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This paper describes the development of electrically insulating hybrid steel–composite cross-arm for medium-voltage overhead distribution power lines with three conductors. The main goal was to achieve substantial weight savings compared to a common steel cross-arm with three ceramic insulators widely used in the Czech Republic. The structure has been designed as a combination of two main composite components – a pultruded horizontal profile and a braided vertical conical tube. These components are connected by a steel locking component which also serves for attaching the whole cross-arm to common concrete utility poles. The hybrid cross-arm is conceived to insulate electrical current in the default configuration thanks to the properties of the glass fiber reinforced composite materials, yet additional insulating elements can be used to further increase the electrical insulation distance. The materials and components used for the construction are chosen by combining finite element simulations of mechanical and electrical behavior with experimental testing and verification of important characteristics on selected substructures. The final design of the cross-arm is then manufactured and subjected to long-term testing in real-life environmental conditions. The achieved reduction of weight exceeds 50% while maintaining the mechanical rigidity and strength of the steel solution as well as the required insulation properties. The future trend of the cross-arm development is aiming at the weight reduction and increase of manipulation capabilities resulting in lowering many additional costs. In this work the designed cross-arm has half the weight of the original one. [ABSTRACT FROM AUTHOR]

To achieve high levels of lightweight design in automotive or aerospace industry, it is necessary to optimize certain composite material systems concerning their lightweight potential. With sandwich composites, which generally consist of a core which is coated with two face-layers, it is possible to reduce the overall part weight while increasing the specific mechanical properties under inhomogeneously distributed loads. Hereby the sandwich core ensures the load transmission while the face layers absorb the tensile and compressive loads occurring at bending stress. The aim is to increase the efficiency of said sandwiches while minimizing the weight per area by using for example (fiber-) reinforced face layers and foamed core materials. Polyphenylene sulfide (PPS) is an important high temperature, engineering thermoplastic polymer. Because of its properties it is used in the automotive, aerospace and electronics industry. As polyphenylene sulfide has a high processing temperature and requires processing know-how, there a few scientific studies on its foaming behavior. To understand the process-structure-property relationship of this material system, an experimental study on the foaming behavior of neat polyphenylene sulfide and long glass fiber reinforced polyphenylene sulfide using the high-pressure foam injection molding process is conducted. As it is also attractive from a manufacturing point of view to reduce the number of manufacturing steps and to operate with the source materials from the beginning of the process an in-line compounding and foam injection molding process is used. This allows the manufacturing of integral sandwich structures with connected, fiber reinforced face and foamed core layers in one shot via "direct thermoplastic foam injection molding". With the findings of this study researchers and manufacturers of neat and fiber reinforced PPS parts are able to identify process parameter boundaries and the influence of these parameters on the sandwich structure and mechanical properties. [ABSTRACT FROM AUTHOR]

Abstract The paper analyses the possibilities and effectiveness of using infrared thermography with optical thermal excitation by reflection approach in the case of multilayer composites reinforced fibers. The advantages and limitations of this technique have been demonstrated via composites reinforced with three types of fibers: glass, aramid, and carbon. The most common defects in such structures are: delaminations, fiber cracks, matrix cracks, and separation of fibers from the matrix. The paper describes some results of the numerical simulations of defects detection using optical IR thermography with the emphasis made on the inspection of multi-layer composite materials. The simulations were conducted in order to determine the detection of defects depending on the depth of their location under the front surface of composite material as well as their geometrical dimensions. [ABSTRACT FROM AUTHOR]

Abstract A hollow concrete column (HCC) is a structurally efficient construction system and uses less material. Conventionally, HCCs are reinforced with steel bars, which are prone to corrosion. This study explored the use of glass-fiber-reinforced-polymer (GFRP) composite bars as reinforcement for HCCs and evaluated the effect of the reinforcement ratio on HCC structural behavior. A total of six HCCs reinforced longitudinally with GFRP bars with different reinforcement ratios (1.78%, 1.86%, 2.67%, 2.79%, 3.72, and 4.00%) were prepared and their behavior was investigated. The different reinforcement ratios were achieved by changing the bar diameter (12.7 mm, 15.9 mm, and 19.1 mm) and number of bars (4, 6, 8, and 9 bars). The results show that increasing the diameter and number of bars enhanced the strength, ductility and confinement efficiency of HCC. For columns with equal reinforcement ratios, using more and smaller-diameter GFRP bars yielded 12% higher confinement efficiency than in the columns with fewer and larger-diameter bars. The crushing strain of the GFRP bars embedded the HCC was 52.1% of the ultimate tensile strain. Lastly, the axial-load capacity of the GFRP-reinforced HCC can be reliably predicted by considering the contribution of the concrete and up to 3000 μ ε in the longitudinal reinforcement. [ABSTRACT FROM AUTHOR]

Abstract This study reports the recently found critical drawback of glass fibers as compared with carbon fibers. GFRP and CFRP composites were placed into a general water bath at 80 °C until saturation for accelerated hygrothermal ageing. After ageing, the impact properties of CFRP composites improved. For GFRP composites, however, their performance under low-velocity impact degraded significantly. After removing the moisture absorbed during ageing, the strength of CFRP laminates increased to 95.75% of its original value. In contrast, GFRP composites only retained 74.65% of the strength after hygrothermal treatment. Compared with CFRP composites, GFRP composites were much more susceptible to hygrothermal attack. Tensile tests on single glass fiber were carried out which proved that both the strength and elongation-at-break decreased significantly after hygrothermal ageing. Decrease in glass fiber strength after ageing is the major mechanism responsible for the reduced performance of GFRP composites. [ABSTRACT FROM AUTHOR]

Abstract The influence of nano-organic montmorillonites (nano-OMMTs) on mode I and mode II interlaminar fracture toughness of continuous glass fibre reinforced ammonium polyphosphate polypropylene (CGF/PP/APP) composites was experimentally studied in this paper. Nano-OMMTs modified CGF/PP/APP unidirectional prepreg composites were fabricated by hot-melt impregnation process. To investigate the effect of nano-OMMTs on the interlaminar fracture toughness of composites, double cantilever beam (DCB) test and end-notched flexure (ENF) test were conducted to obtain the mode I initiation interlaminar fracture toughness (G IC init.), mode I propagation interlaminar fracture toughness (G IC prop.), and mode II initiation interlaminar fracture toughness (G II C init.) of samples, respectively. Through comparison against to the neat CGF/PP/APP composites, with the addition of 6.5 wt% nano-OMMTs into PP matrix, the G IC init. , G IC prop. , and G II C init. of composites can be enhanced from 0.502 kJ/m2, 0.746 kJ/m2 and 0.623 kJ/m2 to 0.712 kJ/m2, 1.119 kJ/m2, and 1.195 kJ/m2, respectively. Finally, it was found that nano-OMMTs in matrix can act as "pins", which could absorb energy in the forms of nano-particles pullout and enhance the interlaminar fracture toughness values in DCB and ENF tests. [ABSTRACT FROM AUTHOR]

Abstract The interlaminar property is essential to the application of fiber reinforced composites for the overall performance. For the purpose of enhancing the interlaminar properties, the unidirectional Glass Fibre Reinforced Plastics (GFRP) laminates were fabricated by adding carbon nanotubes (CNTs) buckypaper to the mid-plane layers under different curing pressures (1 MPa, 2 MPa, 3 MPa). In order to study the enhancement mechanism of the CNTs buckypaper, a series of experiments were designed and conducted based on double cantilever beam (DCB), end notch flexure (ENF) and interlaminar shear strength (ILSS) tests followed by scanning electron microscope (SEM) examination to study the surfaces of the specimens. It was observed that the modes I and II interlaminar fracture toughness of GFRP embedded with CNTs buckypaper were relatively higher than those without CNTs buckypaper. Considerable improvements in interlaminar shear strength of the laminates were obtained with the integration of the CNTs buckypaper between the interlayers. The micro-structure of the enhanced mixed interface of fiber-resin-CNTs responsible for interlaminar strengthening was further studied with the help of SEM. It was found that the higher curing pressure could increase the inter-diffusion of the resin and CNTs offering a stronger interfacial adhesion. The interlaminar characteristics of the specimens cured at 2 MPa pressure were found to be relatively superior with optimal interlaminar fracture toughness and interlaminar shear strength. [ABSTRACT FROM AUTHOR]

Highlights • This paper deals with the mechanical response of glued in rods. • It takes into account the diameter of rods, the material of rods and the direction of the grain of the timber. • It gives an overview and presents known design models and technical approvals comparing the different approaches. • Comparison between the experimental results and the main equations of the literature review. • The authors propose one equation for the case where the bars are parallel to the timbers' grain. • In addition, another one for the case where the bars are perpendicular to the timbers' grain. Abstract We address the mechanical response of glued in rods, taking into account parameters such as rod diameter, the material properties of the rods (carbon or glass fibre-reinforced polymer), and the grain orientation of the timber (rods parallel and perpendicular to the timber grain). All the experimental setups employ pull-out tests, where rods are glued in only on one side of the specimen, and these tests are assessed at the premises of the Laboratory for Composite Materials and Composite Structures at the University Claude Bernard Lyon I. In addition, we give an overview and present known design models, technical approvals and regulations, national standards, and guidance papers, comparing the different approaches among them and with the experimental results. Good agreement was shown and further useful results were observed. [ABSTRACT FROM AUTHOR]

Abstract The effect of impact load with low velocity in thin-walled plates and profiles has been investigated. The paper deals with the relation between damage propagation, size and shape as a function of boundary conditions, layer arrangements and impact energy. The structures under consideration were made of eight-layer Glass Fiber Reinforced Polymer (GFRP) laminate with a quasi-isotropic, quasi-orthotropic and angle ply arrangement of layers. The standardised plates predefined to CAI tests and channel section profiles have been subjected to impact load. Based on the performed tests, the impact characteristics have been obtained and compared with the theoretical model (one degree of freedom mass-spring system). Further, despite it not being mentioned in the ASTM 7136 standard, characteristic curves were identified. It was noted that the impacts introducing matrix damages and the partial fracture of the fibres significantly change the course of the Force-Time histories, particularly after the maximum impact force is reached. [ABSTRACT FROM AUTHOR]

Abstract This paper studies the feasibility of improving structural performance of composites in the presence of stress concentrators. Matrix grading through local deposition of additive-enhanced matrices and fibre steering by varying fibrous architecture are examined independently and in combination on a glass-fibre triaxial braided composite subjected to open hole tensile test. Stiffened and toughened matrices were incorporated through precise point-wise injections of liquid reactive resin into dry preforms (Liquid Resin Printing). Fibre steering was implemented by varying the braiding angle along the length of the braided sleeve. It has been shown that these novel forms of architecture modification enable a significant improvement in composite strength through a variety of deformation mechanisms. This includes local stiffening of composite in the direct vicinity of the stress concentrator and damage accumulation away from the stress concentrators. The experimental observations are explained by using simple finite-element models. [ABSTRACT FROM AUTHOR]

Abstract The current work is concerned with ageing behaviour of sandwich composites constituted of E-glass fibre-reinforced vinylester bonded to PVC foam core being in service near the sea as a reference and basis for materials selection. In order to simulate the properties evolution of sandwich composites as real as possible, the sandwich composites were exposed to the two accelerated synergistic environmental conditions: solar radiation in conjunction with water vapour (condition A) and hygrothermal atmosphere alternating thermal cycling (condition B) for 1680 h, and the weight change and mechanical properties degradation were carried out. The results show that the sandwich composites lost weight in the condition A while gained weight in the condition B, and shear strength of sandwich composites fell finitely, revealing that PVC foam core was insensitive to the two ageing conditions. Meanwhile, the core fracture, interface-core fracture, interface debonding, and piles peel were observed as the increase of exposure time in the flatwise tension test, indicating debonding properties between skin and foam as well as between piles of composites skin were influenced by the two synergistic ageing conditions. [ABSTRACT FROM AUTHOR]

Abstract The present paper aims to provide further understanding of the low-velocity impact behaviour of glass fabric cloth reinforced epoxy composite panels embedded with wire nets (WN-GF/epoxy). WN-GF/epoxy hybrid panels with six layer modes were manufactured by vacuum-assisted injection process. Low-velocity impact tests with various incident velocities were performed, and the perforation thresholds of the designed specimens were found out. Scanning electron microscope was carried out to study the microstructure of the composites after impact. Experimental results show that perforation threshold velocity of the specimens appears a bi-linear increasing tendency with increase of the embedded wire nets, and the increase ratio has an inflection point. Damage pattern observation results show that the enhanced energy abortion ability is mainly due to the increased delamination area of the specimens during the impact events. 3D finite element models were developed in ABAQUS/Explicit to analyze the damage behavior of WN-GF/epoxy composites, and a user subroutine was compiled. The contact load and change of kinetic energy absorption characteristics have been used to validate the finite element results. The penetration process and damage modes of WN-GF/epoxy interply hybrid composites under low-velocity impact are further discussed with the assistant of numerical results. [ABSTRACT FROM AUTHOR]

Understanding the failure mechanisms of the buckling process of glass fiber composites based on acoustic emission (AE) signals is a challenging task. In this work, a method combining AE with digital image correlation (DIC) was used to monitor compressive buckling behaviors of delamination composites. The analysis of AE signals is based on the k-means algorithm and principal component analysis (PCA). According to PCA, three characteristic parameters of AE signals like amplitude, peak frequency, and RA value (rise time divided by amplitude), are selected for cluster analysis by k-means algorithm. The results show that the AE signals of the compression process can be divided into three clusters. The three clusters correspond to three kinds of damage modes such as matrix cracking, fiber/matrix debonding, delamination and fiber breakage. The characteristic frequency of each mode is found by cluster analysis. Besides, the size and position of delamination defects result in the reduction of mechanical properties of the glass fiber composites. The complementary technology combining AE with DIC is effective for damage monitoring of the composites. Clear changes of the displacement fields can accurately detect the damage location and degree of the specimen. [ABSTRACT FROM AUTHOR]

Microvascular channels in fiber-reinforced composites offer various functional enhancements such as damage monitoring and self healing. As an adverse effect, presence of such channels can harm the mechanical behavior of composites. In the present study, 3D models are generated to investigate the stress concentrations in the resin-rich pocket and failure behavior considering different stacking configuration and vascule sizes. Similar results were obtained for [0/90] 4s and [90/0] 4s stacking configuration in spite of having different layer just above the channel and different resin pocket dimensions. The effect of changing the vascule diameter is mostly observed in UD 0 configuration. Utilizing elliptical channels results in reduced stress concentrations compared to the circular ones with the same diameter. While the staking configuration affect the failure in the resin rich zone, it does not affect the failure from the laminate region. [ABSTRACT FROM AUTHOR]

This paper experimentally investigates the lightning strike response of a fiberglass/carbon woven hybrid laminate ([(0/90)g(45/0 2 /−45 2 /90 2 )c]s) under dry and hygrothermal conditions. Damage characteristics are evaluated using various inspection methods and possible damage mechanisms are discussed. Observations demonstrate that as the increase of lightning amplitude, notably enlarged carbon fiber breakage and fiberglass delamination areas as well as fiber bulging are presented in the in-plane and in-depth directions. Compared to a [45 2 /0 2 /−45 2 /90 2 ]s carbon woven specimen, the carbon fiber damage area, internal delamination area and damage depth for the hybrid one are 4.94 times, 2.26 times, 3.21 times greater under 22 kA lightning strike and 3.34 times, 1.97 times, 1.67 times greater under 32 kA strike, respectively. Hygrothermal aging significantly intensifies lightning strike damage on the hybrid specimen with complete detachment of fiberglass layer and massive internal delamination. The possible mechanism for the present damage behavior is analyzed. These results suggest that the fiberglass layer is not expected to protect the integrity of the carbon fiber reinforced polymer structure from lightning strike damage. [ABSTRACT FROM AUTHOR]

The structure and interfacial shear strength (IFSS) of polypropylene-glass fiber/carbon fiber hybrid composites fabricated by a new method called direct fiber feeding injection molding (DFFIM) process were investigated based on the skin-core-skin structure and modified Kelly-Tyson equation, respectively. The fiber dispersion state was evaluated qualitatively by scanning electron microscopy (SEM) and quantitatively by the fiber distribution index (FDI). Carbon fiber agglomeration is more likely to occur in hybrid composites and was present in the core layer of DFFIM composites in this study. However, glass fiber has shown relatively uniform dispersion in hybrid composites with a greater tendency to be distributed in the skin layers. This is attributed to the difference in feeding method, where glass fibers were fed through the hopper while carbon fibers were inserted from the vent hole. The IFSS between fibers and polypropylene of different composites in different layers is discussed by considering fiber length and orientation distributions in the core and skin layers, respectively. [ABSTRACT FROM AUTHOR]

This work presents an assessment of the damage mechanisms occurring during tensile tests in hybrid flax-glass fibres reinforced epoxy composites. The samples were composed of twill flax and glass fibre laminate plies with different stacking sequences and subjected to water immersion at 55 °C. The Acoustic Emission (AE) technique combined with scanning electron microscopy observations was used to identify the typical damage mechanisms and to follow their evolution. This identification was made with a statistical multivariable analysis in which the number of parameters and classes was optimised. Finally, the participation of each damage mechanism to the global failure was evaluated from the hits number and AE energy. The AE results showed that, although the hits number associated with the fibre failure was the lowest, their contribution to the overall failure of composites became predominant regarding the cumulative of AE energy. Furthermore, an additional group of AE signals induced by the delamination between flax and glass fibre layers was detected for the aged hybrid laminates. [ABSTRACT FROM AUTHOR]

Joining of composite materials has been an issue ever since composites were introduced. Among various joining methods, adhesive joining has been widely adopted because the adhesive joining uniformly distributes stresses over larger area, while the mechanical joining of composites induces stress concentration and fiber breakout. However, adhesive joining requires the surface treatment of composite adherends to increase the bonding strength; this treatment is not only costly but also generates much environmental pollution. In this study, a composite adherend that does not require any surface treatment to constitute an adhesive joint is developed. The composite adherends are manufactured by a “soft layer method” to expose bare fibers on the surface during composite fabrication. The bonding strength was significantly improved due to the exposed bare fibers, which was comparable to that of the adhesive joint treated by the conventional peel ply method without generating waste materials. [ABSTRACT FROM AUTHOR]

The fracture toughness of laminated composite materials is a very important property for damage-tolerant design, but remains poorly understood under transverse mechanical loading conditions. In this study, the transverse intralaminar fracture in unidirectional laminates with ductile polymer matrices is investigated. The presence of a ductile matrix calls for the use of concepts of nonlinear fracture mechanics for the accurate determination of the resistance of the composite to crack propagation. Unidirectional glass fiber-reinforced polymers with a polyurethane (PU) matrix are studied and compared to a benchmark epoxy (EP). The fracture toughness of these composites was evaluated through the determination of R-curves using the J-integral method, whereas the underlying fracture mechanisms were investigated through in situ testing in a scanning electron microscope. The results suggest stronger adhesion of the polyurethane matrix onto the glass fibers as compared to the epoxy-based polymer. This enables full exploitation of the ductile behavior of the PU matrix, ultimately endowing the laminate with outstanding fracture toughness. The investigated PU matrix has thus the potential to generate laminated composite parts with significantly higher damage tolerance than its epoxy counterpart. [ABSTRACT FROM AUTHOR]

In this experimental work, the behaviour of unidirectional glass fibre-epoxy composites under a cutting force was studied, in order to better understand the cutting mechanisms of processes involving but not limited to composite materials. Starting from composite panels manufactured by resin infusion process, dedicated specimens were obtained and subjected to a quasi-static punch-shear test. Eight different fibre orientation angles with respect to the direction of the cutting force were considered. For each case, different stress-strain behaviours were observed and discussed. The differences were also investigated, in terms of damage propagation, by means of microscopic observations of the cutting zone. [ABSTRACT FROM AUTHOR]

A scalable technique was introduced to produce high volume lightweight composites using sheet molding compound (SMC) manufacturing method by replacing 10 wt% glass fibers (GF) with a small amount of cellulose nanocrystals (CNC). The incorporation of 1 and 1.5 wt% CNC by dispersing in the epoxy matrix of short GF/epoxy SMC composites with 25 wt% GF content (25GF/CNC-epoxy) produced 7.5% lighter composites with the same tensile and flexural properties of 35GF/epoxy composites with no CNC. The addition of 1 wt% CNC in 25GF/CNC-epoxy SMC composites resulted in increases of 15% in elastic modulus, 11% in flexural modulus and 14% in flexural strength, reaching the corresponding properties of 35GF/epoxy SMC composites. Moreover, it was found that although addition of CNC did not alter the impact energy, removing 10 wt% GF resulted in reduction of impact energy. [ABSTRACT FROM AUTHOR]

Natural fibers are gaining much interest in composite materials because of their potential to replace conventional glass fibers. In particular, unidirectional flax fiber composites have shown excellent mechanical properties with different thermoset matrices. However, few data are available when they are made with a biodegradable thermoplastic matrix. This paper compares the mechanical properties of two types of unidirectional flax composites using polylactic acid as the matrix: one made with layers of aligned flax rovings alone and the other containing an additional paper layer fabricated using paper making techniques. The results show that specific tensile properties of the flax/PLA (252 MPa·cm 3 ·g −1 ) and flax-paper/PLA (217 MPa·cm 3 ·g −1 ) composites are similar to those made using woven glass fabrics impregnated with epoxy (227–278 MPa·cm 3 ·g −1 ). Very high impact strength (600 J/m) was also obtained for UD flax-paper composites compared to unreinforced resin (15 J/m). Together these mechanical properties are promising for industrial applications of this new reinforcement. [ABSTRACT FROM AUTHOR]

In this paper, results of flexure tests aimed at improving the structural behavior of softwood beams reinforced with unglued composite plates and at developing an effective alternative to the use of organic resins are presented. The addition of modest ratios of GFRP ( Glass Fiber Reinforced Polymer ) composite strengthening can prevent tension failure in timber beams. However the application of organic matrices presents problems of reversibility, compatibility and durability with timber and poor performance at high temperatures. The increment in capacity and stiffness and the analysis of the failure modes is the central focus of this paper. The experimental campaign is dealing with a significant number of un-reinforced and reinforced beams strengthened with unbonded GFRP plates. A 3-dimensional finite element model is also presented for simulating the non-linear behavior of GFRP-reinforced softwood beams. The ability of the numerical model to reproduce experimental results for the load–deflection curves is validated. [ABSTRACT FROM AUTHOR]

This study proposes a novel approach to determine the fibre volume fraction in composites using vibration based non-destructive technique with a neural network. Currently, the volume fraction of a glass fibre/matrix based composite material is assessed using destructive techniques. Instead of changing or destroying the structure, a new non-destructive approach based on vibration analysis is proposed. Complete experimental protocols were developed to capture the vibration pattern. An auto-regressive model was developed as a feature extraction tool to classify the fibre volume fractions and as a pole tracking algorithm. The classification performances were within the range of 90–98%. For NDT method to be efficient, the classification results were then compared with destructive burn-out technique. The results of non-destructive test showed good agreement with those obtained through destructive test suggesting that the proposed method is an alternative to ASTM D2584-11 for determining the volume fraction of a glass fibre/matrix composite. [ABSTRACT FROM AUTHOR]

A physics-based model describing the thermal interaction between a lightning channel and a composite structure has been developed. The model includes: (i) spatial and temporal evolution of the lightning channel as a function of the electric current waveform; (ii) temporary and spatially non-uniform heat flux generated at the composite structure, where the heat flux is an explicit function of the electric current waveform and the instant lightning channel radius; (iii) nonlinear transient heat transfer problem formulation for layered anisotropic composites that accounts for temperature-dependent material properties, a moving boundary of the expanding lightning channel, and phase transition moving boundary associated with instantaneous material removal due to sublimation. The model is applied for evaluation of thermal damage of the tip glass fiber reinforced polymer matrix composite panel of the Sandia 100-meter All-glass Baseline Wind Turbine Blade (SNL 100-00) subjected to lightning strike. [ABSTRACT FROM AUTHOR]

A large debate animates the community about the mechanical performance of complex parts obtained by injection moulding (IM) of short glass fibre reinforced polymer (SGFRP) composites. Difficulties to measure exact fibre length and orientation distributions in complex geometries are limiting in terms of basic understanding of damage mechanisms in industrial parts and fatigue design improvement. In this work, we are interested to shed more light on the mechanical performance of short glass fibre reinforced polyamide composites in correlation to local microstructural heterogeneities which are caused by inserts used during IM. For such purpose, we combine X-ray microtomography (μ-CT) and finite element computation to determine the elastic modulus in various positions in an open hole plate configuration with a dissymmetric fibre distribution. Our predictions show that effective properties do not vary significantly around the hole in all space directions. However, stress profiles (through the thickness) far enough from the hole reveal a five-layer quasi symmetric structure evolving to three-layer near the cylindrical insert. We believe that such alteration is responsible for the observed failure of the composite at precisely the positions of highly affected fibre distribution. [ABSTRACT FROM AUTHOR]

Composite materials provide high strength and stiffness-to-weight ratios and adjustable directional material properties. As a result, a growing application of composite materials in weight-relevant and complex-loaded structures is noticeable, especially, for the production of high-speed rotors. However, their material complexity, their parameter scatter and unpredictable impact events contribute to the prediction uncertainty of the operational capability. The in situ measurement of damage-sensitive features can provide information for reliable simulation models. Such models are a prerequisite for the analysis of the damage behaviour in practice-relevant loading scenarios. For this purpose, a novel optical measurement system is introduced measuring the rotor deformation. The system offers a measurement rate of 1.6 kHz with an 7 μm error independent on the rotational speed. Furthermore, the rotor vibration response was identified using a combined excitement-measurement system for different rotational speeds. Both measurement systems were successfully applied on a glass fibre-reinforced composite rotor. Based on the experimental results, a simulation model was adjusted using an advanced mixed numerical–experimental technique. A good compliance between experimental and simulation results was identified for rotor-typical stress states. In addition, the rotor temperature is shown to have a significant influence on the model accuracy and has to be incorporated in future models. [ABSTRACT FROM AUTHOR]

Laminate composites made of glass fibre/epoxy resin filled with acrylic tri-bloc copolymers (Nanostrength) have been successfully manufactured. Microstructure, thermomechanical properties and impact resistance have been investigated and compared with those of a glass fibre/epoxy resin system. Dynamic mechanical analysis (DMA) tests have been conducted to determine the effect of Nanostrength on storage, loss modulus and glass transition temperature. A drop weight tower was used to perform low-velocity impact tests on laminate composites. Addition of Nanostrength to the epoxy matrix led to an increase in both strength and impact resistance of the composite. Moreover, a moderate decrease of storage modulus and glass transition temperature was observed. [ABSTRACT FROM AUTHOR]

This study is focused on the finite element analysis of the hygro-elastic stresses experienced by unidirectional composite structures instrumented by Fiber Bragg Grating (FBG) sensors. One of the significant advantages of this method is that by taking into account the complex geometry of elaborated specimen and their different constituents with heterogeneous hygroscopic and mechanical properties. In the first time, the obtained results were compared with the results obtained owing to an analytical solution deduced from the simplified concentric cylinders approach. This comparison allows us to evaluate the reliability of the concentric cylinders approach which cannot account for every aspects of the actual geometry of the studied samples. Besides the numerical results were compared to the experimental characterization of the mechanical states deduced from the Fiber Bragg Grating measurements. [ABSTRACT FROM AUTHOR]

The objective of this work is to enhance the mechanical properties of unidirectional [0] 8 and quasi-isotropic [0/±45/90] s glass-fiber-reinforced composites by ultrasonically dispersing alumina-nanoparticles in epoxy matrix. The well dispersed alumina-nanoparticles in epoxy resin significantly improved the mechanical properties of alumina-nanocomposites. Results from fracture characterizations of the nano-hybridized unidirectional laminates with different off-axis angles showed high improvements in the interfacial bond strength between nanophased epoxy and glass fiber. Therefore, the flexural strength, modulus, and fracture toughness of quasi-isotropic laminate are significantly improved. Results from free vibration tests showed that the beam free length has high effects on the natural frequencies and the both have insignificant effect on the storage moduli of the composite laminates. For the investigated materials, the storage moduli correlated well ( R 2 = 0.99962) with the flexural moduli. The damping ratios of the nano-hybridized quasi-isotropic and unidirectional laminates are improved by 62% and 60%, respectively. These improvements, which were achieved through conventional processing, are essentially for developing the structural components of automotive and aerospace industry. [ABSTRACT FROM AUTHOR]

In this study, the effect of stitch pattern on the impact and post impact (CAI) behavior of eight ply woven–knit hybrid composite plates consisting of 2D woven fabrics as outer layers and rib knitted fabrics as inner layers was investigated. Layers of the woven–knit hybrid composite plates were sewed in circular, square and diamond shape through thickness in order to prevent delamination and slippage between the layers. Impact and post impact testing were carried out in order to determine the impact and post impact resistance of three different stitched composite structures. Results showed that the impact strength of composite laminate increased 5.5%, 11% and 22% in the case of square, diamond, and circular stitching, respectively. It was also found that the woven–knit hybrid composites with square pattern have the highest CAI strength. [ABSTRACT FROM AUTHOR]

Abstract: This study conducted a finite element analysis of the piezoelectric damage monitoring of unidirectional glass fiber epoxy composites to estimate the electric charge signals from the composite materials against the external load or the internal crack length. To verify the results of the finite element analysis, the calculated electric charges were compared with the results of a previous research. In addition, the crack length during Mode I fatigue tests of the DCB specimen was estimated and compared with the measured data. The estimated electric charge signals from the finite element analyses showed tendency similar to that of the measured ones, and could predict the crack propagation. [Copyright &y& Elsevier]

The paper presents the results of laboratory tests and numerical simulations for thin-walled beams subjected to 3-point bending (3-PB). The beams were made of two parts: an aluminum (7075) omega profile and a GFRP (glass fiber reinforced plastic) panel, which were connected by rivets. The paper focuses on the analysis of deformation states and gradual degradation in these elements. The modeling is described by parameters as the number of rivets, the height of the aluminum profile, and the stiffness of the composite which strongly influence the failure force. The experiments were carried out for two settings of beams on supports, in which the composite panel was located on the side of the supports or on the side of the load acting on the beam axis. The laboratory tests allowed to confirm the correctness of the numerical model. As a result of numerical simulations, the most optimal material and geometric configuration were determined, taking into account the maximum carrying criterion force at 3-PB. The effort of the composite material was also analyzed based on the Tsai-Hill hypothesis. [ABSTRACT FROM AUTHOR]

Abstract: This paper shows the influence of interphase areas in a UD glass fibre/epoxy composite on water diffusion process and mechanical properties thanks to experimental, analytical and numerical analysis. The interphase is first characterized in terms of molecular mobility by micro-thermal analysis and in terms of mechanical properties by force measurements with atomic force microscopy. The measurements show an increase in elastic modulus and in glass transition temperature around each fibre. Optical microscopy reveals an important heterogeneity of fibre distribution inside the matrix with many contacts between fibres. A realistic microstructure including the interphase size (defined by μTA) and fibre distribution has then been considered to model the composite accurately. Diffusion parameters resulting from water absorption measurements on both the resin and the composite were also used in the finite element analysis. The comparison between experimental data, analytical and finite element models shows that the water diffusion coefficient is about five times higher in the interphase area than in the matrix whereas the gradient in modulus around the fibres barely modify the composite mechanical response. [Copyright &y& Elsevier]

Abstract: The phenomenon of diffusion coefficients depending on concentration has been observed and studied experimentally for many years. This is particularly the case for the inter-diffusion in metals [1] but has also been observed on various polymeric systems [2]. This study presents a concentration-dependent diffusion coefficient model for water sorption in a polymer matrix composite. It has been developed by studying the water uptake of a polyphthalamide short glass fiber reinforced material. According to experimental data, we postulate that the diffusion coefficient follows a modified Arrhenius equation with a concentration dependence of the pre-exponential factor but also of the activation energy. This choice is related to the evolution of the glass transition temperature during sorption, which has a profound influence on transport properties. The coupled partial differential equation of the model is solved for some applied examples by using an easy-to-implement finite difference scheme. [Copyright &y& Elsevier]

Abstract: Composites based on poly(etherimide) matrix (PEI) with nanoscale reinforcements (multiwalled carbon nanotubes (MWCNTs)) and multiscale reinforcements (MWCNTs and Glass Fibers (GFs)) are processed by melt blending. For nanocomposite materials, we demonstrate that addition of MWCNTs (i) significantly improve electrical conductivity of PEI matrix, (ii) do not significantly improve the mechanical properties (young and flexural modulus) but strongly degrades the strain at break. In the case of multiscale composite materials PEI_GF_MWCNT, strain at break increases with the nanofiller content, and the electrical properties are the same than that in the case of nanocomposites. TEM observations are performed at the PEI/GF interfaces and clearly show a preferential location and orientation of MWCNTs along and perpendicularly to the GFs which explain the evolution of the strain at break for the multiscale composite materials. [Copyright &y& Elsevier]

Abstract: This paper presents quasi-static crushing performance of empty and polyurethane foam-filled E-glass/vinylester composite tubes with different geometrical characteristics to use in sacrificial cladding structures. Some empty and foam-filled tubes were compressed laterally between two rigid plates. The effects of polyurethane foam-filler on the crushing characteristics and the corresponding energy absorption by the composite tubes are investigated. Composite tubes with three different fiber fabric layers were considered to study the influence of polyurethane foam on the crushing performance. Also, effects of different geometrical characteristics on energy absorption by different tubes are studied. Experimental results show that the presence of polyurethane foam inside the composite tubes suppresses the circumferential delimitation process and fiber fracturing; consequently, it increases the specific absorbed energy by the composite tubes during the flattening process. Also, it is resulted that injection of the polyurethane foam in the tubes causes the more regular deformation mode, comparing with the empty tubes. Finally, experimental results of the present work are compared with some of the previous articles that were investigated the foam-filled composite structures under the different loadings to suggest an optimized energy absorber. [Copyright &y& Elsevier]

Abstract: The fatigue behaviour of small scale composite unit cells consisting of seven fibres and surrounding matrix is developed for use in fatigue simulations. The predicted fatigue properties of these 7-fibre unit cells are compared with the fatigue properties of single glass fibres and of unidirectional composite dog–bone specimens. These predicted results provide stochastic fatigue characteristics that can be used as input for modelling the fatigue behaviour of larger composite structures. [Copyright &y& Elsevier]

Abstract: Glass fiber reinforced plastic (GFRP) composite material is often utilized in marine applications for the construction of boats and ships. In this application it is a usual procedure to protect the material with a layer of gel-coat, but if the gel-coat breaks the material would be in direct contact with a humid environment. The main objective of this study was to understand the degradation that sea-water causes on GFRP mechanical properties. The article deals with two common resin systems in marine applications, i.e. polyester and vinylester, and reports the results of quasi-static and vibration tests at different sea-water exposure times, and fatigue tests in dry and wet environments of unexposed specimens and specimens aged in sea water for 22-weeks. The results show degradation of the material in terms of ultimate stress in the case of quasi-static loading, and no appreciable degradation on the fatigue loading case. Vibration tests demonstrated a stable flexural modulus during the water exposure period. [Copyright &y& Elsevier]

Abstract: The faces of low-observable radomes constructed with sandwich structures are usually made of glass fiber or aramid fiber composites due to their high specific strength and compatibility of stealth characteristics. In this work, a hybrid composite face composed of both E-glass/epoxy and aramid/epoxy composites for low-observable radomes was designed to exploit the better characteristics of both composites. Three design methods, including the characteristic medium thickness, the wavenumber and the dielectric wavelength methods, were applied to satisfy the functional requirements of composite faces. The three design methods were evaluated via three-dimensional (3-D) electromagnetic wave transmission analysis and the free space measurement method. [Copyright &y& Elsevier]

Abstract: Electromagnetic wave reflections from glass fiber reinforced epoxy matrix composites with 0°/90° and ±45° fiber oriented plain-woven glass fabric (PW-GFRP-0/90, PW-GFRP-±45) at incident angles of 30°, 40° and 50° were measured in the frequency range 50–75GHz using a free-space reflection measurement system. The complex dielectric constants of both composites were calculated using a simple transmission line theory. The complex dielectric constants of PW-GFRP-0/90 and PW-GFRP-±45 are similar and were measured to be ε′=4.61±0.01 and ε″=0.16±0.002, respectively. The damage stored in PW-GFRP-0/90 and PW-GFRP-±45 was also evaluated by dielectric constant changes using the same system at an incident angle of 30°. For both composites, ε′ decreased with increasing applied stress and damage parameter. The dielectric constant change is effective for detecting the damage stored in composites and can be used to quantitatively evaluate the damage. [Copyright &y& Elsevier]

Abstract: Since the stainless steel primary barriers of Cryogenic Containment System (CCS) for Liquefied Natural Gas (LNG) ship failed by impact loads due to LNG cavitation during operation, the stiffness and strength of the primary barrier was increased much to endure the impact pressure of LNG, which increased the transfer of impact load to foam insulation panels under the primary barrier. The increase of impact load to the foam insulation panels will induce crack propagation into the insulation foam panels due to the low toughness of insulation foams at cryogenic temperatures. In this work, an impact isolation system was developed using glass fiber composite mats to lower the impact load transmission into the foam insulation panels. An optimal design process for the vibration isolation system was conducted by a numerical method. For reliability of the impact isolation system, cryogenic fatigue tests of the glass fiber composite mats were performed. Also, an impact test equipment with low impact duration was developed to simulate the efficiency of the impact isolation system. [Copyright &y& Elsevier]

Abstract: In this paper effective medium theory based on Clausius–Mossoti relation was used to predict dielectric properties of multiphase composite system. The composite consisted of E-glass fibers (plain weave S-glass, J.B. Martin) embedded in bisphenol A diglycidylether epoxy matrix (D.E.R. 324, D.O.W. chemicals) with hollow ceramic spherical inclusions (SF 14, P.Q. Corp.) at different volume fractions. In many engineering applications, materials with designed dielectric properties are often sought. An important question in engineering design of a composite material is how the overall properties of the composite depend on those of the individual constituents. Mixing the epoxy resin with hollow ceramic inclusions can effectively reduce dielectric constant of the resin, which is often desirable, rendering composites as good candidates for many applications, especially in telecommunications. Compensation for degradation of mechanical properties of matrix (due to inclusion insertion) is obtained by embedding fibers into the mixture. Measurements of dielectric constant and loss tangent for this multiphase composite system were conducted in the region between 0.1 and 100kHz range using DEA 290 (T.A. Instruments) dielectric analyzer and experimental results are presented. Good correlation between analytical model and experimental results was observed throughout all frequency range of investigation. [Copyright &y& Elsevier]

Abstract: Researchers have developed several measures to improve shear resistance capacity of sandwich structures composed of thin composite face sheets of Eglass/epoxy and low density PVC foam core, but these methodologies are either costly or result in damage to the sandwich composites before being placed for production. A novel, innovative cost-effective methodology of introducing fiberglass shear keys attached to face skins to improve the stiffness and strength of the sandwich composite panel is being proposed in this manuscript. Manufacturing of the sandwich composite panels along with experimental and numerical investigations have been performed in this manuscript to demonstrate the potential of the proposed simple methodology which can be utilized for shear-sensitive design and analysis of sandwich composite structures and/or components. A parametric study has also been carried out to determine the effect of change in shape, size, spacing and material of the shear keys on the sandwich composite panel global response. [Copyright &y& Elsevier]

Abstract: This paper investigates the influence of exfoliated nano-structures on sandwich composites under impact loadings. A set of sandwich composites plates made of fiberglass/nano-modified epoxy face sheets and polystyrene foams was prepared. The core was 25mm thick and the face sheets were made of eight layers of woven fabric glass fibers and nano-modified epoxy (≈0.8mm of thickness). The epoxy system was bisphenol A resin and an amine hardener. The fiber volume fraction used was around 65%, while the nanoclay content varied from 0wt.% to 10wt.%. The nanoclay used was Cloisite 30B from Southern Clay. The sandwich panels were submitted to low-velocity impact tests with energies from 5J to 75J. Two sets of experiments were performed, i.e. high velocity+low mass and low velocity+high mass. Damage caused by the two groups of experiments and peak forces measured were dissimilar. The results show that the addition of 5wt.% of nanoclay lead to a more efficient energy absorption. The failure modes were also analyzed, and they seems to be affected by the nanoclay addition to face sheets. [Copyright &y& Elsevier]

Abstract: A multifunctional – hybrid – composite material that incorporates carbon and glass fibres embedded into an epoxy matrix was developed. This material can be made to undergo a shape change without inserting activators. This is accomplished by selective heating of the embedded carbon fibres through the application of an electric current (Joule effect). The resulting shape change was determined and the stresses generated during heating were measured via a 3-point bending device. The possibility of using the reinforcing fibres of a multi-directional composite as activators for inducing specific deformation modes is discussed. [Copyright &y& Elsevier]

Abstract: In this study, the impact behavior of laminated glass–epoxy composites at several temperatures (20°C, −20°C and −60°C) was studied experimentally. Impact tests were performed by using Fractovis Plus impact test machine with a constant impactor mass of 5.02kg. Composite specimens with stacking sequence [0/90/45/−45] S were impacted at varied impact energies ranging from 5J to 70J. Variation of the impact characteristics such as maximum contact load, maximum deflection, maximum contact time and absorbed energy versus impact energy are depicted in figures. Results indicated that the ambient temperature highly affects the impact response of composite materials. [Copyright &y& Elsevier]

Abstract: The present work aims to obtain failure modes and failure loads for multi-pin joints in uni-directional glass fibre/epoxy composite laminates by finite element analysis and validating the results with the experimental work. In multi-pin joints, the effect of variation in pitch-to-diameter ratio (P/D) is also important, in addition to side width-to-diameter (S/D) and edge-to-diameter (E/D) ratios. A two-dimensional finite element model is developed using ANSYS software. Tsai–Wu failure criteria associated with material property degradation is used in the analysis to predict failure load and to differentiate failure modes. The results from experimental and finite element analysis showed that it is very important to consider the affect of E/D, S/D and P/D ratios in designing multi-pin joints. [Copyright &y& Elsevier]

Abstract: In this study, edge-cracked aluminum plates repaired with one-sided composite patches were investigated experimentally for their response to Charpy impact test. Two different materials were used for composite patches, in one the reinforcement was woven glass–fibres while the other had woven carbon fibres as the reinforcement. Further, in each case, two lay-ups were used, viz. 3-ply and 5-ply. Also, the aluminum specimens had edge-cracks of three different lengths. In each case, the energy absorbed during the impact test, when the specimen had a crack of known length and reinforced with patches, was determined and compared with the energy absorbed during impact of the corresponding specimen without crack. It was observed that for a given crack length, the 5-ply patch of carbon fibres performed better than all other patches. For example, when the ratio of the crack length to the specimen width, i.e. l/w is 0.5, the percentage of the absorbed energy for the specimen repaired with 5-ply carbon fibre patch is 2-times as compared to the specimen repaired with 5-ply glass–fibre reinforced patch. [Copyright &y& Elsevier]

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