نشریه علوم و فناوری کامپوزیت
سال هشتم شماره 3 (پیاپی 29، پاییز 1400)

In this study, by developing the finite element model, the pull-out behavior of ribbed steel rebar from ultra-high performance fiber reinforced concrete was investigated. For this purpose, the multi-scale finite element model was simulated in three separate phases, including ultra-high performance concrete in the form of isotropic material, steel microfibers and ribbed rebar. In order to consider more realistic assumptions, Rib pattern of rebar were simulated for the first time and the interaction between concrete with rebar and fibers was modeled by using the adhesive contact zone model. The constant parameters of the adhesive contact zone model were determined by calibrating the results of the finite element model and those of the experimental pull-out test. Finally, after validating the multi-scale finite element results, the maximum pull-out force, the corresponding displacement and the work required for the rebar pull-out were determined using the force-slip curves. Effective parameters such as bond length and rebar diameter, and the volume percentage of steel microfibers, on the bond strength were evaluated. The results show that the bond length has a significant effect on the bonding behavior of the rebar and with increasing the bond length the pull-out force also increases. According to the results, for rebar with a diameter of 20 mm, by increasing the amount of fibers volume from 0 to 2%, the pull-out force and the work increase by about 30% and 22%, respectively.

In this research, the influence of thermal cycling on the fracture toughness and maximum load in mode I delamination of polymer matrix composites (PMCs) is investigated experimentally and analytically. To eliminate the effect of the remote ply orientation on the fracture toughness during delamination initiation and propagation, double cantilever beam (DCB) specimens with a stacking sequence of [0]16 using uni-directional glass fibers and epoxy resin were considered. The specimens were thermal-cycled between 15°C and 65°C for 50-150 cycles. One group of uncycled specimens were tested at the commencement of the investigation as a control group. During the DCB test and receiving universal tensile machine results, the specimens were inspected by 2 real-time cameras to record the delamination length and initial crack tip opening displacement (ICTOD). The strain energy release rate (SERR) approach was used for obtaining the critical fracture toughness (GIC) from the experimental data. By employing the digital image correlation (DIC) method, the initial crack tip separation profile was obtained. The measured bridging laws were used with cohesive elements in ABAQUS commercial software to accurately model the delamination propagation in DCB specimens. Investigation of load variations revealed that critical fracture toughness in mode I of delamination is firmly affected by the thermal fatigue process.

Iron-based metal matrix composites containing hard particles have good resistance to abrasive wear. One of their production methods is infiltration-casting, which has the ability to produce surface composites reinforced parts. In this Article research , the infiltration of molten gray cast iron melt into a porous skeleton of compacted swarfs of 304 stainless steel has been investigated. The molten cast iron melt was pulled into the porous skeleton by capillary forces. At the infiltration temperature of the molten gray cast iron melt, it was in contact with the solid skeleton of 304 steel. Due to the diffusion of chromium as a carbide-maker element and nickel element into the molten gray cast iron, its chemical composition changed and became similar to the composition of white Ni-Hard cast irons. By diffusion and exchange of alloying elements, an iron matrix with hard chromium-iron carbide (M7C3) particles and a ferritic-austenitic matrix were obtained. The microstructure development formation, type, and hardness of different phases were examined with light and electron microscopy, XRD, and microhardness measurement. Due to the relatively low price of gray cast iron and the waste stainless steel turning chips, it can be said that a method has been proposed to produce economical parts reinforced with surface composites.

Honeycombs cores are widely used in nucleate composite structures. One of the important issues in using cores, especially honeycombs, is to ensure proper connection between the layers on both sides of the structure to the core; Because improper connection will lead to separating the layer from the core. The purpose of this study is to investigate and propose the most suitable method for making composite parts with honeycomb core under ASTM C273 and ASTM C297 standards. As a result, it was concluded that the most appropriate way to make a composite piece with honeycomb core is a two-step fabrication method. This is because in the two-stage method, using a vacuum device, after creating a vacuum in the part, the resin rises to the walls of the core and the connection between the core and the layers is established well. The area of connection of the core with the layers in the two-stage method is twice as large as the single-stage method and the manual construction method. So that out of 30 samples that were used for each of the three mentioned construction methods, in the two-stage method, this separation occurred in the honeycomb core itself in 28 samples out of 30 samples, which indicates the suitability of this method. (Separation is acceptable with more than 90%). However, in the one-step method and the manual method, 18 out of 30 samples and also 9 out of 30 core samples failed in the core area under the tensile and shear test, respectively.

In this research, the mechanical properties of polyamide 6/polyolefin elastomer (PA6/POE) blends reinforced with carbon nanotubes have been studied. Nanocomposites consisted of 10 and 20 wt% polyolefin elastomer, 0, 1, 2 and 3 wt% carbon nanotubes (MWCNT) and 5 and 10 wt% maleic anhydride (POE-gMA as compatiblizing agent) were made by an internal mixer based on the full factorial design of experiment. Tensile and impact tests were performed to extract mechanical properties including tensile strength, elastic modulus and impact strength. Scanning electron microscope (SEM) were used to observe the dispersion and distribution quality of MWCNT in the matrix. SEM images showed an acceptable distribution of carbon nanotubes up to 2 wt% in both polyamide matrices with 10 and 20 wt%, while adding 3 wt% of nanotubes in both matrices was associated with nanotubes accumulation. The test results showed that the addition of 2 wt% of carbon nanotubes improved the tensile strength and elastic modulus of the compounds by %21 and %23, respectively. the surfaces extracted from full factorial design method showed that there are no notable interaction between the input parameters in the case of mechanical properties. Finally, the R-squared above %92 for all mechanical properties showed that there is a good agreement between the experimental results and the extracted regression models.

In this paper, the free vibrations of a three-layer doubly curved panel with honeycomb auxetic core and reinforced composite face sheets by carbon nanotubes with different distributions are investigated. The governing equations of the structure are derived based on the new fifth-order shear deformation theory. The sandwich panel consists of three layers, aluminum core layer with cell inclined angle which creates negative Poisson's ratio in the honeycomb structure, composites skins with a uniform distribution (UD) as well as functionally graded (FG) distributions of carbon nanotubes (CNTs) in the simply supported boundary condition. Effective material properties of single-walled (SW) carbon nanotube reinforced composite skins are achieved by applying the extended rule of mixtures. The equations of motion are obtained using the Hamilton principle and solved using the Galerkin residual weight method. The effect of auxetic cell parameters including cell angle, cell aspect ratio, rib thickness ratio as well as distribution and content of carbon nanotubes, length to thickness ratio, and core thickness to total panel thickness ratio are examined and discussed. At the end, free vibration was obtained and compared for other shapes of sandwich panels such as hyperbolic parabolic sandwich panels, cylindrical sandwich panels and sandwich plates.The results showed that the core with negative Poisson's ratio as well as the functionally graded distribution (FG) of carbon nanotubes in the upper and lower composite surfaces reduced the natural frequency of the studied sandwich panel.

In this research, numerical and experimental investigations were performed to study the effect of the adhesive joint on the fractural behavior of the lattice composite plates. Foam format was designed to construct two samples of lattice composite plates, and the hand layup technique was used to prepare the ribs. Finally, the obtained ribs were connected to the composite shell in the interface of ribs with shell using Axson H9940 BK for first sample and LR520 resin adhesives for second sample. The prepared samples were analyzed using the three-point bending test. For this purpose, a test fixture was designed and constructed. The standard test of the crack release energy in the first and second mode, tensile test, and nol test were performed to evaluate the mechanical properties of the carbon cloth, carbon fibrs, resin, and adhesive. The numerical solution of the problem and comparing the obtained results with experimental data revealed that the Axson adhesive connections can tolerate more force in comparison with resin adhesive connections before structure destruction.

Nondestructive evaluation (NDE) of the mechanical properties and thickness loss of bilayer structures is of great importance for health monitoring purposes. This study aimed to propose an NDE method based on the Lamb wave propagation for the inspection of bilayer metal-composite structures. The finite element model of a steel substrate coated with a layered composite material was developed, where the composite coating constituted by chopped strand glass fiber mat and woven roving glass fiber cloth layers. The fundamental antisymmetric Lamb wave mode (A0) with different excitation frequencies were generated and propagated on the modeled bilayer specimens. The dispersion curves for the A0 Lamb wave mode were obtained for the simulated specimens, considering different thicknesses and a range of material properties decay for the metal substrate and composite coating. The obtained results showed that the effect of the thickness and decay in the mechanical properties of the substrate on the A0 Lamb wave mode velocity was more than the effect of the thickness and decay in the mechanical properties of the composite coating. Besides, the estimation of coating thickness was performed more accurately at low frequencies, while the decay in the mechanical properties of the coating and substrate was better evaluated at higher frequencies. It was concluded that the simulated Lamb wave propagation method can be used as a virtual lab for the development of methods for nondestructive evaluation of bilayer metal-composite structures with different material properties and thicknesses.

A composite radome is a durable structure to protect telecommunication radar antennas against mechanical and environmental factors. Due to its placement in front of the antenna and the possibility of affecting the received and transmitted waves, in addition to the mechanical properties, the electromagnetic properties of this structure are very important. In this study, the mechanical properties (tensile strength in the direction of fibers) and the electromagnetic properties (dielectric constant at X-band frequency) for a GFRP composite radome has been studied and optimized. For this purpose, two types of glass fibers E and D series, and two types of epoxy resin and polyester resin have been used. In addition, three common methods of composite fabrication including hand layup, vacuum bag, and vacuum infusion have been used. Also, by using the Taguchi test design method and signal-to-noise analysis, the optimal composition was obtained for each of the properties, which for tensile strength of the combination of E series glass fibers and epoxy resin and vacuum bag fabrication method, the highest tensile strength was 320.9 MPa and for the dielectric constant of D series glass fibers and epoxy resin and vacuum bag fabrication method with the lowest value of 2.85 were determined. After determining the optimal composition for each of the properties, the optimum state was investigated by considering all the mechanical and electromagnetic properties factors together by using the multi-response optimization method (gray relation). Finally, a combination of D-series glass fibers, epoxy resin, and vacuum bag fabrication method was proposed.

Petroleum based polymer composites could be a factor in the development of fire unlike their numerous applications. The research performed on the fire resistance and flammability of composites is not comparable with those conducted on the mechanical behavior of composites demonstrating the importance of studying multi-functional composites of fire resistance behavior. In this work, two-part polyurethane was reinforced with natural zeolite and graphite nanoplatelets (GNP) to examine fire resistance and optimized mechanical behavior of the composite foams. Moreover, poly urea was added with the optimized ratio of 40% into polyurethane composites based on the study hypothesis. The results showed that zeolite and GNP filled nano/microcomposites led to a 200 and 300% improvement in the tensile modulus of the composites, respectively. It was further sown that both fillers resulted in enhancement in the flexural modulus, too. To enhance the mechanical behavior, fiber glass mat was laminated with the optimized polyurethane/poly urea compound leading to a 600 and 200% improvement in the flexural modulus, respectively, in the case of two-layer reinforcement. The poly urea coating on the optimized composite specimens demonstrated the reduced burning rate of 23 mm/min and a 200 s flame time based on the UL94 standard. The mixed compound of polyurethane/poly urea filled with 2 wt% of zeolite led to the burning rate of 4.9 mm/min being in the allowable range above 80% lower than the maximum possible rate set forth in the standard.

The preparation of coatings involving the least material components and fabrication route is one of the today’s challenges in the development of hydrophobic nanocomposite films. This study aims at the fabrication of a hydrophobic polymer nanocomposite coating and the evaluation of the interplays amongst hydrophobicity, microstructure, fillers loading and free surface energy of the coatings. To achieve this, hexamethyldisilazane functionalized nano-silica as the filler and ethylene vinyl acetate (EVA) as the matrix were used. Toluene was used as the solvent. The concentration of 3% was employed as the optimized solvent ratio to create polymer film. The loadings of 10,20 and 30 wt% of nanomaterials were added into the solvent followed by a spray technique due to its single step approach. The analyses showed the coating filled with 30 wt% of nano-silica resulted in a porous interconnected structure, in good agreement with the Cassie-Baxter surface energy model. The 30 wt% nano-silica nanocomposite coatings resulted in a static water angle of ~121 ͦθ exhibiting a 34% increase with respect to that in the case of pure EVA and a sliding angle of 8 ͦθ with 90% reduction compared to the 10 wt% filled coatings due to the high surface roughness. Three surface energy models of Fowkes, Owens-Wendt and Van Oss were utilized were employed through which, unlike from the Van Oss model, the Fowkes and Owens-Wendt models exhibited that with the addition of fillers, the polar component of the free surface energy decreases confirming the increase in the estimated water angle contact.

This research is trying to fabricate an efficient and economical EMIS X-band using Electro-Mechanically Dispersion Technique (EMDT). For this purpose, first, poly-methyl-methacrylate (PMMA)-based nanocomposites produced with the different weight percentages between 0 to 2 %, by two EMDT and ultrasonic probe methods. The structure of samples were evaluated by scanning electron microscope and Raman spectroscopy. The electrical resistivity of samples were also measured to determine percolation threshold. At the final assessments, electromagnetic behavior of the samples were examined. The results shows percolation threshold was occurred at the concentration of 0.2 wt.% in EMDT, while this event was observed at the concentration of 1 wt.% for ultrasonic method. According to the microscopic study and the results of Raman spectroscopy, the ability of EMDT in protecting the structure and length of CNTs was detected as the main factor to decrease 80 % in percolation threshold concentration. The investigation of electromagnetic properties of samples at percolation threshold show that EMDT method, despite having a concentration equal to 20% of the ultrasonic probe method, offers effective EMIS of 38 dB, which is 16% higher than the ultrasonic probe method. However, the advantage of EMDT compared to earlier presented methods relates to reduce 25 times the concentration and simultaneously increasing the absorption from 25 to 36 dB.