GFRP Laminates Research Articles

Finite element prediction of damping of composite GFRP and CFRP laminates – a hybrid formulation – vibration damping experiments and Rayleigh damping

The work reported in this paper describes the development of a hybrid methodology for the prediction of damping properties of vibrating composite laminates; this method could also be applied to homogeneous materials. This hybrid methodology consists of experimental identification of damping, using vibration damping testing methods, and utilization of FEA. The experimentally identified damping property is that of specific damping capacity (SDC), a measure of damping during the 1st mode of resonant vibration of beams. The finite element approach utilizes the concept of Rayleigh damping, and in particular mass proportional damping for the modeling of the damped response of vibrating systems. It is shown that by using such a methodology, damping data can be extracted for cases where application of continuum mechanics analytical solutions cannot provide reliable information. Furthermore, the development of the finite element models is described. The association of damping properties with material reinforcement is highlighted. A series of continuous and woven, cross ply and quasi isotropic GFRP and CFRP coupons were vibrated. The FE damped response prediction was in very good agreement with laboratory observations.

Machining of Fiber-reinforced Thermoplastics: Influence of Feed and Drill Size on Thrust Force and Torque during Drilling

Polymer-based composite materials possess superior properties such as high strength-to-weight ratio, stiffness-to-weight ratio, and good corrosive resistance and therefore, are preferred for high-performance applications such as in the aerospace, defense, and sport goods industries. Drilling is one of the indispensable methods for building products with composite panels. Drilling tests have been conducted on glass fiber-reinforced plastic composite GFRP laminates using an instrumented CNC milling center. Machining parameters such as drill size, feed rate, and cutting speed have been observed for damage-free drilling of GFRP materials. A series of drilling experiments have been conducted on glass fiber-reinforced polyester laminates and the responses experienced such as thrust force and torque as functions of feed rate and drill size have been characterized to develop a semiempirical relationship which correlated well with an established model in terms of cutting parameters. Results indicated that experimental values correlated better with the model of thrust for 6 mm drill size than for 10 mm and torque correlated better for lower feed ranges than for the higher feed ranges.

Fatigue behavior of adhesively bonded joints composed of pultruded GFRP adherends for civil infrastructure applications

This paper presents results from studies on the fatigue behavior of pultruded GFRP laminates, adhesively bonded double lap joints composed of laminates, and full-scale adhesively bonded FRP bridge deck and steel girder connections. The studies show that the roving fibers are determinant in the tensile fatigue life of the laminates. For the bonded joints, the fatigue resistance depends on the through-thickness interlaminar strength of the GFRP adherends. Adhesively bonded joints are more sensitive to changes in the applied maximum stress than the laminates at an amplitude rate of 0.1, however, the degradation rate of strength is almost the same for both. The applied maximum fatigue load affects the stiffness degradation rate of GFRP laminates; the bonded joints do not experience significant stiffness degradation when applied to fatigue loading. Adhesively bonded FRP bridge deck-to-steel girder connections showed no sensitivity to fatigue loading in the longitudinal bridge direction. However, the sensitivity to uplift fatigue loading in the transversal bridge direction was not negligible.

Interpreting acoustic emission signals by artificial neural networks to predict the residual strength of pre-fatigued GFRP laminates

Artificial neural networks (ANNs) were used to predict the residual strength of glass fibre-reinforced plastic beams pre-fatigued in flexure up to different portions of their fatigue life. To this aim, the acoustic emission signals recorded during the tests for the measurement of the residual strength, and the associated applied stress, were provided as input. An optimisation of the network configuration was carried out, using the root-mean-square error calculated in the training stage as the optimisation parameter. The predictive accuracy of the optimised ANN, consisting of two nodes in the input layer, four nodes in the hidden layer, and a single node in the output layer, was tested by the “leave-k-out” method. From the results obtained, ANN provided quite reliable predictions when the applied load was sufficiently far from the failure load, performing better than a previous theoretical model, relying on fracture mechanics concepts. Therefore, ANN was shown to be a valid tool in the non-destructive evaluation of composite materials employed in fatigue-sensitive applications.

Accelerated Testing for Long-term Durability of FRP Laminates for Marine Use

The accelerated testing methodology has been proposed for the long-term durability of polymer composites based on the time–temperature superposition principle to be held for the viscoelasticity of polymer matrix. In this paper, the long-term flexural fatigue life of plain woven carbon fiber–vinylester (CFRP) laminates for advanced marine use was predicted based on the proposed methodology, compared with that of plain woven glass fiber–vinylester (GFRP) laminates for conventional marine use. The applicability of the accelerated testing methodology for these laminates and the advantages of this CFRP laminates are discussed. As results, the flexural fatigue strength of CFRP laminates as well as that of GFRP laminates clearly depends on time and temperature. The time–temperature super-position principle for the viscoelasticity of matrix vinylester resin holds for these flexural fatigue strengths, and then the master curves can be obtained. It is clear from these master curves that while the flexural fatigue strength for GFRP laminates decreases strongly with increasing number of cycles to failure Nf and that for CFRP laminates decreases scarcely with increasing Nf.

Production and Properties of Glass Fibre-Reinforced Polymer Composites with Nanoparticle Modified Epoxy Matrix

AbstractIncreasing the mechanical performance, e.g. strength, toughness and fatigue properties of composites is the objective of many ongoing research projects. Nanoparticles, e.g. carbon nanotubes (CNTs) and fumed silica provide a high potential for the reinforcement of polymers. Their size in the nanometre regime make them suitable candidates for the reinforcement of fibre reinforced polymers, as they may penetrate the reinforcing fibre-network without disturbing the fibre-arrangement.In this work, glass fibre-reinforced epoxy composites with nanoparticle modified matrix systems were produced and investigated. GFRPs containing different volume fractions of the nanofillers were produced via resin transfer moulding. Matrix dominated mechanical properties of the GFRP laminates could be improved by the incorporation of nanoparticles. The addition of only 0.3 wt.% CNTs to the epoxy matrix increased the interlaminar shear strength from 33.4 to 38.7 MPa (+16%). Furthermore, the application of electrically conductive nanoparticles enables the production of conductive nanocomposites. This offers a high potential for antistatic applications and the implementation of functional properties in the composite structures. The effects of different filler types and volume fractions on the electrical properties of the GFRPs were investigated. GFRPs containing 0.3 wt.% of CNTs, for example, exhibit an anisotropic electrical conductivity. Furthermore, an electrical field was applied to the composites during curing. The effects on the resulting electrical and mechanical properties are discussed.

Analysis of Elastic-Viscoplastic Deformation of Plain-Woven GFRP Laminates Using a Homogenization Theory (Domain of Homogenization Analysis)

In this study, a method for reducing the domain of analysis is developed for the homogenization analysis of plain-woven laminates. To this end, by assuming the in-phase and out-of-phase laminate configurations of plain fabrics in laminates, it is shown that the internal structures of the laminates have point-symmetry. Then, the point-symmetry is utilized to the boundary condition for unit cell problems, reducing the domain of analysis to 1/4 and 1/8 for the in-phase and out-of-phase laminate configurations, respectively. Using the present method, the in-plane elastic-viscoplastic deformation of plain-woven GFRP laminates is analyzed. Moreover, the in-plane uniaxial tensile tests of plain-woven GFRP laminates at a constant strain rate are performed at a room temperature. It is thus shown that the present method successfully predicts the in-plane elastic-viscoplastic behavior of plain-woven GFRP laminates. It is also shown that the laminate configurations of plain fabrics have an influence on the viscoplastic behavior of the laminates.

Effect of workpiece vibration on drilling of GFRP laminates

Effect of workpiece vibration on drilling of glass/epoxy (GFRP) laminates using three types of drill, e.g. tipped WC, 2-flute solid carbide and 3-flute solid carbide has been studied. A UD-GFRP laminate of 4 mm thickness was prepared and drilling was carried out using a vertical drilling machine. First, an optimum speed, feed and frequency combination for which thrust, power and tool wear are minimum was arrived at by drilling the specimen using varying speed, feed and frequency. The thrust force was measured using strain-gauge type dynamometer. Tool wear (flank wear) was measured using a Tool Maker’s microscope. The workpiece was subjected to vibration using a variable frequency generator close to the hole being drilled. Frequency used was 220 Hz and amplitude of vibration was 10–15 μm. Thrust, tool wear, temperature, power and AE (RMS value) were measured. It was observed that by vibrating the workpiece during drilling all these five parameters are very much reduced. Also, among the three types of drills performance of 3-flute solid carbide was found to be the best.

The Fatigue Delamination due to Four Different Stacking Sequences of Ankle Foot Orthosis (A.F.O.) in GFRP Laminates

The Fatigue Delamination due to Four Different Stacking Sequences of Ankle Foot Orthosis (A.F.O.) in GFRP Laminates

Accelerated testing for long-term durability of GFRP laminates for marine use

Our proposed accelerated testing methodology for the long term durability of polymer composites is based on the time–temperature superposition principle to be held for the viscoelasticity of polymer matrix. The long term flexural fatigue life of plain woven glass fiber/vinyl-ester (GFRP) laminates for conventional marine use was predicted based on the proposed methodology. As results, the flexural fatigue strengths of GFRP laminates decreases strongly with increasing time and temperature as well as the number of cycles to failure. The long term fatigue strength at any time, temperature and number of cycles to failure can be predicted using the master curves of fatigue strength obtained based on our proposed accelerated testing methodology.

Life prediction methodology for GFRP laminates under spectrum loading

A complete life prediction methodology for glass fiber reinforced plastic laminates accounting for plane stress states and spectrum loading is presented in this paper. Fatigue strength allowables in the laminate symmetry axes are derived by direct characterization for a number of different constant amplitude loading cases. Experimental results from tests inducing plane stress states, conducted using two different loading spectra of variable amplitude, were used to validate the entire methodology. The first loading spectrum is a modified version of the well known in wind turbine rotor blade research activities, WISPERX, while the second one is derived through aeroelastic simulations of a realistic loading case for a small glass fiber reinforced polyester rotor blade. The effectiveness of the methodology is investigated and correlated to several parameters that affect life prediction, such as, type of loading, baseline data, damage accumulation rule, etc.

GFRPの熱環境下での力学特性の評価(クリープ,OS.14 先進材料の力学)

The GFRP composite laminates applied for the electronic parts is exposed to the severe thermal environment when mounted and in service. In that case, the thermal stress leads to delaminations inside the laminates, which causes functional at troubles. Therefore we investigated the interlaminar shear strength of six kinds of GFRP laminates at elevated temperatures and found the relationships between interlaminar shear strength and temperature.

均質化理論に基づく平織GFRP積層板の面内弾 : 粘塑性変形解析(OS7 計算固体力学とシミュレーション)

均質化理論に基づく平織GFRP積層板の面内弾 : 粘塑性変形解析(OS7 計算固体力学とシミュレーション)

Crack blunting and the strength of soft elastic solids

When a material is so soft that the cohesive strength (or adhesive strength, in the case of interfacial fracture) exceeds the elastic modulus of the material, we show that a crack will blunt instead of propagating. Large–deformation finite–element model (FEM) simulations of crack initiation, in which the debonding processes are quantified using a cohesive zone model, are used to support this hypothesis. An approximate analytic solution, which agrees well with the FEM simulation, gives additional insight into the blunting process. The consequence of this result on the strength of soft, rubbery materials is the main topic of this paper. We propose two mechanisms by which crack growth can occur in such blunted regions. We have also performed experiments on two different elastomers to demonstrate elastic blunting. In one system, we present some details on a void growth mechanism for ultimate failure, post–blunting. Finally, we demonstrate how crack blunting can shed light on some long–standing problems in the area of adhesion and fracture of elastomers.

Quantitative damage detection in cross-ply laminates using Lamb wave method

This paper investigates the effects of transverse cracking and delamination on the S 0 mode velocity in GFRP and CFRP cross-ply laminates. We found experimentally that both the stiffness and the velocity decreased as the transverse crack density increased. In contrast, the stiffness decreased but the velocity increased as the delamination length increased. We analytically deduced the relationship between the velocity and the crack density from a combination of a shear-lag analysis and the classical plate theory. We also confirmed that the Lamb wave propagated through the 0° layers in the delaminated regions, and formulized the relationship between the velocity and the delamination length. The predicted crack density and delamination length using the measured velocity were in good agreement with the experimental results. This method is simple and promising for structural health monitoring of composite structures.

인공발(Prosthetic Foot) 스프링용 유리섬유강화 적층재의 적층배향에 따른 층간분리거동 해석

It is considered that the application of advanced composite materials to the prostheses for the disables is important to improve their bio-mechanical performance. Particularly, energy storing foot prosthesis is mostly important to restore gait ability of the disables with low-extremity amputation since it could provide propulsion at terminal stance enhancing the disables ability to walk long distance even run and jump. Therefore, the energy storing spring of prosthetic foot keel under cyclic bending moment use mainly of high strength glass fiber reinforced plastic. The main objective of this study was to evaluate the stacking sequence effect using the delamination growth rate(dA_D/dN) of energy storing spring in glass fiber reinforced plastic under cyclic bending moment. The test results indicated that the shape of delamination zone depends on stacking sequence in GFRP laminates. Delamination area(A_D) tums out that variable types with the contour increased non-linearly toward the damage zones.

Lamb wave evaluation and localization of transverse cracks in cross-ply laminates

This paper investigates the effect of transverse cracks on the S0 mode velocity in GFRP and CFRP cross-ply laminates, and proposes a new AE source location method that considers the change in the S0 mode velocity due to the transverse cracks. We found experimentally that the stiffness and the velocity decreased as the transverse crack density increased. Analytical predictions deduced from the combination of the complete parabolic shear-lag analysis, the classical plate theory and the laminated plate theory are in good agreement with the experimental results. Utilizing this relationship between the velocity and the mechanical damage, we located AE sources of transverse cracks in cross-ply laminates with the calculated in situ velocity. We were able to show that highly accurate source location requires the reduction of the in situ value of the velocity. The present method is simple but quantitative and useful in health-monitoring for detecting and localizing the damage in composite structures.

Post-impact compressive strength of curved GFRP laminates

Low velocity impacts to fibre reinforced plastic composites cause a pattern of damage consisting in general of delamination, fibre breakage and matrix cracking. Such damage is accidental and may go unnoticed; therefore composite structures must be designed assuming impact damage exists. Previous work on flat composite laminates has resulted in a reasonable understanding of the mechanisms of compressive strength reduction. There are, however, many instances where curved laminates are used in structures where impact is likely. Furthermore, due to the mechanisms of strength reduction, it may be expected that curvature would have a significant effect on the behaviour of the laminates. The work described here consists of experimental measurement of the post-impact compressive strength of curved GFRP laminates. The laminates were of 8 plies of 0.3 mm thick pre-impregnated glass fibre/epoxy tape in a (0, ±45, 0°) s lay-up. Each laminate was 200 mm in length by 50 mm wide with the plane of curvature normal to the length. Laminates were impacted on the convex surface of the laminate by dropping a steel mass from 1 m vertically above it. Impacted laminates were loaded in compression and the out-of-plane displacements of the top and bottom surfaces were recorded. Final failure was typically due to fibre breakage occurring through the centre of the impacted area of the laminate. Possible differences in the impact response, and measurable differences in the sizes of the impact damage area, were found to arise from these curvatures, and differences were observed in their post-impact buckling behaviour. However, perhaps unexpectedly, the post-impact compressive strength for a curved laminate was found to be similar to that for a flat laminate. The failure loads for the impact damage laminates are shown to be comparable with those for laminates containing artificial delaminations.

Thermoelastic analysis of cracked woven GFRP laminates at cryogenic temperatures

We deal with the thermal–mechanical response of multi-layered G-11 woven glass/epoxy laminates with cracks and temperature-dependent properties under tension at liquid helium temperature (4 K). The composite material is assumed to be in generalized plane strain. Cracks are located in the fill fiber bundles and are assumed to span the width of the fill fiber bundles. Finite element model is used to study the influence of residual thermal stresses on the mechanical behavior of multi-layered woven laminates with cracks. Numerical calculations are carried out, and the Young's modulus and stress distributions near the crack tip are shown graphically.

Horsetail Creek Bridge: Design Method Calibration and Experimental Results of Structural Strengthening with CFRP and GFRP Laminates

The Horsetail Creek Bridge, constructed in 1912, is located along the Historic Columbia River Highway in Oregon. In a recent rating of bridges, the cross beams of the structure were found to be 50 percent deficient in flexure and 94 percent deficient in shear, mainly due to the traffic loads increase. In order to identify suitable Fiber Reinforced Polymer strengthening system, few commercially available systems were reviewed and alternative designs were carried on. Concurrently, four full size beams were constructed to simulate the retrofit of the bridge. One served as a control, while the other three were reinforced with various configurations of FRP composites. Third point bending tests were conducted. Load, deflection and strain data were collected. Results revealed that addition of either GFRP or CFRP composites provided static capacity increase of 45 percent compared to the control beam. The beam strengthened with CFRP for flexure and GFRP for shear, which simulated the HCB cross beams after the retrofit, exhibited near 100 percent of moment capacity increase. The addition of GFRP for shear alone was sufficient to offset the lack of steel stirrups in the actual bridge, allowing for a conventionally reinforced concrete beam with significant shear deficiency to fail by yielding of the flexural failure. The resulting ultimate deflections of the shear GFRP reinforced beam were nearly twice those of the control shear deficient beam. A design method for flexure and shear was proposed before the onset of this experimental study and used on the HCB. The design procedure for flexure was refined to include provisions for non-crushing failure modes. It allowed for predicting the response of the beam at any applied moment.