Unidirectional GFRP Research Articles

Influence of stitching on the interlaminar fracture toughness energy – modes I and II – of unidirectional GFRP

Abstract The influence of stitching on the delamination resistance of unidirectional glass fiber-reinforced plastic laminates depends strongly on the loading mode. The application of a lock stitch with a stitch density of 3.4 cm−1 perpendicular to the fibers results in an increase of the interlaminar fracture toughness energy, mode I, G IC by a factor of 2.7 compared to an unstitched reference laminate. The Kevlar threads are pulled out of one half of the DCB specimen leaving large frayed fiber bundles sticking out of the other half. The same stitching, however, does not improve the interlaminar fracture toughness energy, mode II, G IIC. The Kevlar threads are not able to deform the surrounding matrix thereby expending additional energy. They fail directly at the mid-plane of the ENF specimens. An increased stitch density of 11.6 cm−1 can be expected to lead to a further significant increase in the G IC value. But the tight stitching pattern causes the bonded metal hinges to tear off. The force introduction would have to be changed to enable testing of corresponding DCB and subsequently ENF specimens.

Modeling and failure mechanism study of unidirectional GFRP corrugated sandwich sheet piles based on discrete element method

In this paper, a DEM model of a unidirectional glass fiber reinforced polymer composite sandwich sheet pile (UGFRP-CSSP) is developed to elucidate the damage mechanisms under longitudinal bending and axial compression at the mesoscale, in order to bridge the gap between existing knowledge and its potential applications as sheet pile walls. A series of four-point bending and axial compression tests are performed on UGFRP-CSSP to validate the DEM model, which is calibrated through a sensitivity analysis of the mesoscopic parameters. The failure behavior, crack progress, and initial crack formation of the UGFRP-CSSP are investigated by correlating the results from physical experiments and numerical simulations. The research reveals that the DEM model accurately depicts the failure behavior under bending and compression of unidirectional (UD) composite elements, offering a mesoscopic perspective on the failure origins. It is identified that the tensile shear strength of the resin is the weak point leading to initial failure, as it is lower than that of the glass fibers. Longitudinal bending failure typically initiates at the skin-core interface and around the central axis of core, with cracks propagating longitudinally due to compression shear failure between fibers. Axial compression failure also occurs predominantly at the skin-core interface, with similar longitudinal cracking attributed to tensile shear failure at the acute angle fiber connections in the long span side. Finally, a new viewpoint was proposed that the fiber modulus may have a decisive impact on the failure behavior of UD composite materials.

Experimental investigation on transverse tension-tension fatigue behavior of pultruded glass-fiber reinforced polymer (GFRP) unidirectional lamina

Inadequate transverse or shear strength is the key factor contributing to the failure of pultruded GFRP structures. In this paper, the static and fatigue performance of 90° pultruded unidirectional GFRP lamina were tested. A series of macroscopic phenomenological models were established to describe the performance variations during cyclic loading, including fatigue life, energy dissipation capacity and cyclic creep strain. The fracture surfaces of specimens were observed by a field-emission scanning electron microscopy (SEM). Experimental results indicated that the transverse fatigue life of GFRP unidirectional lamina can be fitted by S-N-R regression equation. The transverse stiffness degradation can be even ignored during fatigue loading. The results also show that the proposed equations can reasonably reflect the growth trend of dissipated energy per unit volume (UDE) and successfully predict cyclic creep strain increment of pultruded GFRP during cyclic loading.

The use of induction heating for thermographic inspection of composite laminates

In the present paper, an active thermographic technique is proposed for the inspection of a unidirectional GFRP panel with integrated artificial internal defects and an aeronautical CFRP panel with real small production defects.Different inductor geometries were considered by varying the length and the number of spires, finally allowing to identify the optimal solution. Several tests allowed to identify the optimal experimental setup for defect detection as a function of the location of the defect.The acquired thermal maps were processed in Matlab with a suitable analysis algorithm for the determination of the absolute and normalized thermal contrasts. Another important consideration concerns the possibility of obtaining a qualitative indication on the defect depth. Although the thermal contrast depends on the choice of the intact and defective area, the maximum value of the absolute contrast was higher for the more superficial defects than for the deeper ones. Furthermore, the absolute contrast was correlated with the size of the defects. In the study of CFRP panel, the defects characterization was conducted in a comparable way to evaluate the contrasts with a suitable choice of undamaged reference area.The comparison of the proposed technique with the traditional pulsed technique has demonstrated the real potential of application on SMArt GFRP and aeronautical CFRP elements with the same level of reliability as traditional thermographic techniques. However, the induction technique has shown better ability to detect small real defects more accurately in CFRP panels. However, the limitation of the power generator and the size of the inductor could limit the detectability of defects.

Theoretical and experimental analysis of the dielectric properties of 3D orthogonal woven GFRP composites in the terahertz frequency range

A novel method to calculate the dielectric constant (εr′) of three-dimensional orthogonal woven glass fiber-reinforced polymer-matrix (3DOW-GFRP) composites in the terahertz (THz) frequency range is developed and experimentally verified using the THz time-domain spectroscopy (THz-TDS). The dielectric anisotropy is demonstrated through simulation by considering a single propagation direction of the THz waves and three different fiber orientations in unidirectional GFRP composites. Simulation results are compared to those obtained from the classic rule-of-mixture equations to determine the representative equations for the three different cases of fiber orientations in the (x,y,z)-coordinate system and a new model is developed to calculate εr′ of 3DOW-GFRP composites based on electromagnetic modeling principles. As opposed to previously reported lumped circuit models, our model addresses the issue of orthogonality of fiber and considers the shape and spatial disposition of the composite with respect to the polarization direction of the THz waves for accurate determination of εr′. A comparison between the measured and calculated εr′ indicates that the proposed method is highly accurate with ≤ 2.68% maximum error, while the latter reaches 7.82% for the classic rule-of-mixture equations. This method is potentially useful for the design of 3DOW-GFRP with desired εr′ for applications such as electromagnetic shielding and electrostatic discharging structures.

Preliminary Results on Natural Aging of GFRP-Reinforced Masonry Components Exposed to Outdoor Environmental Conditions

Fiber reinforced polymers (FRP) have been widely used for strengthening and repair of masonry structures since the 1990s. Although the short-term performance of such systems has been extensively studied, their durability and long-term behavior are still scarcely known. Therefore, a better understanding of the corresponding degradation mechanisms and durability issues is of major interest, especially in cases of externally bonded systems where the materials are prone to undergo a more severe decay. Different accelerated aging tests have been developed in order to simulate weathering conditions in a shorter time: water immersion, freeze-thaw, hygrothermal cycles, salt crystallization, UV radiation, etc. Nonetheless, accelerated aging data cannot be used to make reliable service life estimations without a clear correlation with real environmental processes. In this study, we present the preliminary results (3/12 years) of a real exposure experimental campaign devoted to analyzing the natural aging of masonry components reinforced with externally bonded glass fibers (GFRP). The aging conditions consist of: (a) direct outdoor exposure, i.e. outdoor hygrothermal variation + sun and rain; (b) indirect outdoor exposure, i.e. outdoor hygrothermal variation + sheltered from sun and rain; (c) laboratory conditions as control group. The specimens consist of solid extruded fired-clay bricks strengthened with unidirectional GFRP sheets following the wet lay-up procedure. The aging effects on bond performance are studied by means of single-lap shear tests. Finally, the effect of brick surface treatment and the application of an external render on bond durability is investigated.

Direct observation of plastic shear strain concentration in the thick GLARE laminates under bending loading

The results of the digital image correlation (DIC) analysis for the three-point bending tests with GLARE1-9/8 laminates (9 aluminum and 8 unidirectional GFRP layers) are presented in this paper. The findings show that, in samples with a small span-to-thickness ratio, an intensive concentration of the plastic shear strain arises inside the GFRP layers, which significantly affects the load-bearing capacity and apparent interlaminar shear strength of GLARE. Previously, such effects were predicted based on numerical simulations. In the present paper, we validate these theoretical predictions based on the full-field DIC analysis of the shear strain distribution across the GLARE thickness. For this analysis, samples with longitudinal and transverse orientation of glass fibers in the GFRP layers were studied and compared. For the transverse samples, we also found that the matrix cracking in the GFRP layers becomes an additional source for the overall non-linear behavior of GLARE, such that the pure failure mode related to the interlaminar shear cannot be achieved.

Performance and analysis of unidirectional GFRP actively confined high-strength concrete under monotonic and cyclic axial compression

For better understanding the response of unidirectional GFRP actively confined high-strength concrete under dynamic load, this paper mainly investigated the influences of active confinement on the cyclic axial compression performance of high-strength concrete cylinders. The active confinement was produced by the innovative combined action from core concrete expansion and external confinement of unidirectional GFRP at different confinement stiffness. A total of 24 unidirectional GFRP confined high-strength concrete cylinders were tested under monotonic and cyclic axial compression. The results show that the strength enhancement ratio of actively confined high strength concrete cylinders is higher than that of ordinary confined counterparts regardless of the loading modes. In addition, owing to the lateral prestress, the brittle macro cracking of the high strength concrete was reduced. The hoop rupture strain of the actively confined concrete cylinder increases with the increase of the active confinement. The results also show that the stress degradation rate of unidirectional GFRP actively confined high-strength concrete is improved remarkably due to the compactness of core concrete. Guided by the experiments, the high-precision models for the compressive strength and stress–strain relationship of unidirectional GFRP confined high-strength concrete under cyclic axial compression were developed.

Comparative study of impact response of GFRP and CFRP composite plates against blunt nosed projectile

Three-dimensional finite element simulations were carried out to explore the perforation behavior of the unidirectional GFRP and CFRP plates against the blunt nosed projectile within sub-ordnance velocity regime. The ballistic performance of both the composite plates of 140 mm × 140 mm × 3.3 mm comprising of four layers (each 0.825 mm thick) having unidirectional fibres was evaluated. The mass, diameter and length of projectile was kept constant as 52.5 gm, 19 mm and 50.8 mm respectively. To carry out the numerical simulations, commercial finite element code Ansys/Ls-Dyna was employed. To simulate the high strain rate behavior of GFRP composite material, enhanced composite damage material model was considered whereas for CFRP, Composite failure material model offered better results. The performances of both the plates were evaluated based on residual velocity, ballistic limit and energy absorption. The numerical results were found to be close enough with the experimental results available in the literature. It was found that GFRP plates underperformed than that of CFRP plates.

Effect of the impact strength of glass fibre reinforced plastic composite using wet layup process

Fabrication of glass fibre reinforced plastic was done using wet layup process and is discussed in the paper. The main purpose of the experiment was to identify the changes in the GFRP when subjected to various chemical resistance test and toughness test. GFRP was combined with different materials to form hybrid natural composite whose characteristics were studied and analysed. The results showed that GFRP had maximum chemical resistance in 5% Sodium hydroxide and minimum in 20% Sulphuric acid. From the Izod impact test, the maximum impact strength was found in unidirectional GFRP and minimum was found in orthogonal GFRP.

Comparison of Pulse Thermography (PT) and Step Heating (SH) Thermography in Non-Destructive Testing of Unidirectional GFRP Composites

Abstract This paper presents two techniques of active thermography i.e. the pulsed thermography technique and the step heating technique. The aim of this article is to compare these two techniques and present the possibilities, advantages and limitations of their use in the context of non-destructive testing of composite materials. The experimental section presents the results of tests carried out on samples of the polymer composites reinforced with glass fiber.

Equistrong Branchy Composite Beams with a Constant Total Area of Variable Elliptic Cross Sections

It is shown that it is possible to create branchy and shaped composite leaf springs with a constant cross-sectional area and three-fold reduced weight, at a given level of accumulated elastic energy, by choosing an appropriate spring geometry. The advantages of branching in comparison with shaping are the absence of cut fibers, exclusion of fiber disorientation, and the possibility of decreasing the spring size by combining its leafs into a bundle. The use of a unidirectional GFRP makes it possible to multiply reduce the spring weight compared with that of a steel analog with the same strength and stored energy requirements. An efficient use of branchy composite springs is possible for transport systems and, in the future, for space-based constructions due to their low weight and the extremely low energy of their production, and these factors allow one, in principle, to create such springs directly in orbit conditions.

Development of lightweight emergency bridge using GFRP–metal composite plate-truss girder

Design of a lightweight emergency vehicular bridge comprising a GFRP–metal composite plate-truss girder and measuring 24 m in span is reported. The said bridge was designed based on optimization of an original 12-m bridge specimen. The bridge, so developed, is intended to be lightweight, structurally sound with modular feasibility, and representative of a construction that is less time consuming overall and fully exploits advantages offered by the use of inherent and complementary pultruded GFRP materials. Conceptual design and considerations of the large-scale structure were first described in detail. Subsequently, full-scale nondestructive tests were performed under on- and off-axis static loadings to evaluate the actual linearly elastic mechanical behavior of the prototype. Experimental results demonstrated that the bridge satisfactorily met the requirements of strength, overall bending stiffness, and torsional rigidity with regards to emergency-bridge applications. Being recognized as the most critical loading case for emergency bridges with major influence on load distribution among truss girders, the lateral live-loading distribution was assigned great importance during design of the unique bridge. Extrusion-type unidirectional GFRP profiles with high-longitudinal but low shear strengths are predominantly suitable for structures subjected to large axial forces, and are, therefore, appropriate for application in the proposed hybrid structural system. Favorable testing results demonstrated that the proposed improved version of the original conceptual design can appropriately be used as a truss girder for a new lightweight emergency bridge with a longer measured span. It is suggested that such a hybrid bridge, which demonstrates reasonably good linearly elastic behavior under service live loads, must also be designed in accordance with a stiffness criterion. Corresponding finite element and analytical analyses were performed and compared against experimental results whilst demonstrating good agreement. The elicited comparisons indicated that the established simplified analytical models and the finite element model (FEM) were both equally applicable for use in preliminary structural calculations and design of the improved bridge under states within its serviceability limit. Results reported herein are expected to make a valuable initial contribution, which in turn, could further lead to development of similar lightweight structural systems.

Mode of failure for reinforced concrete beams with GFRP bars

Crack propagation in concrete structures is a very complicated process. An experimental method to predict the stress distribution of a cracked GFRP reinforced concrete flexural beam is presented. The beam subjected to four-point bending is internally reinforced with unidirectional GFRP bars. The aim of this investigation is to study the flexural performance of reinforced concrete members strengthened using GFRP bars. For the flexural tests performed on reinforced concrete beams strengthened with GFRP bars, the ultimate load of the beams strengthened with GFRP was reasonably increased. The mode of failure for beams reinforced with GFRP sections is slightly different compared with the traditional beam. The GFRP reinforced concrete beams fail either by concrete crushing at the compression zone or rupture of the GFRP reinforcement.

Measurement of tensile strength and infrared thermography on unidirectional GFRP

In this study, matrix is polypropylene, and reinforcement is glass fiber for manufacturing GFRP. Effect of fiber content and fiber orientation angle on tensile strength is evaluated. In addition, IR thermography is applied to detect temperature distribution during crack propagation and temperature change due to load on test specimen, which is not the existing non-destructive analysis method. Using lock-in IR thermography method that can detect minute change of temperature, temperature change from energy dissipation during tensile test and temperature distribution during crack propagation is monitored. Tensile strength of unidirectional GFRP is decreased from 0° to 90°. In addition, 20wt% of fiber content shows the higher decrease of tensile strength than the one with 10wt%. For higher than θ=40° of fiber orientation angle, tensile strength ratio becomes lower than the one for polypropylene (matrix), and there is no effect of fiber reinforcement. Get nearer to 90° of fiber orientation angle forces longer fracture time, and maximum temperature is decreased accordingly. Fracture is occurred right after maximum stress point, and it is related with tensile strength.

Detection and Characterization of Waviness in Unidirectional GFRP Using Rayleigh Wave Air Coupled Ultrasonic Testing (RAC-UT)

In this article, we propose a method of detection and quantification of fiber waviness or “marcels” in unidirectional glass fiber reinforced composite plates using air coupled ultrasonics. Severity of waviness was defined with the help of aspect ratio, which is the geometric characteristic of the marcel. Several wavy samples with different aspect ratios were fabricated and tested with the defined method. Waviness was detected by performing C-Scans and characterized with the help of B-Scans to determine the change in time of flight while traversing along the length of the wave. This method can be effectively implemented as a field technique for marcel characterization in thick composites.

116 マルチスケール一方向GFRPの作製および機械的性質評価

116 マルチスケール一方向GFRPの作製および機械的性質評価

W042003 光ファイバセンサによるFRPの成形モニタリング(【W04200】知的材料・構造システムの現状と今後の展開,機械材料・材料加工部門企画,ワークショップ)

It has been proven that fiber-optic refractive-index sensors can be used for monitoring cure process of thermosetting resin. In the present paper, process monitoring of GFRP was conducted by embedded refractive-index sensors. The sensors were embedded in 0° and 90° directions of unidirectional GFRP laminates and in resin of prepregs in order to investigate effect of reinforcing fibers on refractive index of resin. The laminates were manufactured at 140℃ and pressure of 0.5 MPa by a hot-press molding device. The experimental results showed that the effect of reinforcing fibers had remained small until cure process finished, however thermal residual stress caused by reinforcements affected refractive index of prepreg at cooling. Therefore, it was found that cure index of FRP prepregs could be measured by refractive-index sensors. In addition, it appeared that the refractive-index sensors were available to monitor air voids in resin qualitatively during molding.

316 一方向GFRPの成形ひずみ測定 : モデル材による検討

316 一方向GFRPの成形ひずみ測定 : モデル材による検討

An Investigation into the Response of GFRP-Reinforced Glue-Laminated Tudor Arches

This paper presents the results of an experimental and computational investigation tailored to examine the response of glass fiber-reinforced-plastic-(GFRP-) reinforced glue-laminated curved beams and arches. The main objective was to ascertain the viability of GFRP as an effective reinforcement for enhancing the load carrying capacity and stiffness of such curved structures. The study included optimization of the length and thickness of the GFRP reinforcement. In doing so, first a parametric finite element study was conducted to evaluate the influence of unidirectional GFRP reinforcement applied onto the arch using eleven possible configurations and different thicknesses. Subsequently, an experimental investigation was conducted to verify the results established by the finite element method as well as the integrity of actual GFRP-reinforced glue-laminated curved structures. The results indicate that GFRP can be considered as an effective and economically viable solution for strengthening and stiffening glulam arches, without adding any appreciable weight to the structure.