Glass Fibre Reinforced Polymer Sandwich Research Articles

Compressive performance of full-scale GFRP composite sandwich wall panels with wood core

This paper investigated the compressive behaviors of sandwich wall panels consisting of glass fiber-reinforced polymer (GFRP) face sheets and Paulownia wood core experimentally and numerically. Vacuum-assisted resin infusion molding (VARIM) process was used to produce GFRP sandwich wall panel specimens. Four full-scale wall panels with different heights were tested under axial loading to study their failure modes and load-bearing capacities. The results revealed that buckling was the main failure mode of GFRP wall panels under the axial compression and the ultimate load-bearing capacities of specimens were improved the decrease of the height-to-thickness ratio. Experiments showed that decreasing the height-to-thickness ratio from 51.5 to 38.6 can effectively improve the ultimate load-bearing capacities of the wall panels by 124%. The experimental results were compared with finite element analysis (FEA) and analytical results were in good agreement. In addition, an equation was used to predict the load-bearing capacities of the wall panels suitable for the specimens with smaller height-to-thickness ratio.

Bending and eigenfrequency analysis of glass fiber reinforced honeycomb sandwich shell panels containing graphene-decorated with graphene quantum dots

The present study employs a finite element (FE) formulation to examine the free vibration and bending analyses of sandwich shells made of a glass fiber reinforced polymer (GFRP) composite honeycomb core and graphene decorated with graphene quantum dots (GDGQD) reinforced by GFRP composite face layers. The mechanical characteristics of the GFRP composites with GDGQD reinforcement were evaluated using experimental tests. The FE formulation was developed to compute the vibration and bending responses of the sandwich shells via MATLAB software. The Lagrange method is used for dynamic analysis, whereas the principle of potential energy is used for static analysis. The FE model's validity is compared to numerical data reported in the literature in terms of frequencies and deflection, and the results demonstrate excellent agreement. The impacts of the end conditions, core-to-facesheet thickness ratio, curvature ratios, and weight percent of GDGQD are studied further in relation to the bending and dynamic properties of GFRP sandwich shells.

Inner defect detection of glass fiber reinforced polymer sandwich panel using pulsed terahertz imaging based on smoothing and derivative

We conducted an experimental investigation to localize and detect internal defects in a sandwich panel made of glass-fiber-reinforced polymer (GFRP) using terahertz (THz) time-domain spectroscopy and signal processing techniques, which included smoothing and derivative analysis to enhance the defect detection performance. A simulated defect was created using a peel ply and scanned using a pulsed THz scanner with a reflection geometry specifically designed for this purpose. The reflected THz wave was analyzed to improve defect detection performance through time-domain signal processing using smoothing and derivative analysis. To quantify the performance of the proposed signal processing method, we introduced the Defect to Noise Ratio and evaluated its effectiveness accordingly. As a result, noise was removed and defect detection performance was enhanced, enabling the visualization of foreign insert defects in the GFRP sandwich panel.

Modal properties investigation of a carbon/glass fiber-reinforced polymer composite sandwich structure with lozenge-like core: An experimental and finite element simulation

The vibration properties of a carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP) composite sandwich panels with lozenge-like core and free-free boundary conditions are experimentally determined in this research. The sandwich panels manufactured for this study are composed of CFRP, GFRP, and epoxy resin. A modal experiment is performed using a modal hammer with an acceleration sensor to obtain modal properties of the structure, including natural frequencies, damping ratio, mode shapes, and frequency responses. Eventually, the comparison of vibration characteristics of the CFRP sandwich panel with the GFRP sandwich panel with almost identical geometry is discussed in detail. Furthermore, a finite element simulation is developed to validate the precision of the experimental data. It is recognized that the finite element results are in remarkably good agreement with those obtained by modal experiments. Hence, the finite element analysis and experiment method could be utilized to predict the modal properties of the CFRP and GFRP composite sandwich panels.

Local residual random forest classifier for strain-based damage detection and localization in aerospace sandwich structures

To ensure the structural integrity of large aerospace structures during operation, structural health monitoring is a major challenge. The monitoring can be performed by distributed strain measurements using strain gauges or fiber optical sensors. In this work, an advanced local residual classifier for strain-based damage detection and localization is introduced. The key principle is that a change in the relationship between a strain sensor and its neighbors indicates the presence of damage. After defining a sensor grid with sensor locations and their orientation, the relationship can be obtained from numerical simulations of the healthy structure. Here, local regression models are estimated between each master sensor and its neighboring sensors. Then, residuals of the predicted and measured strains are evaluated using a random forest classifier. The evaluation of the residuals has the advantage that the method is independent of the load level, as well as the fact that it is independent of certain environmental influences that are uniformly distributed over the entire structure. In addition to the numerical healthy strains, synthetically generated damage data are used for training the classifier. The synthetic data are obtained by statistical modifications of the healthy strains. This procedure avoids time-consuming and expensive damage simulations. The health monitoring approach is applied to a glass fiber reinforced polymer sandwich structure, imitating an aircraft spoiler, with a hole in the face layer considered as damage. The validation is performed by numerical finite element simulations as well as physical experiments under random loading conditions. The results demonstrate the high potential of the presented approach for strain-based structural health monitoring in composite sandwich structures.

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.

A framework for physics-driven generation of feature data for strain-based damage detection in aerospace sandwich structures

In recent years, structural health monitoring has been increasingly applied to composite sandwich structures, as typically used in aerospace applications. In addition, machine learning approaches are increasingly popular for damage detection, localization and size estimation, due to their great advantages in pattern recognition and anomaly detection. However, a major disadvantage of machine learning techniques is that these algorithms generally require large amounts of realistic data. In general, these data are expensive or even impossible to obtain within a feasible time. In order to overcome this hindrance, this work introduces a computationally inexpensive framework for physics-driven feature generation of strain data for the training of ML-based SHM methods using sub-structuring and the concept of reanalysis. First, the global FE model is subdivided into a monitored part, i.e., a smaller submodel, and a global model. Second, the stiffness matrix of the submodel is extracted from the finite element software. Then, static condensation is performed to further reduce the computational effort. Afterwards, selected eigenvectors are derived in terms of displacements of master nodes and the corresponding strains are calculated. Finally, a statistically varied linear combination between the different characteristic eigenvector load cases is performed based on the superposition principle. This procedure enables the efficient generation of a large number of different physics-driven determined strain solutions for a subsequent training of a ML algorithms. The proposed framework is evaluated by means of a damage detection approach, based on an artificial neuronal network classifier algorithm. The applied approach utilizes strain measurements from selected positions as physical quantity and is demonstrated using a composite sandwich structure imitating an aircraft spoiler. The key principle of the damage detection algorithm is based on the fact that a change in the relationship between sensors indicates the presence of damage. Additionally, to the numerical healthy strains resulting from the framework, synthetically generated damage data are used for training the neuronal network classifier. The synthetic data are obtained by statistical modifications of the healthy strains, to avoid time-consuming and expensive damage simulations. The feature generation framework and health monitoring approach are validated using experiments and numerical simulations of a glass fiber reinforced polymer sandwich structure with a hole considered as damage. The presented numerical and the experimental results clearly show the high potential for the efficient approach for damage detection in a sandwich structure.

Compressive and bending behavior of foam-filled composite sandwich panel with novel M-shaped core reinforced by nano-silica

The present paper has experimentally determined the flexural strength of a glass fiber reinforced polymer sandwich panel with a novel M-shaped lattice core reinforced by nano-silica (nano-SiO2). For this purpose, a polymer composite sandwich panel with an M-shaped core made of glass-epoxy fiber has been fabricated in this experiment using the vacuum-assisted resin transfer molding method. Afterward, polyurethane foam (PU) was injected into the empty space of the sandwich panel. In order to determine the flexural strength of the sandwich panels, they have been subjected to three-point bending and compressive loads. From the results of the study, it was figured out that: (a) Adding 1–3 wt% of nano-silica into the glass fiber had the most desirable effects on the enhancement of flexural strength of the sandwich panels (b) PU has improved the flexural strength of sandwich panels (c) The new M-shaped core can well postpone the buckling and failure of the composite sandwich structures.

Thermal and mechanical performances of GFRP sandwich structures with integrated amorphous silicon photovoltaic cells

Applications of photovoltaic (PV) cells in buildings are often excluded from locations with major mechanical strains because of their brittleness and possible damages at small mechanical strains. Combinations of glass fibre reinforced polymer (GFRP) components and amorphous silicon (a-Si) photovoltaic cells show potentials, while the effects from elevated temperatures and mechanical loads on the electrical performance of the integrated PV cells need to be well understood. In this study, specimens were fabricated by adhesively bonding a-Si PV cells to GFRP sandwich structures. The specimens were then exposed to outdoor natural sunlight. The sunlight intensities and the temperature responses at different locations of the specimens were continuously monitored and their correlation was quantified. An approach was proposed to calculate the sunlight intensities received by the integrated PV cells and their temperature responses based on geographic information (e.g. longitude, latitude and altitude) of such applications. The electrical performance of the integrated PV cells was then investigated under four-point bending until structural failure. Two loading scenarios were considered where non-uniformly distributed tensile or compressive strains were introduced into the integrated PV cells. Reductions in voltage of the integrated PV cells were highlighted due to non-uniform distribution of strains.

Numerical analysis on flexural behaviour of GFRP sandwich roof panel with multilayer core material

In this study, the flexural performance of Glass Fibre Reinforced Polymer (GFRP) sandwich roof panel with multilayer polyurethane foam core was investigated using commercial FEM software ANSYS WORKBENCH. The GFRP sandwich panel is composed of thin GFRP face sheets at top and bottom infilled with Polyurethane (PU) foam core of two different densities. GFRP sandwich panels with different stacking sequences of the multilayer core was considered to determine the best sequence in which the different layers of core material can be laid on top of each other. The optimum panel size was obtained from parametric study conducted with number of GFRP plies and thickness of core as parameters. Alternatively, the flexural behaviour of GFRP sandwich panel with three different web core configurations were also studied, which includes trapezoidal shaped GFRP webs infilled with PU foam (Type 1), rhombus shaped GFRP webs infilled with PU foam (Type 2), and wave shaped GFRP webs infilled with PU foam (Type 3). Based on the finite element analysis, GFRP sandwich panel with Type 3 core configuration (wave shaped GFRP webs infilled with PU foam) with lesser deformation represented a feasible model to be used as a roofing panel for small residential buildings and prefabricated modular constructions.

An experimental investigation of static and fatigue behaviour of GFRP sandwich beams with a web reinforced wood core

Three-point static and fatigue bending experiments were performed on 51 composite sandwich beams with glass fibre reinforced polymer (GFRP) face sheets and paulownia wood cores. Specimens with no lattice, one lattice and two lattices were prepared, and the fatigue bending failure mode, fatigue life distribution and stiffness degradation model of the sandwich beams were obtained. The results show that the specimens with no lattice exhibit two brittle failure modes, i.e., interface debonding and core shearing. The failure modes of the specimens with one lattice are local buckling and crushing, while the specimens with two lattice webs only exhibit tensile failure of the bottom face sheet. Moreover, the specimens with webs exhibit ductile failure characteristics. Based on the Weibull model, the Weibull distribution equations of each specimen under different survival rates were established, and the engineering fatigue curve was drawn. Furthermore, the secant stiffness curve of the specimens with webs was established based on the fatigue cumulative damage theory, and the fatigue lives of three specimens were predicted.

Life cycle assessment of alternative building floor rehabilitation systems

The aim of this study is to quantify and compare the environmental impact from cradle-to-gate of different systems for building floor rehabilitation.The Life Cycle Assessment (LCA) method was used for the evaluation of the potential environmental impact of the production of each floor system in eight categories and a monetisation method was used for the weighting of the results and for their expression into one single indicator. Five functionally equivalent (from a structural standpoint) systems were assessed, including: (i) traditional solutions, such as timber floors and reinforced concrete (RC) slabs; (ii) less conventional solutions, such as beam-and-block and steel–concrete composite slab systems; and (iii) an innovative glass fibre reinforced polymer (GFRP) sandwich panel system. The environmental impacts of these systems are compared in this paper for the first time.It was found out that timber is the most environmentally friendly solution, since it presents the lowest total values in all environmental impact categories (under 1% of the impact of the solution with highest impact in each category, RC or GFRP), except for primary renewable energy consumption, for which the RC solution presents the least consumption. On the other hand, it was found out that: the steel–concrete composite floor is the least environmentally friendly solution in abiotic resource depletion; the RC solution is the least performing option in terms of global warming potential and ozone depletion potential; and the GFRP system presents the worst behaviour in five environmental impact categories – non-renewable energy consumption, photochemical ozone formation potential, acidification potential, eutrophication potential and Eco-costs. However, if the most important inefficiencies identified during the production of the latter floor system are at least partly amended, its impacts would be substantially reduced, particularly regarding the Eco-costs indicator, which considers four environmental categories at once – in that case, the RC solution would become the worst solution and GFRP sandwich panels would have an aggregated result of 80% of the RC. The final part of the paper presents a qualitative assessment of the gate-to-grave performance of the different solutions.

Hybrid sandwich panels for building uses: focus on glass fibre reinforced polymer and mineral matrix interface.

Sandwich structures are being used increasingly in civil engineering because of their high strength, stiffness, and stiffness-to-density ratio. The studied sandwich structure was made of glass fibre reinforced polymer (GFRP) skins, and GFRP reinforced core. Two case studies are presented in this paper: a use as lightweight floor in building rehabilitation, and an application as façade panels. In both cases, the GFRP sandwich structure can be associated with a mineral matrix because of conventional construction methods and acoustic floor insulation in the first case, and because of architectural issues in the second. To design the hybrid sandwich panel and ensure monolithic mechanical behaviour, a finite element method (FEM) that predicts the interface failure was introduced. To implement the FE model by mechanical interface properties, pull-off and push-out tests were performed to assess the mode I and mode II stress limits. Four GFRP surface roughnesses and two configurations with chemical additions were tested. The three configurations that performed the best were tested by submitting the hybrid sandwich structure to three-point bending loading. The prediction of the interface failure by the FEM was assessed by comparison to the experimental data. Finally, full-scale panels were experimentally tested and designed for the named two uses cases thanks to the FE model.

Impact Properties of Aluminum Foam – Nanosilica Filled Basalt Fiber Reinforced Polymer Sandwich Composites

This paper investigates the effect of nanosilica on impact and energy absorption properties of sandwich foam-fibre composites. The materials used in this study are closed-cell aluminum (Al) foam (as the core material) that is sandwiched in between nanomodified basalt fiber reinforced polymer (as the face-sheets). The face sheets were made of Basalt Fibre, nanosilica and epoxy polymer matrix. The sandwich composite structures are known to have the capability of resisting impact loads and good in absorbing energy. The objective of this paper is to determine the influence of closed-cell aluminum foam core and nanosilica filler on impact properties and fracture behavior of basalt fibre reinforced polymer (BFRP) sandwich composites when compared to the conventional glass fibre reinforced polymer (GFRP) sandwich composites. The drop impact tests were carried out to determine the energy absorbed, peak load and the force-deflection behaviour of the sandwich composite structure material. The results showed that the nanomodified BFRP-Al foam core sandwich panel exhibited promising energy absorption properties, corresponding to the highest specific energy absorption value observed. Also, the result indicates that the Aluminium Foam BFRP sandwich composite exhibited higher energy absorption when compared to the Aluminium foam GFRP sandwich composite.

The temperature and pre-crack length effects on delamination resistance of woven GFRP sandwich composites

In this paper, the temperature and pre-crack effects on the delamination resistance of woven glass fibre reinforced polymer (GFRP) sandwich composites under Mode I loading was experimentally investigated and numerically modelled. For this purpose, the sandwich composite panel was fabricated with vacuum assisted resin infusion molding (VARIM). The mechanical properties were obtained at room and high temperature. Double cantilever beam (DCB) test was adopted for the measurement of delamination resistance by evaluating the strain energy release rate (SERR) value to obtain the fracture properties of woven GFRP sandwich composites. The three analytical methods, the experimental compliance calibration method (CCM), Modified Compliance Calibration Method (MCC) and Modified Beam Theory (MBT) have been used. In the finite element analysis virtual crack closure technique (VCCT) was used. The results obtained from 3D finite element analysis for various delamination lengths compare well with the experimental results. It is seen that there is an agreement between experimental and numerical results so that VCCT analysis is to be an appropriate method for analysing a SERR value of sandwich composites. DOI: http://dx.doi.org/10.5755/j01.mech.22.5.16229

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.

Compressive strength after blast of sandwich composite materials

Composite sandwich materials have yet to be widely adopted in the construction of naval vessels despite their excellent strength-to-weight ratio and low radar return. One barrier to their wider use is our limited understanding of their performance when subjected to air blast. This paper focuses on this problem and specifically the strength remaining after damage caused during an explosion. Carbon-fibre-reinforced polymer (CFRP) composite skins on a styrene–acrylonitrile (SAN) polymer closed-cell foam core are the primary composite system evaluated. Glass-fibre-reinforced polymer (GFRP) composite skins were also included for comparison in a comparable sandwich configuration. Full-scale blast experiments were conducted, where 1.6×1.3 m sized panels were subjected to blast of a Hopkinson–Cranz scaled distance of 3.02 m kg−1/3, 100 kg TNT equivalent at a stand-off distance of 14 m. This explosive blast represents a surface blast threat, where the shockwave propagates in air towards the naval vessel. Hopkinson was the first to investigate the characteristics of this explosive air-blast pulse (Hopkinson 1948 Proc. R. Soc. Lond. A 89, 411–413 (doi:10.1098/rspa.1914.0008)). Further analysis is provided on the performance of the CFRP sandwich panel relative to the GFRP sandwich panel when subjected to blast loading through use of high-speed speckle strain mapping. After the blast events, the residual compressive load-bearing capacity is investigated experimentally, using appropriate loading conditions that an in-service vessel may have to sustain. Residual strength testing is well established for post-impact ballistic assessment, but there has been less research performed on the residual strength of sandwich composites after blast.

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.

Thermal Field Distributions of Girder Bridges with GFRP Panel Deck versus Concrete Deck

Temperature effects have always been considered to be one of the critical issues that affect the performance of traditional concrete and steel bridges. Currently, glass fiber-reinforced polymer (GFRP) composite panels, with the advantages of high strength, light weight, good corrosion resistance, long-term durability, etc., have been applied to slab replacements in bridge engineering. The thermal behaviors of these GFRP bridges, however, have not been fully understood. More specifically, the current temperature design specifications in the AASHTO LRFD may no longer be valid for GFRP bridges. Therefore, this paper presents the thermal field distribution behaviors through studying an as-built bridge with GFRP sandwich panels. First, a transient-state numerical modeling method is proposed. This method is capable of predicting the temperature field of a GFRP bridge based on local environmental conditions. Then, a parametric study is conducted to analyze the thermal responses of bridges with different structure combinations (i.e., GFRP panels, concrete slabs, concrete beams, and steel girders). The mechanical and physical properties of GFRP panels are predicted using the micro-macro mechanics theory and validated by a field measurement and an experimental study. Based on the results from the present study, the temperature distribution patterns for GFRP panel bridges are proposed, referring to the available AASHTO LRFD sign code. The thermal strains and stresses caused by temperature gradients along the depth of bridge superstructures are examined.

Connection development and in-plane response of glass fiber reinforced polymer sandwich panels with reinforced cores

This paper presents the findings of an experimental research project that was conducted in two phases to study the behavior of glass fiber reinforced polymer (GFRP) sandwich panels with reinforced cores under in-plane loading conditions. The tested panels consisted of two GFRP face skins separated by a polymeric foam core. The foam core was reinforced with different configurations of through-thickness fiber insertions and through-thickness GFRP web skins. In the first phase the performance of three different types of structural connections was tested — namely, bolted, bonded, and so-called enhanced bolted connections. The findings indicate that bolted connections to thinner and more flexible panels exhibited lower strength and a higher degree of nonlinear behavior compared with the bonded connections to the same panels. In contrast, the bolted connections to thicker and stiffer panels were generally stronger and stiffer than their bonded counterparts. The findings further indicate that the ultimate strength of the connections can be increased by up to 26% by bonding a steel reinforcing plate to the face skins of the panel prior to bolting. In the second phase three full-scale sandwich panels (1400 mm × 1400 mm) with different panel configurations were tested under proportional, biaxial, in-plane loading. The applied loads were selected to simulate the in-plane loading conditions in sandwich panels that are subjected primarily to in-plane loads, such as webs of deep beams and shear walls in lightweight structures. The full-field strains and displacements of the panels were measured using a digital image correlation-based (DIC) non-contact measurement system. The findings indicate that the through-thickness core reinforcements effectively prevented localized buckling, debonding, and separation of the panels’ face skins. The findings also demonstrated that slender panels exhibit shear–compression buckling failures, whereas less slender panels exhibit shear–tension rupture failures.