impact-damaged composite sandwich structure is considered to be unsafe if the damage will propagate to a specified characteristic location near the damage zone under the desired compressive load. The critical far field stress corresponding to the initiation of damage propagation at the specified characteristic location is used as the only characteristic value in this criterion. For a sandwich structure with low-velocity impact damage, this critical far field stress can be determined through either Sandwich Compression After Impact (SCAI) test or numerical analysis by using a modified analytical model or a nonlinear finite element model. Both the experimental and numerical approaches are briefly introduced with examples in this paper. At the end of this paper, the feasible methodology is introduced for the engineering applications in damage-tolerant design of composite sandwich structures and the residual safety evaluation of impact-damaged composite sandwich panels. [ABSTRACT FROM AUTHOR]

This research has been carried on to analyze the capability of a new composite to be effectively used as shielding of gamma radiation. The preparation of a metal-organic composite formed by tungsten and Kraft lignin is presented. Samples have been characterized through X-rays and scanning electron microscopy measurements. The results led to the study of the different phase formations. The microscopic analyzes indicate that two different phases are present in the composite. The absence of oxidation in the process even after the temperature treatment imposed to form the sample has also been noticed. Measurements of the attenuation have been performed to study its ability to absorb gamma radiation. A sample of cobalt 60 (Co-60), for which the peak energies are at 1173 keV and 1332 keV, was used as a source of gamma radiation in the experiment of attenuation. The measured attenuation of gamma radiations when the composite is placed to act as a shield is only 16% smaller than the attenuation obtained for standard pure tungsten. This is a clear indication that the new metal-organic composite is suitable for the fabrication of devices dedicated to shielding radiation, with the advantage of being easier to manipulate. [ABSTRACT FROM AUTHOR]

Atheoretical investigation of initial local buckling and post-buckling behavior of composite I-sections is presented. The equilibrium equation for initial buckling is solved both exactly and with an approximate method, namely, Galerkin’s method. In Galerkin’s method, the out-of-plane deflection of each plate element is approximated by a weighted sum of polynomial functions. The post-buckling response is studied as an extension of the approximate analysis with Galerkin’s method where now both equilibrium and compatibility equations must be solved. The bending deflection in the post-buckling regime is assumed to be a magnification of the deflection function used in initial buckling analysis. No mode-shape change is thus allowed in the post-buckling region. A polynomial type of function is also adopted for stress distribution in order to take into account the deviation from uniform in-plane load distribution in the plate elements following the onset of local buckling. The paper provides an efficient and accurate method for predicting the post-buckling behavior of composite structural sections composed of plate elements. Galerkin’s methodwas previously applied to isotropic flat plates only. The present approach is tested against a commercial code, STAGS, with a very good agreement in results and a very large saving in computer time for post-buckling analysis of an I-section. [ABSTRACT FROM PUBLISHER]

Filament wound curved composite parts which are autoclave cured tend to exhibit "spring-in," a permanent deformation due to residual stresses (Stover, 1993). Spring-in is the tendency of a circular part to reduce it's radius of curvature upon radial cutting. The causes of spring-in and the associated residual stresses must be understood to ensure dimensional accuracy and safety of the final cured part. The purpose of this paper is to isolate and quantify the principle causes of spring-in in filament wound unidirectional autoclave cured carbon fiber/epoxy composite hoops.This paper first explores the causes of spring-in as given in the literature: through thickness inhomogeneity (Radford, 1993), part compaction (Meink, 1998), and anisotropy with initial curvature (Kollar, 1992). Next, the development of the residual stress profile through the fabrication and cure process is given along with the governing equations for the spring-in phenomenon. Finally, an experimental procedure is developed to obtain the residual stress profile in a thin composite part. This methodology involves progressive cutting (i.e., stress relief) and associated strain gage monitoring. The strain gage data is then combined with a finite element (FEM) analysis to determine the through-thickness residual stress profile.From the residual stress profile experiments, it was found that the principal cause of spring-in is not material anisotropy as was previously thought. Instead, the principle cause of spring-in was found to be the gradual thermal expansion of the mandrel during cure, which causes a "tension-lag" in the curing part. In addition, the compaction of the part during cure was found to have a nominal contribution to total spring-in. [ABSTRACT FROM PUBLISHER]

This paper aims to develop a numerical nonlinear progressive damage model for laminated composite materials considering in-plane and out-of-plane shear stresses in combination with cohesive interface elements to predict the structural response and the failure mechanisms of laminated composite materials. For this purpose, the constitutive models for intralaminar and interlaminar damage modes have been developed as a numerical code by a UMAT subroutine and implemented in commercial finite element software. This model, which is based on the continuum damage mechanics approach, enables to predict the gradual degradation of material properties with five distinct damage parameters for different failure modes; three of these damage factors apply the shear damage contribution as a separate damage mode by a separate damage factor into the model and characterize it by shear damage dissipation energy, and two parameters for fiber and matrix in transverse directions. Also, a series of experiments have been performed to characterize and validate the nonlinear behavior of glass/epoxy laminate. This model is used to predict the behavior and the final strength of open-hole tension specimens. A reasonably good agreement was also achieved between numerical predictions and experimental observations in terms of shapes, orientations and sizes of individual intraply damages induced around the notch and also the final strength of open-hole tension specimen. [ABSTRACT FROM AUTHOR]

This paper reports the dielectric relaxation studies of carbon nanotubes loaded in polyester polymer matrix. The study was carried out in the frequency range between 100 Hz and 1 MHz at constant temperature, T = 300 K. The frequency dependence of the electrical data was treated in the frameworks of the impedance Havriliak-Negami formalism and by using the universal Jonscher power law. The imaginary and real parts of the dielectric permittivity change with concentration of the carbon nanotubes. This work consists in studying the influence of these nanoparticles on the dielectric properties, describing the electrical relaxation and the conduction mechanisms. [ABSTRACT FROM AUTHOR]

The paper investigates new composites fully based on wastes of polyethylene terephthalate, rubber, high-density polyethylene, and wood, aiming at multifunctional, environmental-friendly materials, for indoor and outdoor applications. The rubber: polyethylene terephthalate: high-density polyethylene: wood ratio and compression molding temperatures are optimized considering the output mechanical properties, focusing on increasing the waste polyethylene terephthalate content. To investigate the durability in the working conditions, the water-stable composites, with good tensile and compression strengths were exposed to surfactant systems, saline aerosols, and ultraviolet radiations. The results prove that surfactant immersion improves the interfaces and the mechanical properties and a pre-conditioning step involving the dodecyltrimethylammonium bromide surfactant is recommended, prior application. The interfaces and the bulk composites were investigated by X-ray diffraction, Fourier-transform infrared, differential scanning calorimetry, contact angle measurements, scanning electron microscopy, atomic force microscopy, to identify the properties that influence the mechanical behavior and durability. The composites containing 30% of polyethylene terephthalate, obtained at 160℃ and 190℃ have a good combination of mechanical properties and durability that is enhanced by the plasticizing effect of water and surfactants. The compressive strength of the composite processed at 190℃ was 51.2 MPa and the value increased to 58.4 MPa after water immersion. The ultraviolet and saline exposure slightly diminished this effect; however, long time testing (120 h) ended up with values higher than those corresponding to the pristine composite: 55.3 MPa after ultraviolet and 57.1 MPa after saline exposure. [ABSTRACT FROM AUTHOR]

Under environmental conditions, triaxial braided composites exhibit complex behavior and damage mechanisms. This paper investigates the damage mechanisms of these complex composites under varying environmental conditions. Tensile, compressive, and shear specimens of triaxial braided composite material were tested at room, hot (100℃), and hot/wet conditions (60℃/90% relative humidity). The strain field was studied using a digital image correlation system and the effect that the specimens’ edges have on the strain field was quantified. For the tension specimens, the environmental conditions caused reductions in the elastic and failure properties, whereas the compression specimens exhibited degradation exclusively in the failure properties. An increase in temperature rather than humidity was found to be a driving factor for the degradation of the mechanical properties. A non-destructive, flash thermography technique was used to characterize surface/subsurface damage in the specimens. Scanning electron microscopy was conducted to determine the microstructural modes of failure. [ABSTRACT FROM AUTHOR]

A technique is developed in this paper to utilize a four-point-bend test for piecewise construction of the stress-strain curve for the outer fibers (tension and compression) of the sample. The generated flexural stress-strain curves are compared to uniaxial tension and compression stress-strain curves. Initial agreement is excellent while in the latter stages the flexural curves are stiffer and exhibit significantly higher failure stresses. This technique fully allows for the material softening (nonlinear material behavior) due to the onset of damage well below ultimate fracture as well as different stress-strain behavior in tension and compression. The potential importance of this technique in providing corrected estimates of both tensile and compressive elastic moduli and failure stress, cf. ASTM D 790-91, is discussed. [ABSTRACT FROM PUBLISHER]

The penetration of the polymeric melt into the fine interstices (typically of the order of a few microns or less) between the fibers of a thermoplastic prepreg depends upon the shape, volume fraction and geometry of packing of the fibers, the rheology of the polymeric melt and the interfacial surface tension. A model for preimpregnation would help determine optimal processing variables. In this paper the methods currently used to prepare thermoplastic prepregs are briefly reviewed and a preimpregnation model (which is generic to most processes involving melt impregnation) for describing the flow of resin into fiber-tows is developed. Such flows are usually modelled using Darcy's Law, but the approach of this paper represents an improvement in that empirical parameters (like permeability) are not required and detailed flow profiles can also be calculated. The effect of different geometries of packing on processing variables (fiber spacing, impregnation time, pressure drop) and flow characteristics (velocity, planar elongation and shear rates) is calculated. The fiber spacing (which depends upon the fiber volume fraction and geom etry of packing) has been found to be a critical parameter. The degree of shear-thinning of the polymeric melt on the pressure drop during flow through reinforcement is found to be very significant. Numerical values for the various flow and processing variables are calculated based upon experimental data reported in the patent literature. The value of the model is that it lets one predict the processing parameters required to completely impreg nate the tow and whether effective impregnation is feasible for a given polymer- reinforcement system. [ABSTRACT FROM PUBLISHER]

In this study, nanoalumina (Al2O3) and nanosilica (SiO2) particles are evaluated as tougheners for a high glass-transition temperature (Tg) epoxy system in correlation with liquid composite molding (LCM) processability. The aim of this paper is to directly compare the effectiveness of nanoalumina and nanosilica of the same nominal particle size as epoxy tougheners on the same neat resin system. The epoxy resin system used in this study was Dow D.E.R. 330 amine cured epoxy with a Tg of 150℃. Both particle types are observed to be Tg neutral and increase fracture toughness of the base epoxy system. Between the two particle types, nanoalumina is found to be more effective than nanosilica in terms of achievable fracture toughness at a given particle loading. As resin viscosity increases with particle addition, the addition of fewer particles with the use of nanoalumina is also beneficial to LCM processing where a lower viscosity is preferable. [ABSTRACT FROM AUTHOR]

A probabilistic model for estimating the fatigue life of laminated composite plates is developed. The model is based on lamina-level input data, making it possible to predict fatigue properties for a wide range of laminate configurations. Model parameters are estimated by Bayesian inference. The reference data used consists of constant-amplitude cycle test results for four laminates with different layup configurations. The paper describes the modeling techniques and the parameter estimation procedure, supported by an illustrative application. [ABSTRACT FROM AUTHOR]

Multi-layered glass and carbon-reinforced polymer composites may exhibit unique properties comparatively with the benchmark, proven they are being tailored bounded by several requirements. The paper herein approaches issues on the influence of the various contents and orientation of UD carbon fibre constitutive on the mechanical, dynamical and thermal expansion if embedded along with glass fibres in different stacking sequencing within an unsaturated polymer resin. The results show that the architectures with the highest content of carbon fibres (e.g. GF:CF(60:40) – 0° and 90°) provide the best tensile and flexural properties, and behave better under dynamical loading conditions and temperature variations, no matter the orientation directions. In addition, it was shown that a thorough understanding can be attained, with respect to the UD carbon fibre content, and different orientations influence on the overall composite material properties, taking into account the data retrieved from dynamical and thermal expansion runs. [ABSTRACT FROM AUTHOR]

The design of silicon nitride reinforced by silicon nitride (SiC) nanoparticles is attractive due to its improved high-temperature strength and resistance to oxidation. The high-temperature mechanical behaviors of these composites are difficult to be investigated because it is hard to measure these structural changes in the mechanical test. In this paper, we applied the molecular dynamics simulations to study the effects of size and quantity of SiC clusters on mechanical properties of silicon nitride-based composites. The results show the model with a SiC diameter of 2.4 nm has the largest stress and Young’s modulus, and the stress and Young’s modulus tend to be smaller when the quantity of SiC clusters increases. With an increasing temperature, the Young’s modulus of the structure with the largest clusters decreases before 1300 K and then jumps to 97 GPa at 1500 K, both the loss and jump in Young modulus is related to the interfacial effects between matrix and clusters. [ABSTRACT FROM AUTHOR]

To lighten structures, many metallic components, such as aircraft wings, are being replaced by composite components. To join these components with the rest of the structure, various joining techniques are used. When using multiple bolted joints, bypass vs. bearing loading is developed around each joint. The ratio of bearing to bypass loading is known to affect the level of load at which failure occurs. There have been many models created to predict failure within composites but very little work has been carried out to investigate how well numerical models predict failure within bolted joints subjected to bearing and bypass loading. In addition, few models have been developed that account for the through thickness stresses that are developed underneath the bearing load. This paper compares a range of failure criteria and degradation models utilizing a three-dimensional model and compares how well they predict failure for bearing vs. bypass loading for a supported-pin-loaded joint. [ABSTRACT FROM AUTHOR]

In the present paper, a novel superabsorbent hydrogel composite based on gelatin have been prepared via graft copolymerization of acrylic acid (AA) in the presence of kaolin powder using methylenebisacrylamide (MBA) as a crosslinking agent and ammonium persulfate (APS) as an initiator. Certain variables of the graft copolymerization (i.e., the monomer, the initiator, and the crosslinker concentration as well as kaolin/gelatin weight ratio) affected on the ultimate water-swelling capacity were optimized. The composite structure was confirmed using Fourier transform infrared (FTIR) spectroscopy. A new absorption band at 1728cm-1 in the composite spectrum confirmed kaolin-organic polymer linkage. The effect of kaolin amount and MBA concentration showed that with increase of these parameters, the water absorbency of the superabsorbent composite was decreased. The swelling measurements of the hydrogels were conducted in aqueous salt solutions. Results indicated that the swelling ratios decreased with an increase in the ionic strength of the salt solutions. This behavior can be attributed to 'charge screening effect' for monovalent cations, as well as 'ionic crosslinking" for multivalent cations. The effect of solutions with different pHs on the swelling of the superabsorbent was also studied and the hydrogels exhibited pH responsiveness, so a swelling-deswelling pulsatile behavior was recorded at pHs 2 and 8. Finally, the swelling kinetics of the synthesized composite with various particle sizes was preliminarily investigated as well. [ABSTRACT FROM AUTHOR]

As composite materials are gaining wide acceptance in aircraft structures, the repair of damaged composites is becoming an important issue. Composite repair, however, has been performed more or less in a manual fashion, an integrated repair system is desirable. In this paper, an Internet-based advisory service (Repair Advisory Service, RAS) for composite repair is proposed to increase the efficiency of the repair process. Based on the client--server architecture, RAS offers time-effective sharing of engineering knowledge between remote locations, easy maintenance and update of the application software, and ubiquitous access to the services. The web browser is used as the user interface, which provides an easy access to the service and software friendly environment. The RAS system provides integrated design and repair tools for repair technicians and engineers as follows: (1) A design software for scarf repair and lap repair estimates the tensile failure and shear failure of repaired structures. (2) A structural repair manual is provided in the PDF document for viewing, and a web search engine can search the document. (3) A fabrication module of RAS generates a CNC toolpath to cut repair patches. (4) A CNC-based prepreg cutting machine has been developed to implement an automated composite repair. A scenario for the web-based remote repair was proposed, and a test repair was performed on an aircraft wing. The RAS service is open to public and available at http://fab.snu.ac.kr/ras. [ABSTRACT FROM AUTHOR]

The fabrication of nano reinforced metal matrix composites (MMCs) is scientifically essential to further upgrade the thermal, electrical, and mechanical properties of current MMCs. Here, we present a green molecular mixing (MLM) of silver (Ag) and graphene (G) to overcome these challenges. Experimental results show that the green MLM allows to homogeneously mix 3 vol. % of G as observed with traditional MLM while adding a control on the oxidation steps. Metal and GO reductions can be precisely control by varying the solution temperature. Highly thermal conductivity and strong elastic modulus nanocomposites are fabricated making it an eco-friendlier mixing process. [ABSTRACT FROM AUTHOR]

This study used the vacuum bag-only process to fabricate angle composite parts on a complex-shape mold by employing interleaved unidirectional prepreg and dry fiber. The study focuses on the thickness variation along the angle part by analyzing the thickness distribution within the angle part by the statistic method. The corner thickness deviation (CTD) and angle deviation (AD) of the angle part were used to quantify the quality of the part by different process parameters and mold design. The results indicated that the CTD and AD were both linearly decreased with the increase of the part thickness. Besides, the part fabricated with the convex mold presented higher dimensional stability and uniformity as compared to that with the concave mold. In addition, we investigated the effect of caul plate applied at the corner regions, and the results showed that the lead caul plate could effectively improve the thickness change in the corner area in both the concave and convex mold. In order to fully understand the thickness variation of the part at the corner region, a semi-empirical model was proposed to forecast the corner thickness deviation of angle part. In this study, the prediction errors of the semi-empirical model for the parts by the concave and convex molds are 10% and 25%, respectively. [ABSTRACT FROM AUTHOR]

Hybrid carbon fiber and glass fiber reinforced thermoset composites are commonly used in structural applications throughout the energy, aerospace, automotive, and marine industries. The differing fiber types can be combined into a hybrid composite with the intent to efficiently utilize the beneficial properties of each type of reinforcement. The fiber fraction of each individual fiber type influences the mechanical properties of the overall composite. Measurement of the fiber fraction of each individual fiber type allows understanding and prediction of the mechanical properties as well as a method of quality control in manufacturing. A combination of a carbonization in nitrogen method for the measurement of fiber content in carbon fiber reinforced composites and an oxidation method to measure the glass fiber content are used for the first time. It has been demonstrated that the approach is effective at measuring the carbon fiber and glass fiber contents in hybrid glass and carbon fiber thermoset composites in both epoxy and vinyl ester resin systems. [ABSTRACT FROM AUTHOR]

Toner composite is an important element in laser printing and copying processes which is included magnetite, carbon black and binder. The emulsion aggregation is a popular method for toner preparation that allows tight control on toner properties. This study was done to determine the effects of the basic ingredients (magnetite, carbon black, and styrene-acrylic copolymer) that affect the properties of toner composites. Particle size analysis (PSA), scanning electron microscopy (SEM), and field emission scanning electron microscopy (FE-SEM) were used to study particle size, shape, and morphology of the toner composites. It was found that using carbon black, magnetite and styrene-acrylic copolymer with small particle size produced toner composite with small particle size. It was also shown that using a styrene-acrylic copolymer with high viscosity resulted in smaller and narrower particle distribution in the resultant toner. The results determined by SEM confirmed that carbon black, magnetite and styrene-acrylic copolymer particle size did not affect the toner’s shape, and that it remained spherical. [ABSTRACT FROM AUTHOR]

In this study, a finite element strategy is implemented for more accurate simulation of nonlinear failure behavior in multidirectional laminated composites. The considered approach is based on the parameters which could affect the SERR2. Accordingly, interaction between delamination and matrix cracking as well as mixed stress distribution around the fiber due to mismatch in properties of adjacent plies with different fiber angle orientation influence the strain energy release rate. Thus, in the present research the prediction of the damage behavior of cross-ply laminates is improved considering the effects of matrix cracking and inhomogeneous fiber-matrix modeling. For simulation of the delamination and the matrix cracking damage, cohesive interface elements are implemented. They have been used perpendicularly inside the 90o plies for the matrix cracking. CZM3 approach, ABAQUS subroutine and Python programming were utilized for automatic modeling of matrix cracking. The resultant load–displacement curve in DCB4 test of cross-ply laminates indicated that by considering delamination and matrix cracking simultaneously, load–displacement curve of cross-ply laminates became nonlinear earlier and the obtained results had an acceptable compatibility with experimental results. The obtained results for double cantilever beam (DCB) test specimen with cross-ply lay-up indicate that the numerically reproduced GIc parameter of 90/0 interface can be improved about 20% using this simulation. Moreover, by simulating periodic matrix cracking inside 90o plies, crack jumping, stick-slip phenomenon and local hardening behavior are anticipated such as experiments. [ABSTRACT FROM AUTHOR]

The combination of natural fibers and renewable source matrices is an option to replace materials from non-renewable sources used in the manufacture of composites. One example is the use of pinus sawdust and sisal fibers together with a matrix of polyurethane (PU) foam derived from vegetable oils. For the application of composites in sectors such as building or furniture industry, one of the necessary processes is drilling, which allows assembling through different fastening systems. The aim of this work is to investigate the drilling process of the composites of PU foam derived from vegetable oils matrix with only pinus sawdust and hybrid pinus sawdust and sisal fibers taking into account surface damages of the holes and temperature generated during the process. Results showed that drilling parameters influence on the generation of damages mainly at the edges of the holes. Lower cutting speed and feed rate were most appropriate for drilling these types of composites, and temperatures generated during drilling showed relationship with the generation of damages. [ABSTRACT FROM AUTHOR]

Two new equations are developed for effective electrical conductivity of concentrated particulate composites using a differential scheme along with the solution of an infinitely dilute dispersion of particles in a continuous matrix. The proposed equations are evaluated using 16 sets of experimental data on the electrical conductivity of two-phase particulate systems. The following model developed in the article describes the experimental data very well: (σ/σm)1/3 (σd-σm)/(σd-σ)= (1-Φ/ σm-αΦm where σ, σm and σd are electrical conductivities of composite, matrix, and dispersed phase (filler) respectively, Φ is volume fraction of filler, Φm is the maximum packing volume fraction of filler, and α is a constant of the order of unity. In the special case of α = 1 and Φm = 1, this model reduces to the well-known Bruggeman equation for the electrical conductivity of two-phase particulate systems. The predictions of the proposed model are significantly different from the predictions of the existing GEM (General Effective Media) model. [ABSTRACT FROM AUTHOR]

A method for the determination of the effective constant coefficients of the transverse coupled hygrothermal diffusion is presented. Onsager's reciprocal relation [2] is utilized to reduce the number of the diffusivity coefficients of constitutive materials from four to two. The so called cross coupling diffusivities are expressed in terms of the thermal conductivity and the coefficient of diffusion. The representative volume element of the composite is analyzed by applying a two-dimensional finite element model. It is assumed that the composite is undamaged, and there is no resistance in diffusion and heat conduction at the interface of the fiber and the matrix. The method is tested numerically with a set of different cases. The coupling affects most clearly to the heat flux in the tests. The average coupled heat flux is significantly greater than the uncoupled heat flux. [ABSTRACT FROM AUTHOR]

Polymers constitute the most employed reinforced matrices to achieve composite materials. Carbon and glass fibers are the reinforcements that are mainly utilized to improve specific properties for both thermoplastics and thermosets creating a new class of materials, which has been applied in several industrial fields. Various products made of fiber reinforced polymers are available on the market and at their own end-of-life, they must be reclaimed and remanufactured, properly, in an ideal recycling circular economy. The aim of this review is to point out the progress on fiber reclaiming from these materials, providing an overview on the most employed strategies and highlighting their main technological limits. Specifically, first, mechanical, thermal and chemical reclaiming processes have been contextualized introducing their peculiarities. Subsequently, the attention has been focalized on the new research trends proposed in the last years showing the direction, where the research is moving to. The processes have been also classified in terms of tensile strength of the reclaimed fibers and in terms of the energy required to be performed. Finally, design for reclaiming and remanufacturing treatments have been also considered pointing out the different approaches that can be pursued as valuable solutions to strengthen the recycling capabilities encouraging as much as possible the recovering of the polymeric matrix composites. [ABSTRACT FROM AUTHOR]

Polymers were introduced in agriculture in the 1950s and, nowadays, mulching is one of major applications of plastics in this sector. In the last decades, difficulties in recovery of non-degradable plastic films have prompted the intense use of biodegradable mulch films. Therefore, the aim of this work was to develop mulch films based on a biodegradable polymer (poly(butylene adipate-co-terephthalate)), sugarcane and carnauba wax. Mulch films containing 2.5 and 5.0% of sugarcane residue, with and without 2% w/w carnauba wax were prepared by flat extrusion. To evaluate the performance of mulch films during application to the soil, they were maintained for 60 days under real conditions of use. During this period, the temperature and humidity of the soil protected by mulch were measured. In addition, the chemical, physical and morphological properties of the films, after weathering, were determined. Results show that the mulch films developed have promising properties. When larger moisture retention is needed, PBAT mulch films containing 2.5% and 5.0% sugarcane residue are the most indicated and, when higher temperatures are demanded, PBAT mulch films containing carnauba wax and 2.5% and 5.0% residue are the best option. After being exposed to high temperatures and moisture, the mulch films photo degraded and their transmittance was reduced. Besides, the sugarcane residue added to the polymer swelled, which led to breakage of the polymeric matrix. [ABSTRACT FROM AUTHOR]

In this work, The hybrid hydroxyapatite (HaP), chitosan (CH) and graphene nanosheets (GNS) mixtures were applied by electrospinning on the surface of GNS and Si3N4 (SN) binary powder reinforced hybrid titanium (Ti) metal composites surface to improve composite biosurface functionallity. The surfaces of coated materials were characterized and antibacterial tests were carried on for their suitability in the industry by performing artificial body fluid tests. The hybrid nano fiber coatings formed a homogeneous structure on the composite. According to bioactivity tests and microstructure analysis, it was seen that HaP, which has the best results in the change of pH (pH= 11.80) values. The lowest mass change (0.0005 g) was observed on the 10th day of pure titanium. The highest mass change (0.0210 g) was obtained as on the HaP coated hybrid titanium composite. According to the antibacterial test result the hybrid nanofiber containing silver (Ag+) doped HaP on Ti composites showed the best antibacterial property aganist the E.coli. The fabricated electrospin coated hybrid composites can be a potantial candidate for dental, orthopedic implant applications and tissue engineering. [ABSTRACT FROM AUTHOR]

To understand the crucial effect of core, the mechanical performances of composite sandwich panel (CSP) with different core configurations are numerically evaluated under the planar compression loading. The failure behavior of metal core and fiber face-sheets are modeled with the ductile damage model and linear damage evolution model respectively. The numerical evaluation model validated with W-shaped core CSP is used to predict the mechanical behaviors and failure mechanisms of CSP with X-, Y- and A-shaped cores. The damage deformation, energy-absorption (EA) and energy absorption efficiency (EAE) are revealed to understand the compression performances, and effects of core thickness are further studied. Results show that the evaluation model is proved to be accurate due to a consistent match between simulated and available experimental results. It is found that compared to traditional W-shaped core, the CSPs with these three cores have higher resistance to deformation and energy absorption properties. Increasing the thickness of each core, the peak load, average load and EAE obviously increase but the stability decreases. The A-shaped core with enough thickness is recommended as the most potential core applied in composite sandwich structures. [ABSTRACT FROM AUTHOR]