Unidirectional Glass Fiber-reinforced Polymer Research Articles

Cavitation erosion of glass fibre reinforced polymer composites with unidirectional layup

Glass fibre reinforced polymer (GFRP) composites are increasingly used in marine applications and can be subjected to aggressive environmental effects, one of which is cavitation. This study investigates the behaviour of unidirectional GFRP composites exposed to cavitation erosion generated using an ultrasonic transducer. Cavitation erosion tests were performed in accordance with the ASTM G32 standard. All specimens were preconditioned to eliminate the influence of water absorption on the mass loss caused by cavitation. The erosion process was monitored with a microscope and the mass loss was measured at regular periods. The tested specimens were scanned with X-ray computed microtomography. The research findings indicated that the erosion process was affected by several parameters including specimen thickness, distance between fibre bundles, bundle shape and distribution. The initiation and development of erosion damage were highly influenced by the surface condition. Cavitation erosion traced parts of fibre bundles located closer to the surface creating trenches and valleys on the surface. The regions with thick epoxy layers above and between fibre bundles were much less susceptible to erosion damage. Several erosion mechanisms were identified and discussed. The research findings also highlighted the difficulties in characterising ultrasonic cavitation erosion of GFRP composites using acoustic impedance and mean erosion depth.

Fatigue life reduction of GFRP composites due to delamination associated with the introduction of functional discontinuities

This paper reports on an experimental investigation into the fatigue life of Glass Fibre Reinforced Polymer (GFRP) when essential design discontinuities are introduced to a GFRP part to enable correct function. Specifically, the impact of high speed drilled holes on fatigue life is investigated. Fatigue life is a critical mechanical property, in particular, for industrial applications where both minimal weight and high reliability are sought.The paper investigates the impact of high speed drilling parameters on the delamination created around the hole, and subsequently on the static strength and fatigue life of GFRP composite laminates. Delamination damage in GFRP specimens is monitored using novel Acoustic Emission (AE) and image processing techniques. The progress of delamination under fatigue testing is used to predict GFRP mechanical performance and associated GFRP mechanical properties are proposed.What follows is an outline of the experimental method used. First, the extent of delamination after high speed drilling was measured in both unidirectional and woven GFRP specimens under different feed rate and cutting speed parameters. Quasi-static three point bending tests were then performed to investigate the effect of delamination on strength and to assist the determination of appropriate fatigue load magnitudes. Then, three point fatigue bending tests were completed. In this step, Acoustic Emission and image processing techniques were applied simultaneously. Experimental results indicated that drilling parameters have negligible effects on the static strength of GFRP specimens, however, the fatigue life of GFRP specimens varied significantly with the changes of drilling parameters. The experimental results also showed that AE and image processing techniques produced consistent data, offering a validation to the data itself and to indicating that both techniques have merit when completing experiments of this type more generally to determine the mechanical behavior (and associated mechanical properties) of specimens.

A comparative study of mode I delamination behavior of unidirectional glass fiber-reinforced polymers with epoxy and polyurethane matrices using two methods

The mode I delamination behavior of a unidirectional glass-fiber reinforced polymer with a prototype elasto-plastic polyurethane matrix (GF/PU) was investigated and compared with that of glass/epoxy (GF/EP) using double cantilever beam specimens. Fracture resistance was assessed using experimental data based on the strain energy release rate (ERR), G, calculated by means of the modified compliance calibration, and the J-integral, calculated using the applied force and arm rotations measured by digital image correlation. The fracture energies given by the two methods differed by ∼5%. In both systems large scale bridging was observed and fracture resistance at initiation and steady state was about 4 times greater in GF/PU and correlated to adhesive failure in GF/EP and cohesive in GF/PU. Since the fiber sizing was the same, the higher ERR of GF/PU was attributed to strong glass/PU interface allowing higher local matrix strains and larger fiber bundles to develop. Traction-separation relations were determined by the direct and an indirect method using Fiber Bragg Grating sensors to construct numerical models to simulate the delamination process. The simulated force-displacement and R-curve data were in good agreement with experiments.

Investigations on performance characteristics of GFRP composites in milling

Milling of glass fiber reinforced polymer (GFRP) composites becomes essential in order to enhance its surface quality by improving its dimensional tolerances, and minimizing the surface defects. In present work, investigations were done to optimize the four important milling parameters, namely, spindle speed (N), feed rate (f), depth of cut (t), and type of milling tool on performance characteristics (i.e., machining force (Fm) and surface roughness (Ra)). Taguchi L25 orthogonal array was used for experimental planning, and analysis of variance (ANOVA) has been used to identify the contribution of each considered parameters on performance characteristics of unidirectional (UD) GFRP composites. Optimum combination of parameters, i.e., spindle speed 1950 rpm, feed rate 1 mm/s, depth of cut 1 mm, and type of milling tool as two-fluted brazed carbide tipped end mill tool, were identified to achieve minimum values of machining force and surface roughness. Scanning electron microscope (SEM) was used to study the surface morphology of UD-GFRP composite laminates. Minimized subsurface damages were found, when milled with customized two-fluted brazed carbide tipped end mill tool.

Unidirectional GFRP composite connections between precast concrete wall panels under simulated seismic loads

A method of connecting precast concrete walls with Glass Fiber Reinforced Polymer (GFRP) composite sheets is investigated. The method can be used either as a seismic retrofit of existing welded steel plate connections or in new construction. The specimens consisted of two precast concrete wall panels connected with either two or four layers of unidirectional GFRP composite sheets. Eight specimens were subjected to quasi-static cyclic loads to explore the effect of the following variables on the connection capacity: concrete surface preparation; application of surface pressure to the GFRP laminate; number and orientation of GFRP composite layers; and use of carbon fiber reinforced polymer (CFRP) spike anchors. The performance of the composite connection was acceptable in terms of lateral load, lateral displacement, and shear transfer capacity. Application of surface pressure during curing of the GFRP laminate and the use of CFRP spike anchors improved the lateral load and displacement capacity of the composite connection significantly. An empirical expression is proposed to determine the capacity of the composite connection.

Finite element analysis of initial imperfection effects on kinking failure of unidirectional glass fiber-reinforced polymer composites

The compressive kinking behavior of non-slender unidirectional glass fiber-reinforced polymer (GFRP) specimens has been analyzed by finite element (FE) models. The experimentally observed imperfections, including the initial fiber waviness throughout the entire specimen volume and the scattered resin/interface defects, were taken into account in the FE models. The birth-and-death method was employed to simulate the progressive damage to the material. The consideration of the coexistence of initial fiber waviness and initial resin/interface defects was found to be essential for accurate modeling of the kinking failure process. Kinking was initiated due to the disproportional increase of the fiber microbuckling at the locations of initial defects. The numerically obtained peak load, fiber microbuckling amplitudes, kink band angle and width, and compressive strain concentrations at the kink band edges were well predicted compared to the experimental results. The number of defects was less significant than the fact that defects existed that served as initiation points of the kink band formation.

Quantitative analysis of QSI and LVI damage in GFRP unidirectional composite laminates by a new ultrasonic approach

Abstract Our work is focused on a new experimental approach for the comparison between Quasi Static Indentation (QSI) damage and Low-Velocity Impact (LVI) damage in polymer composites starting from the results of ultrasonic goniometric immersion tests. In particular, the comparison is performed through the analysis of the additional anisotropy induced by the damage in unidirectional Glass Fiber-Reinforced Polymer (GFRP) composites due to QSI and LVI damage tests performed with a low level of the employed energy. To this aim, we ultrasonically reconstruct the acoustic curves (velocity curves and slowness curves) before and after the damage. Ultrasonic experiments are performed by using a goniometric ultrasonic immersion device designed and built at our laboratory, aimed at the mechanical characterization of anisotropic materials. We highlight differences and similarities between QSI and LVI damage starting from the analysis of the variations of the acoustic behavior and by using a suitable anisotropic damage model developed in the framework of the Continuum Damage Mechanics theory. The proposed experimental approach can be suitably developed for in situ investigations on low-velocity impact damage in polymer composite components.

Strengthening Steel Members with Holes Under Tension Using Unidirectional GFRP Sheets

Steel structure sections are often connected using bolts and rivets, which necessitates the need for holes. These holes provide a zone of failure during loading. In the present work an attempt is made to strengthen the steel members under tensile loading with hole provision for bolts, using Glass Fiber Reinforced Polymer (GFRP) sheets. The performance of FRP wrapping is assessed by changing the angle of orientation of fiber. The experimental investigation is conducted for one hole, two hole and staggered hole arrangement and the results are compared with finite element analysis. The difference between experiment and finite element modeling are within allowable limits. From the study it is found that GFRP wrapping with 0° orientation of fiber is the most suitable in terms of ultimate strength for all the three hole configurations.

Dynamic effects of a single fiber break in unidirectional glass fiber-reinforced polymer composites: Effects of matrix plasticity

In a unidirectional composite under static tensile loading, breaking of a fiber is shown to be a locally dynamic process, leading to stress concentrations in the matrix and neighboring fibers and debonding of the interface, which can propagate at high speed over long distances. In our previous work, a fiber break within a two-dimensional fiber array embedded in elastic epoxy matrix (with cohesive interface) was modeled to quantify the effects of these dynamic stresses. The results indicated that the elastic limit of the polymer matrix can be exceeded. In this study, the effects of matrix plasticity on dynamic stress concentrations due to a single fiber break are investigated. For the range of matrix yield stresses considered, the dynamic stress concentrations are significantly higher than corresponding values predicted by a quasi-static model with a pre-broken fiber. Based on the ratio of shear yield strength of the matrix and mode II peak traction of the interface cohesive law, two distinct regimes of damage are shown to exist. Only matrix yielding occurs when this ratio is less than 1.0, while both interfacial debonding and matrix yielding occur when it is greater than 1.0. At higher fiber break strengths, where the elastic matrix model predicts unstable interfacial debonding, reduction in matrix yield strength leads to a transition to stable debonding and arrest. Reducing the matrix yield strength also leads to a lowering of the peak dynamic stress concentrations in adjacent fibers, while spreading the stress concentrations over a larger volume of the composite microstructure.

Mode I transverse intralaminar fracture in glass fiber-reinforced polymers with ductile matrices

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.

Sensing uniaxial tensile damage in fiber-reinforced polymer composites using electrical resistance tomography

This work describes the application of electrical resistance tomography (ERT) in sensing damage in fiber-reinforced polymer composites under uniaxial quasi-static tension. Damage is manifested as numerous matrix cracks which are distributed across the composite volume and which eventually coalesce into intralayer cracks. Hence, tensile damage is distributed throughout the volume, and could be more significant outside the sensor area. In this work, tensile damage of unidirectional glass fiber-reinforced polymer composites (GFRP) and plain weave carbon fiber-reinforced polymer composites (CFRP) is sensed by utilizing a spray-on nanocomposite sensor, which is then instrumented by boundary electrodes. The resistance change distribution within the sensor area is reconstructed from a series of boundary voltage measurements, and ERT is implemented using a maximum a posteriori approach and assumptions on the type of noise in the reconstruction. Results show that this technique has promise in tracking uniaxial damage in composites. The different fiber architectures (unidirectional GFRP, plain weave CFRP) give distinct features in the ERT, which are consistent with the physical behavior of the tested samples.

Validation of a Representative Volume Element for unidirectional fiber-reinforced composites: Case of a monotonic traction in its cross section

Based on a Finite Elements Analysis homogenization technique which uses a Representative Volume Element with a random distribution of the fibers, this article presents the adequate characteristics to obtain a mesoscopic model which predicts the mechanical behavior of a unidirectional glass fiber-reinforced polymer composite under monotonic transverse traction. A parametric study is made and finally an appropriate model was chosen to present the comparison between experimental and numerical results. The mesh and the RVE size/fibers size ratio are the most sensitives attributes. Considering the numerical model is in the framework of elasticity and elasto-plasticity, it is in good agreement with the macroscopic response. To improve the accuracy of the model, a further study accounting the interface and damage phenomenons as matrix cracking and fiber/matrix debonding is suggested.

Experimental investigation of kink initiation and kink band formation in unidirectional glass fiber-reinforced polymer specimens

The failure mechanism of non-slender glass fiber-reinforced epoxy prismatic specimens subjected to axial compression and different temperatures was investigated. Digital image correlation was used to map and derive the failure modes. The failure mode changed from splitting failure at ambient temperature to kinking at the onset of the glass transition temperature. The kink initiation mechanism could be clearly observed. Based on the initial waviness of the fibers, the wave amplitude disproportionally increased at one specific location up to fiber microbuckling and surrounding matrix failure; the wavelength of the initial imperfection was thereby maintained. The kink band then rapidly propagated through alternating horizontal and inclined segments leading to an overall inclined kink band of 31° on average. The kink band width doubled from around three to six times the average wavelength of the initial imperfection.

Optically-derived mechanical properties of glass fiber-reinforced polymer laminates for multifunctional load-bearing structures

This paper demonstrates how the translucency of glass fiber-reinforced polymer (GFRP) laminates allows the derivation of their mechanical properties through optical measurements. Spectrophotometric, goniophotometric and tensile experiments were performed on unidirectional and cross-ply hand lay-up GFRP laminates with fiber volume fractions ranging from 0.15 to 0.45. An analytical model to predict the directional fiber volume fractions—and thus the mechanical properties of GFRP laminates—has been developed based on the total and diffuse transmittance and directional light scattering of the laminates. It is demonstrated that structurally optimized GFRP laminates can meet the requirements for GFRP skylights and the encapsulation of photovoltaic cells into translucent GFRP laminates.

Mechanical behaviour of FRP-confined masonry by testing of full-scale columns

The vulnerability of masonry constructions under seismic forces, or more generally under the mechanical actions during the centuries, has been highlighted in the last years by several events that caused the loss of significant heritage buildings. Faced with this difficulty, the use of composite materials, fiber reinforced polymers (FRP) may be a solution for mitigating the vulnerability of masonry buildings. This solution has been tested in the laboratory by researchers in the last decade. In particular, studies regarding elements such as walls, arches and vaults, strengthened with FRP materials are available. A few numbers of studies are known for columns, which have been tested only as small or middle scale samples. The current state of the art does not report studies on FRP-confined masonry columns tested in real scale. The research presents the results of an experimental program performed on full-scale masonry columns strengthened with different composite systems. The same kind of study had been previously performed by the authors on medium scale masonry columns, using the same materials for both the masonry core and for the FRP system. Prismatic columns with a square cross section were subjected to compression tests according to the following test schemes: two control unconfined columns; column with continuous wrapping by using unidirectional glass FRP (GFRP) sheets; column with discontinuous wrapping by using GFRP unidirectional sheets; column with continuous GFRP wrapping and internal carbon FRP bars bonded in the transverse directions; column wrapped with continuous alkali resistant GFRP grid and steel spikes bonded together in lime based matrix. The experimental results are presented and discussed in the paper along with the comparison with the results obtained from the experimental tests on medium scale specimens. The comparison between experimental data and theoretical predictions provided by the analytical model found in the guidelines of the CNR technical document is also illustrated.

Environmental Aging Effect on Tensile Properties of GFRP Made of Furfuryl Alcohol Bioresin Compared to Epoxy

AbstractBiocomposites are emerging as a possible sustainable alternative in structural applications, and as a result, a major focus should be on their durability. This study focuses on the environmental aging of bioresin glass fiber–reinforced polymer (GFRP) through immersion in saltwater at elevated temperatures. The resin is furfuryl alcohol based, derived from renewable resources, such as corncobs and sugarcanes. Deterioration was quantified by tensile testing of unidirectional bioresin GFRP coupons at various stages of exposure, and compared to conventional epoxy GFRP coupons under the same conditions. A total of 150 specimens were exposed to three different environments, namely 23°, 40°, and 55°C water with 3% salt concentration, for up to 300 days. It was found that the bioresin GFRP retained 80, 44, and 39% of its original strength at the three temperatures, respectively. On the other hand, the epoxy GFRP exhibited 86, 72, and 61% strength retentions, respectively. No reductions occurred to the You...

SQP법을 사용한 복합재 조류력 발전용 블레이드의 스파 캡에 대한 두께 최적화

In this study, the thickness optimization for uni-directional (UD) glass fiber reinforced polymer (GFRP) laminates of the spar cap of composite tidal blades was performed under the tip deflection constrains. The spar cap was composed of GFRP composites and carbon fiber reinforced polymer (CFRP) composites. The stress distributions in the blade as well as its material costs for the optimized results were additionally investigated. The optimized thickness was obtained by interacting a sequential quadratic programming (SQP) algorithm and an ABAQUS software to calculate an objective function. It was confirmed that the thickness of UD GFRP increased with a decrease of the restrained tip deflection when a thickness of UD CFRP laminates was constrained to 9 mm. The weight of the optimized spar-cap increased up to 96.2% while the maximum longitudinal tensile stress decreased up to 24.6%. The thickness of UD GFRP laminates increased with a decrease of the thickness of UD CFRP laminates when the tip deflection was constrained to 126.83 mm. The weight increased up to 40.1%, but the material cost decreased up to 16.97%. Finally, the relationships among the weight, internal tensile stress, and material costs were presented based on the optimized thicknesses of the spar cap.

Microwave Absorption of Structural Polymer Composites Containing Glass-Coated Amorphous Microwires

A new kind of glass fiber reinforced polymers (GFRP) incorporating ferromagnetic microwires are developed for microwave absorption applications. Strikingly, an orientation preference to the absorption due to the polarization effect of glass fibers is observed for the unidirectional GFRP laminates; there also exists a remarkable dependence of absorption on the number of laminates. These observations are rationalized by an equivalent circuit model. With wires addition, we established here the correlation of the microwave absorption performance with the local properties of wires such as wire geometry, the mesostructure including inter-wire spacing, wire orientation, wire pattern (random or continuous) and wire concentration. All these observed effects are elucidated within the framework of microwave absorption theory and equivalent circuit models.

Micro-mechanical fatigue modelling of unidirectional glass fibre reinforced polymer composites

A multi-scale numerical fatigue modelling methodology is proposed for the characterization of axial tension–tension fatigue behaviour of unidirectional Glass Fibre Reinforced Polymer (GFRP) composites. The methodology is developed to link the dominant fibre failure mechanism to composite fatigue properties, by a three-step geometrical up-scaling process. This study aims to provide composite fatigue predictions by only using the material properties of fibres and matrix. Compared to the experimental results, the fatigue model over-predicts fatigue lives and gives reasonable estimations of axial stiffness degradation curves of the unidirectional GFRP composite dog-bone specimens.

Micromechanically derived lowest threshold stress intensity factor for macroscopic stress-corrosion cracking in unidirectional GFRP composites

Observation by scanning electron microscopy of a typical fracture surface in glass fiber-reinforced polymer (GFRP) composites shows that the fracture surface of each fiber is characterized by a mirror region surrounded by a hackle region. This indicates a two-stage fracture process: (1) a ‘‘slow’’ (meaning time-dependent) fracture of a portion of the glass fiber is followed by (2) a ‘‘fast’’ unstable fracture across the remainder of the glass fiber. On the premise that the time-dependent failure of GFRP composites in acid environments is controlled by the initiation and slow growth of a crack from a pre-existing inherent surface flaw in a glass fiber, a micromechanical model of stress-corrosion crack growth (sometimes called the Sekine–Miyanaga–Beaumont model) was constructed [1–3]. Using this model, an equation was derived for the macroscopic crack growth rate as a function of the apparent crack tip stress intensity factor for the mode I. If tougher and more ductile matrices are used, there exists a lowest threshold value of the stress intensity factor. By assuming that the polymeric fibrils of matrix or ligaments stretched between the crack surfaces behave according to an ideal cohesive force model, i.e., the Dugdale model [4], the lowest threshold stress intensity factor was given in an explicit form [3]. However, the examination in the case of a more general cohesive force model, i.e., the Barenblatt model [5], has been left until now, although this examination will give a physically definite description of the lowest threshold stress intensity factor. We begin this examination with a brief review of our previous article [3]. In bulk glass, the stable crack growth rate due to stresscorrosion cracking da/dt can be given by