Glass Fiber Reinforced Polymer Skins Research Articles

Fire performance of pultruded wood-cored GFRP sandwich components for building construction

Fire safety is significantly required for glass-fiber reinforced polymer (GFRP) sandwich composite structures since the GFRP skin and the core are normally sensitive to high temperatures. This study presents an investigation on the thermal responses, insulation performance, and charring behavior of the pultruded wood-cored GFRP sandwich (PWGS) components subjected to the ISO-834 fire. The PWGS specimens protected with calcium silica (CS) boards were exposed to one- or three-sides fire with the fire duration being 30 or 60 min. It was found that the GFRP skins without CS board protection were fully charred after the 30-min fire exposure. Nevertheless, the PWGS section exhibits good insulation. The average temperature rise of the unexposed surface of the PWGS sections without protection did not exceed 140 °C, satisfying the insulation requirement in specifications such as EN 1995-1-2, ASTM E119 and GB/T 9978.1. Further, the PWGS specimen with a 25 mm-thick CS board can survive without clear damage during the 60 min thee-sides fire exposure. In addition, the charring rate of the GFRP skin and the Douglas-fir core were summarized and discussed.

Mechanical Properties of Sandwich Composite using Glass Fiber Reinforced Polymer as A Skin and 3D Printed Polylactic Acid as A Core

Recently, 3D printing technology has become a practical method to realize products rapidly. It is suitable for making small quantities of products. Although it is capable of printing with a high level of geometric complexity, there is a lack of tensile strength due to its process where the products are printed layer by layer. However, this technology is potentially to be combined in a composite manufacturing process. Mostly, a composite product is made by using a mold. This mold is relatively expensive and can only create a product with less complexity. Nevertheless, the composite product has main advantages such as light, strong, and flexible. Therefore, combining these two technologies is a new breakthrough in realizing products with high complexity, light, strong, and flexible. This study aims to determine the mechanical properties of sandwich composite filled with 3D printed product as a core. Several parameters were varied including core thickness and skin thickness. The skin material was a Glass Fiber Reinforced Polymer (GFRP) while the core material was 3D printed Polylactic Acid (PLA). The tensile and bending tests have been done in accordance with ASTM D638 and ASTM D790. The results showed that the addition of GFRP skin on the sandwich composite could significantly increase the tensile strength but did not have an impact on the flexural strength. The highest flexural strength of 50.36 MPa was achieved at 3 layers of GFRP skin while a remarkable tensile strength of 55.74 MPa was obtained at 4 layers GFRP skin. Moreover, the addition of core thickness also does not have an impact on flexural strength. The flexural strength of the 3D printed core was around 20 MPa for all thickness. However, when 2 layers of GFRP skin were used, a remarkable flexural strength of 57.67 MPa was obtained but the flexural strength was then decreased when using 10 and 15 mm cores.

Manufacture and behaviour of innovative 3D printed auxetic composite panels subjected to low-velocity impact load

This paper investigated the low-velocity impact behaviour of innovative 3D printed auxetic composite panels experimentally. The panel consisted of glass fibre reinforced polymer skins and a 3D printed glass fibre – polyamide 12 (with fibre weight content of 30%) composite core. The core was extended from the 3D re-entrant auxetic unit cell, which consisted of four umbrella shaped elements arranging in a specific manner and the auxetic behaviour was caused by ‘folding up’ or ‘opening’ of these elements. A total of 15 composite panels were fabricated and tested under low-velocity impact, the main parameters of which were the diagonal rod radius of the core and the impact energy. Test results revealed that the auxetic behaviour of the composite panel could be observed using high-speed camera. Besides, as the impact energy increased, the panel failed more brittlely or even destroyed completely. In addition, the larger impact energy resulted in larger damage area of the skin and higher peak impact load of the composite panel. Moreover, with the same impact energy, the increased rod radius resulted in a stiffer structure, which had larger impact resistance and thus the damage was minor, leading to larger impact load. The peak impact load to normalized weight ratio was similar for specimens with different rod radius, indicating that increasing rod radius could not improve the material efficiency in resisting impact load.

Flexural Behavior of Slab-Rib Integrated Bridge Decks with GFRP Skin and Polyurethane Foam Core

This paper presents experimental and analytical studies on flexural behavior of slab-rib integrated Sandwich composite decks. The influences of layers of glass fiber-reinforced polymer (GFRP) facesheets, foam densities, and the existence of webs and cross beams are discussed herein. The test results showed that the existence of vertical webs remarkably improved the debonding of the facesheets from the foam core, thus increasing the ultimate load by 59% compared with the specimens without webs. However, the existence of horizontal webs has insignificant effect on the failure mode and ultimate load. Increasing the number of layers of GFRP facesheets from 2 to 4 and 6 results in 100% and 214% increments in ultimate loads, respectively, while the specimen with lower density of foam had a higher ultimate load than the specimen with higher density of foam due to deformation compatibility between GFRP skins and foam core with low density. The analysis software Abaqus Explicit was used to simulate the flexural behavior of test specimens, and the numerical results agreed well with the test data. The verified finite element model was extended to analyze the influences of the number of GFRP layers on the top of decks and the height of vertical webs. Based on equivalent method and compatibility of shear deformation, the flexural and shear rigidities were estimated. Then, analytical solution for displacement of the slab-rib integrated Sandwich composite decks subjected to four-point load was derived out. Comparison of analytical and experimental results shows that the displacements can be precisely predicted by the present theoretical model.

Terahertz time domain spectroscopy and imaging application for analysis of sandwich panel with embedded fibre Bragg grating sensors and piezoelectric transducers

Sandwich structures are commonly used in many objects, e.g. deck components in bridge deck panel. Safety requirements result in development of structural health monitoring (SHM) systems that are implemented into sandwich structures. Such systems based on different types of sensors that can be embedded into the structure. The paper presents THz time domain spectroscopy and imaging application for internal structure analyses of a sandwich panel with glass fibre reinforced polymer skin and polyurethane foam fulfilment. In the adhesive bond between skin and fulfilment, an array of fibre Bragg grating (FBG) sensors and piezoelectric (PZT) transducers was embedded. The analyses were focused on detection and localisation of embedded sensors and transducers and their influence on THz wave propagation. The analysis contains differences in THz wave parameters, like refractive index and absorption coefficient and image analyses. Methods that allow to detect and localise of transducers and fibre optics with FBG sensors as well as damage (debonding between GFRP skin and foam fulfilment in sandwich structure) were proposed.

On-axis and off-axis compressive behavior of pultruded GFRP composites at elevated temperatures

In this study, the on-axis and off-axis compressive behavior of pultruded unidirectional glass fiber reinforced polymer (GFRP) coupons at elevated temperatures were investigated. The coupons with fiber orientations of 0°, 30°, 60° and 90° to the compressive loading axis were tested from 20 °C to 180 °C. The specimens under on-axis compression clearly exhibited a kinking failure mode at elevated temperatures, while the in-plain shear failure occurred in the specimens of 30° and 60° off-axis compression, and the out-of-plain shear failure occurred in 90° off-axis specimens. The failure modes were hardly affected by the temperatures except that the GFRP skin wrinkling captured in off-axis compression specimens above 60 °C. As expected, the compressive strength and elastic modulus were significantly reduced with the increase in temperature. Moreover, the compressive strength of on-axis and off-axis specimens exhibited similar retention rate at each targeted temperature, while the elastic modulus of on-axis specimens showed a higher retention rate than those of off-axis specimens. The analytical studies showed that the degradation of modulus and strength of all specimens generally followed the inverse rule of mixture. In addition, the off-axis strength and elastic modulus were estimated by the Hankinson’s formula.

Effects of the core density on the quasi-static flexural and ballistic performance of fibre-composite skin/foam-core sandwich structures

Polymeric foams are extensively used as the core materials in sandwich structures and the core material is typically bonded between relatively thin fibre-composite skins. Such sandwich structures are widely used in the aerospace, marine and wind-energy industries. In the present work, various sandwich structures have been manufactured using glass-fibre-reinforced polymer (GFRP) skins with three layers of poly(vinyl chloride) foam to form the core, with the densities of the foam layers ranging from 60 to 100 kg/m3. This study has investigated the effects on the quasi-static flexural and high-velocity impact properties of the sandwich structures of: (a) the density of the polymeric-foam core used and (b) grading the density of the foam core through its thickness. The digital image correlation technique has been employed to quantitatively measure the values of the deformation, strain and onset of damage. Under quasi-static three-point and four-point bend flexural loading, the use of a low-density layer in a graded-density configuration reduced the likelihood of failure of the sandwich structure by a sudden force drop, when compared with the core configuration using a uniform (i.e. homogenous) density layer. The high-velocity impact tests were performed on the sandwich structures using a gas-gun facility with a compliant, high-density polyethylene projectile. From these impact experiments, the graded-density foam core with the relatively low-density layer located immediately behind the front (i.e. impacted) GFRP skin was found to absorb more impact energy and possess an increased penetration resistance than a homogeneous core structure.

Mechanical performance of GFRP-profiled steel sheeting composite sandwich beams in four-point bending

This study proposed a novel GFRP-profiled steel sheeting composite sandwich beam (GPSS beam) composed of glass fiber reinforced polymer (GFRP) skins, a lightweight polyurethane (PU) foam core and a 0.9-mm-thick profiled steel sheeting. The interaction between the GFRP skins and the profiled steel sheeting was enhanced by the use of stainless steel core rivets. The flexural performance of the GPSS beams was studied through experimental and analytical methods. Eight beam specimens, including a control specimen, were tested in 4-point bending tests to verify the efficiency of the profiled steel sheeting to improve the stiffness, ultimate load-bearing capacity and energy dissipation capacity of the beams. Test results demonstrated that the bending stiffness and ultimate load-bearing capacity can increase up to 306% and 158%, respectively compared to those of the control specimen. The energy dissipation ability of GPSS beams was increased greatly by using stainless steel core rivets. Additionally, unlike the control specimen, the GPSS beams collapsed in a ductile manner. Analytical formulas for predicting the bending stiffness and ultimate load-bearing capacity of GPSS beams were proposed and captured these effects reasonably accurately. The flexural stiffness of the GPSS beams with the profiled steel sheeting were equivalent to those of the same size RC beams with a moderate to heavy reinforcement ratio of 1.0–2.4%, but the GPSS beams were 25.5–65.1% higher in ultimate load-bearing capacity and 5–8 times lighter in weight.

Increasing flexural performance of hybrid sandwich panels by using strain hardening cementitious base composite and glass fiber-reinforced polymer

The present paper explores different techniques for increasing flexural performance of composite sandwich panels made of hand-layup glass fiber reinforced polymer skins and low density closed cell polyurethane foam core. An experimental program compares the performance of simple panels face to the use of transversal and longitudinal internal glass fiber reinforced polymer ribs and the installation of a strain hardening cementitious base composite top layer over the panels. Based on the experimental results, finite element models are also developed to simulate the flexural behavior of tested panels and conducted an in depth analysis of the techniques studied. Obtained experimental and numerical results show that the use of internal glass fiber reinforced polymer ribs (especially in longitudinal direction) along with the use of strain hardening cementitious base composite significantly and effectively increases the flexural performance of sandwich panels. The high stiffness to weight ratio capacity of the panels attained by applying the proposed method enables its use in rehabilitation of historical places. Furthermore, the proposed techniques could be helpful for strengthening available composite sandwich panels.

The influence of different types of core materials on the impact behaviour of sandwich composites

Sandwich structures are popular in applications in which the weight of the component affects the efficiency, especially in the aviation and aerospace industries. This study aims to understand the impact behaviour of sandwich structures with different core materials. Sandwich structures are manufactured with glass fibre reinforced polymer skins and balsa wood, polyethylene terephthalate (PET) and polyvinyl chloride (PVC) core through resin infusion under flexible tools. Three different core materials were tested and compared using the damaged area of the back face of the sample. The effect of the core materials on the mechanical behaviour of the structures is crucial. The results showed that the microstructure of the core materials plays an important role, because althoughthe density of balsa wood is greater than the density of PET and PVC, the structures having PVC and PET as core materials undergo less damage than those having balsa wood as a core material. Keywords: Sandwich composite, impact behaviour, core materials.

Structural behavior of load-bearing sandwich wall panels with GFRP skin and a foam-web core

Abstract An innovative load-bearing sandwich wall panel with glass fiber-reinforced polymer (GFRP) skins and a foam-GFRP web core (GSFW wall panels, where “GS” denotes GFRP skin and “FW” denotes foam-GFRP web core), which was manufactured using a vacuum-assisted resin infusion process, was developed in this paper. An experimental study involving nine specimens was conducted to validate the effectiveness of this panel for increasing the axial strength under edgewise compression loading. The effects of web thickness, web spacing, web height, and skin thickness on axial stiffness, displacement ductility, and energy dissipation were also investigated. The test results demonstrated that axial strength, axial stiffness, displacement ductility, and energy dissipation could be improved by increasing the web thickness, web height, and skin thickness. An analytical model that considers the confinement effect of foam and the local buckling of GFRP skin was proposed to predict the ultimate axial strength of GSFW panels. A comparison of the analytical and experimental results showed that the analytical model accurately predicted the ultimate axial strength of GSFW wall panels under edgewise compression loading. To simulate the low velocity impact by blindings that are rolled by the wind, an impact test was conducted and the residual axial strength of the wall panels after impact was also investigated.

The effect of foam core density at various slenderness ratios on axial strength of sandwich panels with glass-FRP skins

Abstract In this study, the effect of the density of a polyisocyanurate (PIR) foam core on the behavior of axially loaded sandwich panels with glass fibre reinforced polymer (GFRP) skins are examined, for different panel heights. Thirty-six test specimens with a cross-sectional foam core area of 100 × 50 mm2 and symmetrical 100 mm wide GFRP skins were fabricated with heights of 750, 1000, 1250, and 1500 mm to examine various slenderness ratios (kLe/r). Three PIR foam core materials with densities of 32, 64 and 96 kg/m3 were used. An out-of-straightness assessment was first conducted to ensure that eccentricities were within acceptable limits. All samples were tested to failure in axial compression with pinned end conditions. It was shown that doubling and tripling the core density led to increases in peak load of 71 and 170%, respectively, due to the enhanced composite action and reduced shear deformations. On the other hand, slenderness had insignificant impact on peak loads, contrary to what one would expect from Euler's theory. The reason being the variety of governing failure modes. Columns with low (kLe/r) of 22–25 failed largely due to localized single skin buckling, while those with high (kLe/r) of 51–61 failed primarily by global buckling followed by secondary core shear failure with skin debonding. Columns with intermediate (kLe/r) experienced a combination of localized and global failure modes.

A Methodology for Enhancing the Shear Response of Sandwich Composite Panel: Sandwich Composite Panel with Stair Keys

Objectives: In composite sandwich panel, delamination between the foam core and face sheet is due to inability to withstand shear load. Hence a methodology of introducing shear keys for enhancing the shear response to resist delamination is analysed in the present study. Methods/Statistical Analysis: The conventional sandwich panel under analysis is composed of Glass Fibre Reinforced Polymer (GFRP) skin with PVC (Polyvinyl chloride) foam core impregnated with epoxy resin. The present study is to introduce stair keys in between GFRP and PVC which are made by overlapping GFRP face sheets, with the length and depth to get the required step size. Parametric Finite Element (FE) investigation using ANSYS software has been performed to evaluate the foam core performance by grooving the foam material as stepped shapes. The effect of different length of the stepped grooves in foam material are analysed and its results are compared with conventional sandwich panel. The stair key response for number of steps ranging from n = 1, 3, 5 and 7 are investigated. Findings: The FE results showed an improvement in the shear stress response of the sandwich panel with stair grooved foam core with stair keys compared to conventional model. Also the stair inserts between the GFRP skin and the foam core increases the initial shear stiffness and ultimate shear strength of the proposed model of the sandwich panel. Application/Improvements: The present methodology acts as a new model of peel stopper mechanism, where the shear failure due to debonding of face sheet and foam core is rerouted to foam core. Also it is free from initial damage as there is no loss of solidarity of the bulk material at the foam core, cost effective and easy to design and manufacture compared to other pinning and stitching process.

Flexural behavior of an FRP sandwich system with glass-fiber skins and a phenolic core at elevated in-service temperature

This paper presents the effects of elevated in-service temperature on the flexural behavior of sandwich composites consisting of glass-fiber-reinforced-polymer (GFRP) skins and a phenolic core. The glass transition temperatures of the GFRP skin and phenolic core were obtained from dynamic mechanical analysis (DMA). Subsequently, the flexural behavior of the GFRP skin and the sandwich beams were assessed with a 3-point static bending test at temperatures ranging from room temperature up to 180°C. The results show that the variation trend of the GFRP skin’s stiffness from 3-point static bending test is in general agreement with the corresponding storage modulus results from the DMA. The GFRP skin, phenolic core, and sandwich beams retained more than 80% of their mechanical properties at 80°C. Moreover, the flexural behavior of the sandwich beams was governed by the GFRP skins up to 80°C, while the phenolic core played a major part on overall behavior beyond this temperature. The full-composite action between the skin and core was lost due to the disintegration of the phenolic core at high temperature.

In-Plane Bending and Failure Mechanism of Sandwich Beams with GFRP Skins and Soft Polyurethane Foam Core

AbstractThe out-of-plane bending of sandwich panels has been widely investigated, but studies on their behavior under in-plane loading are quite scarce. In this study, 18 sandwich beam specimens were tested under three- and four-point in-plane-bending. The specimens are composed of low-density polyurethane core and glass fiber-reinforced polymer (GFRP) skins on both sides. The shear span-to-depth (a/h) ratio was varied from 1.33 to 4.67. Two different skin thicknesses were also used, providing beam depth-to-skin thickness (h/t) ratios of 47 and 94. It was found that the ultimate moment reduced by approximately one third as the a/h ratio increased from 1.33 to 4.67 in sandwich beams with an h/t ratio of 94 and decreased by about one quarter in beams with an h/t ratio of 47. Doubling the skin thickness [i.e., from h/t of 94–47] increased the moment capacity significantly. The percentage increase ranged from 55 to 95% as a/h increased from 1.33 to 4.67. Failure of sandwich beams occurred by skin wrinkling in...

Axial Loading Tests and Simplified Modeling of Sandwich Panels with GFRP Skins and Soft Core at Various Slenderness Ratios

AbstractExperimental and analytical studies were conducted to investigate the axial strength and failure modes of sandwich panels with glass fiber reinforced polymer (GFRP) skins and soft polyurethane core. A total of 45 specimens of 78×150 mm cross-section and of lengths Le varying from 500 to 2,400 mm [i.e., slenderness ratio (KLe∶r) of 15–70, where r is the radius of gyration] were tested concentrically using pin-ends (K=1). The effects of skin thickness and an internal GFRP rib on axial behavior were studied. The study first assessed the level of out-of-straightness and concluded that all specimens generally fall within an acceptable limit of span/1,000. A model based on sandwich panel theory, accounting for excessive shear deformations of the soft core, was used to predict axial strength at a wide range of KLe∶r and then used in a parametric study. It was shown that short panels with KLe∶r of 15–17 experienced local failure, outwards skin wrinkling in non-ribbed panels, or skin crushing in ribbed pa...

Independent analytical technique for analysis of the flexural behaviour of the composite sandwich panels incorporated with shear keys concept

The current investigation presents an independent fibre-modelling approach for the analysis of the composite sandwich panels reinforced with shear keys methodology under static bending load. The sandwich panel under investigation is composed of glass fibre reinforced polymer skin with polyvinylchloride and polyurethane foam core, while the semi-circular shear keys inserts are made of chopped strand glass fibre impregnated with epoxy resin. The constituent materials were investigated through a series of quasi-static tests, while the flexural response of the sandwich panel with and without shear keys has been evaluated under four-point bending test. The experimental stress–strain curves of all the constituent materials were considered in the analytical model, while the total deflection is assumed to comprise two main components, one due to flexure and the other due to shear deformation. An incremental approach is used, where the concepts of force equilibrium and strain compatibility are satisfied in every loading step. The numerical procedure was executed using MATLAB code programming with the predictions of the damage mode. A significant improvement in the flexural stiffness and strength of the panel incorporated with shear keys accompanied with a good correlation with the analytical results were observed.

Mechanical behavior of a glass fiber-reinforced polymer sandwich panel with through-thickness fiber insertions

Fiber Reinforced Polymer (FRP) sandwich panels have been commonly used in aerospace, marine and transportation applications. Their lightweight, high strength and superior durable characteristics make them a strong candidate for the construction industry. In specific applications, the core material between the FRP skins of the sandwich panels provides insulating properties that are beneficial for building applications. The panels used in this study are composed of Glass Fiber Reinforced Polymer (GFRP) skin layers, glass fiber insertions and a foam core. This paper presents the results of a computational and experimental study on the mechanical behavior of a 3-dimensional GFRP sandwich panel with through thickness fiber insertions. Samples of the GFRP skin and glass fiber insertions were cut out from larger panels and are tested to determine basic material properties of the components of the sandwich panels. The experimental results on the components were used to create a nonlinear finite element model that can predict the large-scale mechanical deformation of the sandwich panels. The model can predict force–displacement behavior of the panel accurately under a variety of experimental conditions. The finite element model was used to determine the bending capacity of sandwich panels of various span lengths to create load tables for design engineers. This model is further used to predict the mechanical performance and failure load of sandwich panels for specific load conditions. A parametric study was also performed to determine the correlation between the density and pattern of fiber insertions on panel behavior. The results show that increasing the density of fiber insertions makes the panel stiffer. An alternating pattern for the insertions also increased the overall panel stiffness. Varying the overall panel thickness between a minimum and a maximum thickness indicted that an optimal overall thickness could be established. The results of the parametric study agreed with the results in the literature.

High cycle fatigue under reversed bending of sandwich panels with GFRP skins and polyurethane foam core

The effect of fully reversed loading (R=−1) on high cycle fatigue performance of sandwich panels composed of polyurethane foam core and Glass Fiber Reinforced Polymer (GFRP) skins is studied and compared to that of similar panels tested under fully unloaded (R=0) conditions. Fatigue life curves are established and compared based on maximum loads of 30–70% of the ultimate monotonic strength (Pult). It was shown that panels consistently fail in shear of the foam core. The fatigue life reduces significantly at R=−1, to about 10% of that at R=0. In order to achieve at least 2 million cycles – the commonly acceptable fatigue life in structural engineering – the maximum service loads should be limited to 30% and 45% of Pult, respectively, for the cases of R=−1 and R=0. It is estimated that the threshold loading levels at infinite fatigue life are 23% and 37% of Pult, for the cases of R=−1 and R=0, respectively. By the end of fatigue life, up to 25% reduction in stiffness occurs. The transition between high and low cycle fatigue occurs between 200 and 5000 cycles. A 3-D Haigh diagram is also established for design purposes.

Effect of Internal Ribs on Fatigue Performance of Sandwich Panels with GFRP Skins and Polyurethane Foam Core

This paper investigates the cyclic fatigue behavior of sandwich panels composed of glass fiber reinforced polymer (GFRP) skins connected by longitudinal Z-shape ribs and low-density polyurethane foam core. Eighteen 1,143×635×78-mm panels with ribs of two different flange widths, including three control static tests, were tested in fatigue under fully reversed loading (R=−1) and fully unloaded (R=0) conditions, to maximum loads (Pmax) of 20–70% of their ultimate static strength (Pult). Fatigue life curves were established and compared with those of panels without any ribs. It was shown that internal ribs increased static strength and stiffness by 80 and 66%, respectively, but were only beneficial within the low cycle fatigue range. Under Pmax of 50% Pult, number of cycles to failure (Nf) at R=−1 was only 6% of Nf at R=0 in the ribbed panels. Fatigue failure consistently initiated by flange debonding of the rib from skin. In panels loaded to higher than Pult of their nonribbed counterparts, final shear failure of foam core occurred simultaneously with rib debonding, whereas in panels loaded below this level, additional fatigue life was attained after rib debonding, until core shear failure, but at significantly lower stiffness. To achieve an Nf of 2M cycles, Pmax should not exceed 35 and 21% of Pult at R=0 and −1, respectively. A three-dimensional (3D) failure surface (Haigh diagram) was established for the ribbed panels. It predicts fatigue life for a given mean and amplitude loads.