Blast Performance and Damage Assessment of Composite Sandwich Structures

Dentin hypersensitivity (DH) is a common irritating condition. A precise sensitive test for its assessment can greatly aid in appropriate treatment planning. This meta-analysis aims to compare the air blast and tactile tests for assessment of the efficacy Nd:YAG laser therapy versus non-laser treatments for DH in short-term and long-term follow-ups. For this review, an electronic search of the literature was carried out in three databases by two researchers for English articles published until March 10, 2021. Pooling of the data extracted from the selected articles was performed according to the PRISMA statement by the random-effect model. The mean difference (MD) and 95% confidence interval (CI) of pain score before the treatment onset and during the follow-up period according to the visual analog scale (VAS) were calculated. The level of heterogeneity was assessed by the I2 test, and a funnel plot was drawn to assess the publication bias of the reviewed studies. Of 152 articles primarily retrieved, 9 randomized clinical trials (RCTs) using the air blast test and 4 RCTs using the tactile test were subjected to quantitative synthesis. In the short-term follow-up and immediately after treatment, the air blast test showed superiority of laser therapy compared with non-laser treatments (SMD: 0.55, 95% CI: 0.05-1.04, p= 0.03). However, this difference was not significant according to the tactile test (SMD: 0.48. 95% CI: 0.01-0.96, p= 0.06). In the long-term follow-up, the difference between laser therapy and non-laser modalities was not significant according to both air blast (SMD= -0.38, 95% CI: -1.43-0.67, p= 0.48) and tactile (SMD=0.0, 95% CI: -0.38-0.38, p= 0.99) tests. Comparison of laser therapy and non-laser modalities in the short-term reveal-ed higher sensitivity of the air blast test due to its mechanism of action compared with the tactile test. Further studies are required to interpret the results in the long-term follow-up.

Sandwich composites are of interest in marine applications due to their high strength-to-weight ratio and tailorable mechanical properties, but their resistance to air blast loading is not well understood. Full-scale 100 kg TNT equivalent air blast testing at a 15 m stand-off distance was performed on glass-fibre reinforced polymer (GFRP) sandwich panels with polyvinyl chloride (PVC); polymethacrylimid (PMI); and styrene acrylonitrile (SAN) foam cores, all possessing the same thickness and density. Further testing was performed to assess the blast resistance of a sandwich panel containing a stepwise graded density SAN foam core, increasing in density away from the blast facing side. Finally a sandwich panel containing compliant polypropylene (PP) fibres within the GFRP front face-sheet, was subjected to blast loading with the intention of preventing front face-sheet cracking during blast. Measurements of the sandwich panel responses were made using high-speed digital image correlation (DIC), and post-blast damage was assessed by sectioning the sandwich panels and mapping the damage observed. It was concluded that all cores are effective in improving blast tolerance and that the SAN core was the most blast tolerant out of the three foam polymer types, with the DIC results showing a lower deflection measured during blast, and post-blast visual inspections showing less damage suffered. By grading the density of the core it was found that through thickness crack propagation was mitigated, as well as damage in the higher density foam layers, thus resulting in a smoother back face-sheet deflection profile. By incorporating compliant PP fibres into the front face-sheet, cracking was prevented in the GFRP, despite damage being present in the core and the interfaces between the core and face-sheets.

In this study, we experimentally investigate the impact damage and failure mechanisms of foam-core sandwich composites with carbon fiber reinforced polymer (CFRP) and/or glass fiber reinforced polymer (GFRP) face sheets. Specimens are conditioned and impacted over a wide temperature range (from room temperature of 23°C down to -70°C) using Instron CEAST 9350 impact test machine. This work also explores the use of hybrid composite face sheets, with various configurations of layering CFRP and GFRP in the face sheets of foam-core sandwich composites. Results show that exposure to low temperature causes more severe damage, particularly in specimens with CFRP face sheets, due to their extreme brittleness. GFRP face sheet specimens are able to achieve larger deformation and absorb greater impact energy even at low temperature conditions. There is a significant benefit in using hybrid configurations, by striking a balance between brittle (CFRP) and ductile (GFRP) materials and harnessing their mechanical advantages of high strength and high fracture toughness, respectively. Results also demonstrate that hybridization is a solution to reduce temperature effects in impact deformation. Careful placement of CFRP and GFRP in layup order can harness the best impact performance. X-ray micro-computed tomography (μCT) images reveal substantial delamination at the interfaces between CFRP and GFRP layers. It is also observed that CFRP layers fail by brittle fracture, while delamination is the major damage mode within GFRP layers. Other complex failure mechanisms in the composite face sheets (such as matrix crack and fiber breakage) and foam core (core crushing, core shearing and interfacial debonding) are also presented.

In this study, we investigate the behaviour of foam-core sandwich composites, specifically with regards to bending after impact (BAI), in the presence of carbon fiber reinforced polymer (CFRP) and/or glass fiber reinforced polymer (GFRP) facesheets. To accomplish this, we utilize an Instron 5582 universal testing machine to conduct 3- point bend test on specimens that have previously undergone impact testing. The 3-point bend tests are conducted at both room temperature (23°C) and low temperature (-70°C) to evaluate the effect of low temperature (LT) on the BAI behaviour of the composite materials. Moreover, we explore the effect of different stacking sequence and layup configuration of GFRP and CFRP layers on sandwich composites under flexural loading. We analyse the force-displacement curves and compare peak stress values to gain insight into the influence of the CFRP and GFRP hybrid facesheets. Results suggest that altering the arrangement of CFRP and GFRP layers has a significant impact on the flexural strength of the hybrid composite. At LT, the specimens experience more flexural damage due to increase in brittleness. However, the peak stress at LT increases due to the increase in the compressive strength of the CFRP layers. Specimens featuring a GFRP layer on the outside (referred to as ‘glass first’ specimens) and specimens with alternating CFRP/GFRP layers, with CFRP positioned on the outermost layers (referred to as ‘CGCG’), when impacted at 15J, experience core shear and core debonding at LT flexural test. Overall, GC is found to be the optimal configuration. This configuration provides excellent strength from the inner CFRP layer and while the outer GFRP layer offers damage resistance during impact test.

The depletion of fossil fuels and the rise of environmental concerns have increased the importance of renewable energy sources, positioning wind energy as a key alternative. Modern wind turbine blades are predominantly manufactured from composite materials due to their light weight, high strength, and resistance to corrosion. In offshore applications, approximately 95% of the composite content is glass fiber-reinforced polymer (GFRP), while the remaining 5% is carbon fiber-reinforced polymer (CFRP). GFRP is favored for its low cost and fatigue resistance, whereas CFRP offers superior strength and stiffness but is limited by high production costs. This study investigates the durability of adhesively bonded GFRP and CFRP joints under marine exposure. Seven-layer GFRP and eight-layer CFRP laminates were produced using a 90° unidirectional twill weave and prepared in accordance with ASTM D5868-01. Specimens were immersed in natural Aegean Sea water (21 °C, salinity 3.3–3.7%) for 1, 2, and 3 months. Measurements revealed that GFRP absorbed significantly more moisture (1.02%, 2.97%, 3.78%) than CFRP (0.49%, 0.76%, 0.91%). Four-point bending tests conducted according to ASTM D790 showed reductions in Young’s modulus of up to 9.45% for GFRP and 3.48% for CFRP. Scanning electron microscopy (SEM) confirmed that moisture-induced degradation was more severe in GFRP joints compared to CFRP. These findings highlight the critical role of environmental exposure in the mechanical performance of marine composite joints.

Prolonged exposure to adverse conditions affects the performance and promotes the degradation of reinforced concrete (RC) structures, requiring repair and strengthening to preserve their integrity and functionality. Synthetic fibers, including carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP), are frequently employed for retrofitting owing to their superior strength-to-weight ratio and simple installation. Natural fibers, such as kenaf, have been increasingly incorporated into fiber reinforced polymer (FRP) composites as sustainable alternatives in the construction industry due to their lightweight nature and low carbon footprint. Yet, many investigations focused on synthetic fibers, thus, the purpose of this study is to develop finite element (FE) models for RC beams and columns retrofitted with CFRP, GFRP and kenaf fiber reinforced polymer (KFRP), to examine the impact of natural fibers and comparing the findings with those derived from synthetic fibers. The models consider element types, mesh discretization, solution methodologies, and nonlinearities. Concrete behavior is represented using Concrete Damage Plasticity (CDP), whereas fiber laminates employ the Hashin damage model. The FE model's load-displacement behavior, ultimate strength, and failure mechanisms were verified against existing experimental and numerical findings. The findings indicate that FRP wrapping substantially enhances the load-carrying capacity of RC beams, with ultimate load increases ranging from approximately 13% for KFRP to 66% for CFRP, whereas the corresponding improvements for RC columns are notably smaller, remaining below 7%. Although KFRP exhibits lower mechanical performance than GFRP and CFRP, its sustainability and cost-effectiveness support its use in applications where environmental and economic considerations are prioritized.

The current age has been impacted by a serious issue caused by terrorist air blast violent attacks. The ability to overcome this crisis to ensure safety for all is a goal that governments, law enforcement organizations, and security forces throughout the world are always working to improve. The development of blast-resistant structures is an ongoing objective of research. The choice of materials is crucial to developing these protective structures. AL-6XN steel is mostly favored for creating blast-proof structures because of its high ductility and great strength. Composite materials have an excellent strength-to-weight ratio. However, they are continuously replacing conventional materials. The lightweight carbon fiber-reinforced polymer (CFRP) composite laminate has the highest strength. Therefore, it was considered for the present study and compared its blast performance with that of an equivalent AL-6XN steel plate. Analyzing a structure’s blast mitigation properties by experimentation is exceedingly expensive, risky, and environmentally harmful. Therefore, this chapter focuses on a numerical analysis of the blast performance of protective sheets that is both efficient and affordable. The quarter portions of the metallic plate and composite laminate are modeled using Abaqus CAE to study the significant influence of the materials on air blast mitigation. The computational cost in terms of storage and computation time is greatly decreased by the use of quarter models. The finite element models of an AL-6XN steel plate and CFRP composite laminate were subjected to blast loads of 1 kg–3 kg of TNT for a 0.3 m stand-off distance (SoD). To analyze the dynamic response of the modeled sheets under blast loads, the Conventional Weapons Effects Program (CONWEP) module of Abaqus/Explicit was used. In air blast loadings, the structures are subjected to high impulse and deform with a high strain rate. However, to predict the plastic deformation behavior of the metallic plate, the Johnson–Cook plasticity model was used. For predicting the damage behavior of the CFRP composite laminate, Hashin damage criteria and the Puck–Schurmann criterion were used. To verify the applicability of the numerical approach followed, the obtained deflection results were compared with the available literatures’ experimental data. The blast performance comparison of the steel plate and the composite laminate deals with their peak deflection, deformation modes, kinetic energy, and energy absorption. The obtained results showed that the composite laminate has the greatest blast mitigation capability, due to its smallest deflection and the highest capability to absorb energy. Hence, it can be used as a protective sheet in military vehicles and naval industries to save solders’ lives from blast attacks.

Influence of curing on the mechanical performance of FRP laminates

When a composite laminate is subjected to humidity, moisture diffusion occurs depending on the number and thickness of the lamina. Water diffusion changes the mechanical response of laminates and usually causes a significant reduction of the mechanical properties of the composite specimens in ocean structures. One of the most important mechanical properties of laminates is flexural stiffness which should be considered in the design procedure. Despite the extensive research on single cycle aging of composites, cyclic aging of these materials is less explored. The aim of the current research is to investigate the variation of mechanical properties of carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) composites as a substrate in adhesive joints with the same initial flexural stiffness values subjected to cyclic wet/dry aging conditions for long-term structural applications. The matrix used in the CFRP and GFRP composites are based on epoxy and vinyl ester, respectively. Both unaged and cyclically aged samples were characterized by tensile and three-point bending tests. In order to simulate the moisture absorption condition of composites in adhesive joints, one side of the composite laminates was sealed with aluminum foils and three sides were exposed to humidity. The interaction between the composite thickness and the number of aging cycles was also investigated. The experimental results show that in cyclic aging condition, the reduction of flexural stiffness in CFRP is more than GFRP laminates and GFRP laminates is more suitable for ocean applications.

Strengthening of shear critical RC beams with various FRP systems

Glass fiber reinforced polymers (GFRP) have become a preferable material for reinforcing or strengthening reinforced concrete structures due to their corrosion resistance, high strength to weight ratio, and relatively low cost compared with carbon fiber reinforced polymers (CFRP). However, the limited fatigue life of GFRP hinders their use in infrastructure applications. For instance, the low fatigue life of GFRP caused design codes to impose stringent stress limits on GFRP that rendered their use non-economic under significant cyclic loads in bridges. In this paper, we demonstrate that the fatigue life of GFRP can be significantly improved by an order of magnitude by incorporating Multi-Wall Carbon Nanotubes (MWCNTs) during GFRP fabrication. GFRP coupons were fabricated and tested under static tension and cyclic tension with mean fatigue stress equal to 40% of the GFRP tensile strength. Microstructural investigations using scanning electron microscopy (SEM) and Fourier Transform Infrared (FTIR) spectroscopy were used for further investigation of the effect of MWCNTs on the GFRP composite. The experimental results show the 0.5 wt% and the 1.0 wt% MWCNTs were able to improve the fatigue life of GFRP by 1143% and 986%, respectively, compared with neat GFRP.

In the present work, first an attempt has been made to prepare carbon, glass and carbon-glass fibre-reinforced polymer composites, and later, machining of these composites is done on abrasive water jet machine (AWJM) to compare and optimize the machining parameters. Actually, the carbon fibre-reinforced polymer (CFRP) composites, glass fibre-reinforced polymer (GFRP) composites and carbon-glass fibre-reinforced polymer (CGFRP) composites are prepared through vacuum bagging process by using epoxy resin as the polymer matrix. The machining experiments are conducted to analyse the effects of the predominant machining parameters, i.e. cutting speed rate, feed rate and stand-off distance on the required machining characteristics, i.e. surface roughness (Ra), kerf top width (kw) and material removal rate (MRR). The range of values of each parameter is set at three different levels, and Taguchi’s L9 orthogonal array is used to design factors so that all the interactions between the response variables and machining variables can be investigated. Based on the experimental values, second-order regression equations are fitted between each of the response parameters and the machining parameters using Minitab 18 software. The equations are then optimized by defining the three equations of Ra, kw and MRR as the three objectives of a multi-objective optimization problem (MOOP) using a multi-objective optimization algorithm called Elitist Non-dominated Sorting Genetic Algorithm (NSGA-II). Single best compromise solutions with respect to the MOOPs of GFRP, CFRP and CGFRP composites are also determined from the Pareto optimal solutions obtained by NSGA-II. Finally, confirmation tests are conducted on specimens of GFRP, CFRP and CGFRP composites machined at their corresponding optimum parameters given by the GA. It is observed that the optimum values of Ra, kw and MRR of all the optimization problems are closer to the corresponding experimental values of confirmation tests.

A contrast investigation of the seismic behavior of unreinforced masonry walls with window openings strengthened with Carbon Fibre Reinforced Polymer (CFRP) sheets and sprayed Glass Fibre Reinforced Polymer (GFRP) is presented. Three wall specimens in the scale of 1/2 were tested by the horizontal cyclic loading combined with constant gravity loads. The seismic strengthening effects by two different FRP retrofitting schemes are compared in aspect of the hysteresis response, deterioration of rigidity and ability of energy dissipation. The experimental results indicate that the increasing degree of the improvement of seismic behavior of the SGFRP-strengthened wall are significantly superior to that of the wall strengthened with epoxy-based CFRP; and the overcoat of sprayed GFRP can bond tightly and work well together with the masonry to play a critical role in earthquake resistance.

The main objective of this paper was to illuminate the effect of marine environmental condition on durability of reinforced concrete (RC)-corroded columns strengthened with carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) layers. Small-scale columns were prepared and corroded by an accelerated corrosion process. After strengthening, compressive strength tests were carried out on control and weathered specimens. In this research, a marine simulator was designed and constructed similar to the tidal zone of marine environment in south of Iran which was selected as a case study in this research. Mechanical properties of wrapped specimens were studied after placing them inside the simulator for 3000 hours. Marine environment decreased ultimate strength by 4.5% and 26.3% in CFRP and GFRP-wrapped columns, respectively. In some corroded-columns, strengthening was carried out after replacing damaged cover by self-compacted mortar. In this method, by confining with one layer of CFRP and GFRP, 4.2% and 22.4% reduction in ultimate strength was observed, respectively, after exposure. Furthermore, the elastic-brittle behavior has been verified in this retrofit method. Also results of tension tests revealed, the ultimate tensile strength was degraded by 2% and 28.8% in CFRP and GFRP sheets, respectively, after applying marine exposure.

When glass fibre-reinforced polymer (GFRP) bar splices are used in reinforced concrete sections, they affect the structural performance in two different ways: through the stress concentration in the section, and through the configuration of the GFRP–concrete bond. This study experimentally investigated a new method for increasing the bond strength of a GFRP lap (two GFRP bars connected together) using a carbon fibre-reinforced polymer (CFRP) sheet coated in epoxy resin. A new splicing method was investigated to quantify the effect of the bar surface bond on the development length, with reinforced concrete beams cast with laps in the concrete reinforcing bars at a known bending span length. Specimens were tested in four-point flexure tests to assess the strength capacity and failure mode. The results were summarised and compared within a standard lap made according to the ACI 318 specifications. The new method for splicing was more efficient for GFRP splice laps than the standard lap method. It could also be used for head-to-head reinforcement bar splices with the appropriate CFRP lapping sheets.