نشریه علوم و فناوری کامپوزیت
سال هفتم شماره 3 (پیاپی 25، پاییز 1399)

In this research, the effect of imposed stress on the extent of progression of cyclization, oxidation and dehydrogenation reactions during thermal stabilization of polyacrylonitrile fibers as a heat-resistant material in the polymeric composites were investigated. In this study, specific stress was determined based on the maximum entropy stress value. The progress rate of each of the stabilization reactions was identified separately through the using of thermal, chemical, elemental, X-ray diffraction, physical and mechanical analysis. Based on the results, stress had a deterrent effect on the development of reactions and also the competition between cyclization and dehydrogenation occurring under the influence of stress is very evident. With increasing tension, the temperature of the lowering of the weight due to thermal degradation has decreased. This result showed that the thermal stability of the samples decreases with the increase of the specific stresses during the thermal stabilization process. By applying a minimum specific stress of 0.6 cN/tex, the highest rate of stabilization reaction, especially the nitrile group cyclization, and the thermal stability of the oxidized polyacrylonitrile fibers were observed for application in polymer composites.

In this study, the effects of stacking sequence and thermal cycling on the flexural behavior of fiber metal laminates (FML) including glass and Kevlar fibers were investigated. The FML samples were composed of aluminum 2024-T3 sheet and epoxy composite. The polymer composites consisted of glass fibers and Kevlar fibers. These composites were prepared in 2 different fibers arrangement by hand lay-up method. Each thermal cycle was carried out for 2 min between -150 and 100 °C. The FML samples were cycled for 20, 40 and 60 times and their flexural performance was evaluated before and after thermal cycling. The highest values of flexural strength and modulus, and fracture energy were related to sample that Kevlar fibers were the bottom layer of composites. With applying 40 thermal cycles to the mentioned sample, the flexural strength and modulus and fracture energy values were respectively increased to 8, 9 and 35 percent compared to the samples without cycling. While a decreasing trend was observed for samples with 60 cycles compared to the samples with 40 cycles. When the glass fibers were the bottom layer of composites, flexural strength and modulus and fracture energy values were respectively increased to 10, 14 and 9 percent with applying 40 thermal cycles, compared to sample without thermal cycles. But in 60 cycles, flexural properties were reduced. Results of this research indicated that post curing, compressive stress and deboning between components of FML were three main mechanisms for changing the flexural properties of samples during thermal cycling.

In this research, the structure and mechanical properties of polyethylene (PE)/ethylene-vinyl acetate copolymer (EVA)/nanoclay microcellular foams obtained through extrusion physical foaming by using carbon dioxide were investigated. The effect of PE/EVA concentration, organically-modified nanoclay and temperature profile on the properties were evaluated. The foaming process of these systems has significant complexity due to the occurrence of two concurrent phase transitions including upper critical solution temperature (UCST) phase behavior of PE/EVA blends and gas phase transition in the cell nucleation and growth process. Owing to the UCST phase behavior of PE/EVA mixtures, an increment in the process temperature profile has a profound impact on the morphology and mechanical properties of the foams and results in higher miscibility of the polymeric constituents of the blends and nanocomposites. Moreover, the nanoclay compatibilization mechanism leads to better miscibility of the polymeric phases. At higher temperature profile, lighter foams with higher cell density are obtained. Increasing the EVA content in the blends improves the nanoclay dispersion state and as a result, causes an improvement in the foam structure. By increasing the temperature profile and adding nanoclay, the elastic modulus of the foams improves and worsens respectively, due to the better PE/EVA miscibility state and nanoparticle influential role in the bubble formation.

Composites are commonly used in pressure vessels. One of the critical problems in manufacturing composite vessel under the external pressure is sealed opening design and its considerations. It's required to review the body resistance in opening locations. In this paper, a special design process for opening closure presented and the influence of such cutouts on vessel body studied. Because of the importance of body resistance, the vessel material and structure mechanical performance were analytically evaluated. Then, Mechanical behavior of the vessel structure was investigated based on the parameters affecting the definition of different opening designs through finite element simulation and analysis in ABAQUS commercial code/software. Hence, the effect of the presence of openings, including the consequences of creating asymmetry in the structure, has been determined in a large aspect ratio. Finally, the results represents, the importance of complying practical requirements and implementation of reinforcement rings in the opening locations to get the suitable design as one of the most effective solutions to compensate the opening reduction consequences i.e.; buckling strength reduction and highly increasing asymmetry.

In the present study, mesoporous silica/iron oxide nanocomposite (MCM-41/γ-Fe2O3) was prepared with hydrothermal method and then the application of the nanocomposite as an adsorbent for the removal of Ni(II), Cd(II), Cr(III), Pb(II) and Zn(II) ions from aqueous solution was investigated. Furthermore, the effects of the solution pH, contact time and adsorbent dosages on removal percentage were studied. The results indicated that the adsorption of these ions on the surface of the adsorbent increase with increasing of solution pH, contact time and adsorbent dosage. The maximum percent removal (in the condition of: pH=5, t=50min, w=0.16g) of Ni(II), Zn(II), Cd(II), Cr(III) and Pb(II) ions was reached 53, 79, 61, 89 and 99.5%, respectively. The synthesized mesoporous silica/iron oxide nanocomposite was characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), vibrating sample magnetometer (VSM), nitrogen adsorption-desorption (BET) and Field Emission Scanning electron microscopy (FE-SEM). The concentrations of metal ions of the solutions were measured by atomic absorption spectroscopy (AAS). According to the results, MCM-41/γ-Fe2O3 nanocomposite exhibited a large surface area 461.19m2/g, a total pore volume 0.4128cm3/g, a mean pore diameter 3.58nm, a narrow pore size distribution and the superparamagnetic behavior.

In this research, a hybrid aluminum matrix nanocomposite was made using tungsten disulfide nanoparticles and carbon nanotube. Ultrasonic was used for mixing powders in acetone. Then hybrid particles and aluminum powder were mixed by mmechanical stirrer for 2 h and ball mill for 5h. The final mixed powder were compressed by hot pressing. Microstructural analysis of the specimens was performed by Optical Microscopy (OM) and Field Emission Scanning Electron Microscopy (FESEM). The results showed that the reinforcement phases were properly adsorbed on aluminum particles, there was a good distribution of the reinforcement particles in the aluminum matrix, and nanoparticles maintained their structure. The density of samples was measured by Archimedes method and the relative density of hybrid samples was found to be 96 to 98%. Micro hardness test showed that hybridization had a positive effect on the hardness and the hybrid nanocomposite micro hardness increased with increasing carbon nanotube content up to 20% that of Al/ WS2 nanocomposite. Compressive strength measurements showed that hybridization increased the final compressive strength up to 17%. Wear test showed the friction coefficient of the hybrid nanocomposite decreased up to 50% compared to the pure aluminum.

In recent years, due to achieving desired properties, increasing application of Composite in various industries is observable. Exact determination and evaluation of failure mechanisms lead to modified designs and helps assessment of loading in composite structures, specially sandwich panels. In this paper, failure mechanisms during bending of Foam / Glass - Polyester Sandwich panels, which degraded by pre-impact of various energy, have been investigated by acoustic data analysis. 4 different Lay-up techniques for laminates with 4 types of with and without pre-impacts and totally 16 samples were studied. Matrix crack, fiber de-bonding, fiber breakage and foam crack were the mechanisms were investigated through sample failure photos, acoustic frequency count analysis, extracted energy in each frequency range and force-time diagrams. Results showed that brittle failure mechanisms containing matrix crack and foam crack release more condensed and in lower frequency range with higher energy acoustic counts in comparison with ductile failure mechanisms like as fiber de-bonding and fiber breakage. Ductile failure mechanisms release more scattered in higher frequency range but with lower energy acoustic counts. Acoustic Results which applied to assess failure mechanisms validated by visual pictures and force diagrams also lead to a new frequency range determination for foam crack (190 to 220 kHz).

Acrylonitrile butadiene styrene (ABS) has comparatively good mechanical properties, but its low fluidity limits the injection molding of thin parts. In this research, acrylonitrile butadiene styrene/thermoplastic polyurethane (ABS/TPU) blends and ABS/TPU nanocomposites containing multi-walled carbon nanotubes (MWCNT) were produced by employing a twin-screw extruder and injection molding. The morphology of fracture surfaces was studied by scanning electron microscopy. The tensile, flexural and impact properties as well as melt fluidity of different specimens were evaluated. The addition of TPU to ABS substantially increased the melt flow index (MFI), but decreased the mechanical properties. The presence of carbon nanotubes in ABS/TPU blend improved mechanical properties and expanded the plastic deformation of fractured surfaces. The maximum tensile and flexural strengths were obtained by applying 0.3 and 0.5 wt.% MWCNT, respectively. The notched impact strength in nanocomposite containing 0.1 wt.% CNT showed about 95% increase in comparison with ABS/TPU blends. The appropriate dispersion of carbon nanotubes and their adhesion to polymer matrix were considered as the most important factors in improving mechanical properties.

Radar cross section (RCS) is an indicator of the radar detectability of an object. The greater this value, the more visible is the object is to the radar. Basic methods of RCS reduction in composite materials are accomplished through modifications in the design, use of adsorbents additives and composite layup. In this research, the design and development of a GFRP composite material with low RCS has been accomplished. For this purpose, at first, the GFRP samples were manufactured using the Infusion process. In the next step, a layer containing epoxy reinforced by 1, 3 and 5 Vol.% of aluminum, alumina and iron oxide powder has been applied on the GFRP samples. Finally the manufactured samples were subjected to X-band VNA test at four frequencies of 8.5, 9.5, 10.5 and 11.5 GHz. In order to save money and time, Responsive Surface Method (RSM) of Design of experiments (DOE) was used to find the optimum sample. The results show that the maximum absorption was in the sample containing the AL powder (5%) and minimum was observed in the sample having 1 Vol.% of alumina due to their conductivity. Overall in all samples, the absorption increased with the increase in the volume percent of the additives.

Fiber metal laminates (FMLs) are one of the most widely used hybrid polymeric composites in the aerospace and marine industries that are fabricated using the bonding between the metallic and polymeric layers. The combination of the advantages of both metals and composites is the main reason for the usage of the FMLs. Due to the application of FMLs under different temperature conditions, in the present study, the Charpy impact behavior of smart FMLs in comparison to fiber metal laminate was investigated at temperatures of -45, +25 and 90 °C. FML samples were made of two layers of 6061-T6 aluminum alloy and four layers of glass fiber-reinforced epoxy (GFRE), which shape memory alloy (SMA) wires by zero and 5% pre-strain were placed in the middle layers. The investigated parameters in this study were the number of SMA wires, the pre-strain effect of SMA wires, and also the effect of temperature on the energy absorption values of the FMLs. The results showed that the presence of two SMA wires at temperatures of -45, +25 and + 90 °C respectively caused the increment in the energy absorption by 14, 20 and 8%, compared to the without SMA wire samples.

Low-density polyurethane foams as the low-cost and lightweight materials are used to produce some structural parts of marine vessels. However, these foams have low mechanical properties and they are susceptible to water and moisture penetration. The presence of initial defects or impact-induced damages on the external surface of the marine vessels can lead to the water penetration into the foam core, which generally results in core decay and, subsequently, delamination in core/skin interfacial bonding. Thus, the main objective of this research is to reduce the weaknesses of low-density polyurethane foam-based structures by introducing a novel material in this field. For this purpose, a sandwich panel with a hybrid core consisting of agglomerated cork and low-density polyurethane foam reinforced by a composite grid from epoxy/glass fibers structure is successfully designed, fabricated, and tested. Results of 100-days aging indicated that the water absorption of novel specimens was less than half of the conventional polyurethane-based specimens. In addition, replacing the stiffened cork/polyurethane hybrid core instead of the polyurethane core resulted in 506, 814 and 144.94 % increase in maximum flexural load, initial flexural stiffness and flexural toughness, respectively. In the following, the structural behavior of both the novel and conventional structures was investigated by numerical simulation of bending test. The results of numerical analysis showed that the flexural stiffness of novel panel was enhanced about 884%, comparing to the conventional structure.

Aluminum foils have been extensively used in packaging and household applications to protect foods and pharmaceutical products from environmental effects. In recent years, sever plastic deformation processes have been highly regarded due to the production of ultra-fine-grained metal materials. The high applicability of nanostructures, due to their unique physical and mechanical properties, reveals the importance of investigations on new forming methods. Accumulative roll bonding (ARB) process is one of the best and most practical methods for forming metal sheets, which mechanism is the plastic deformation of material through the passage between two or more rollers. In this investigation, thin aluminum foils with a thickness of two hundred microns were produced using accumulative roll bonding method in five passes without lubricant or additional heat treatment between passes and at ambient temperature. To investigate the mechanical properties, uniaxial tensile test and microhardness test were used, and to investigate the microstructure and fracture surface area, scanning electron microscopy was used. The ultimate tensile strength at the end of the fifth ARB pass reached 393 MPa, about 5.9 times larger than of the initial sample. Also, compared to the previous research, the obtained strength was highest due to the lower thickness of the layers and the penetration of surface oxides into the metal matrix during preparation. Furthermore, by increasing the number of accumulative roll bonding passes, the thickness of the layers decreases and the bonding quality between layers is improved. Investigation on tensile fracture surface after five passes exhibits ductile failure mechanism.

Nowadays, due to the need of composite structures with thin walls in industry and due to low weight ratio to high strength, has received more consideration than before. Composite laminates are cause of more problems and damages in machining process. This problems are especially in thin wall composite structures. One of the best ways for preventing damage during machining process in thin wall composite structures is use of machining process with high speed. In this study effect of milling parameters in high speed on surface roughness, material removal rate, and deflection in thin wall composite structures was studied. For this purpose, firstly were made samples of glass- epoxy composite with thicknesses 2mm, 4mm and 6mm and then milling process with high speed and changing spindle speed, feed rate, and cutting depth was done. To get test results using an experimental design. Comparison of the optimization results on composites with different thicknesses by the response surface methodology, showed that the optimal values of surface roughness 2.12 μm and material removal rate 5.99 mm3/min and deflection of 0.082 mm is for the sample with a thickness 6 mm. In composites with a thickness 6 mm due to higher rigidity, better results were obtained. Also, the predicted error rate was calculated in comparison with the experimental values obtained for the surface roughness parameters, material removal rate and deflection rate 6%, -5.22% and 2.5%, respectively, which indicates the favorable agreement between the experimental results and statistical analysis.

A mechanical joint has been used to joint metal cylinder to composite pipe made by filament winding so that by creation bump grooves, on the steel cylinder a guided path for manufacturing the composite pipe, during filament winding was made. Considering the modulus difference between composite carbon-epoxy pipe and steel cylinder, it’s predictable that the main parameter of joint strength under internal pressure is the mentioned modulus difference and fracture at joint edge. Finite element simulation and with hashin criteria for predicting failure initiation and progressive damage criteria that definet by UMAT subrotine achieving damage evolution has been used. The numerical solution results has been compared with experimental test results which are in good agreement with each other. Besides using microscopic imaging, the failure zone has been investigated. For numerical modeling the element with various orders has been used and numerical solution precision and speed, using this elements compared to experimental results has been investigated and continuum element with reduced integral 3D high order proposed.

In this paper, dynamic analysis of the composite cylindrical shells subjected to oblique low-velocity impact by a spherical impactor is investigated. The equations of motion based on classical shell theory (CST) have been extracted using the Newton method. The boundary conditions are considered to be simply supported. The displacement components are written as double Fourier series expansions according to the boundary conditions. In order to obtain the natural frequency and cylindrical shell response under low-velocity impact loading, the equations of motion are solved using the Galerkin weighted functions method. The contact force history is improved by the mass-spring modeling method and predicted using the Hertz linear contact law. For verification purpose, the results are compared with the Abaqus finite element software and the latest available literature and good agreement is observed between the contact force history parameters like maximum contact force and the contact time. In this study, the effect of shell geometrical parameters including ratio of length-to-radius (L⁄R) and ratio of thickness-to- radius (h⁄R) and also the effect of impactor parameters including velocity (v_0), mass (m_i) and angle of impact (γ) on the impact response is investigated.

In this paper, the free vibration of a composite shell with and without a rectangular cutout was studied based on the first-order shear deformation theory. The equations were derived in a general form and can be converted to Donnell`s, Love`s, and Sanders` theories. To investigate the perforated shell a physical domain was decomposed into several elements with uniform boundary and loading conditions in each element edges. In each element, the governing equations were discretized in both longitudinal and circumferential directions by the use of generalized differential quadrature method (GDQM) as well as the boundary conditions at the cutout edges, and the compatibility conditions at the interface boundaries of adjacent elements. Assembling these discretized relations, a system of algebraic equations was generated. Finally, the natural frequencies were calculated by an eigenvalue solution. To validate the presented method, the results of GDQM were compared with the available ones in the literature and also with the ABAQUS finite element model. Then a parametric analysis was performed to investigate the effects of different parameters on the vibrational behavior of the shells with and without cutouts. This study illustrated that small cutouts (c/L<0.3) had no significant effect on the natural frequency of the shell. This was independent of both shell material and layup. In addition, decreasing the length to radius ratio or increasing the shell thickness decreased the effect of cutout on natural frequency. Moreover, circumferential cutouts had less effect than longitudinal ones.