فهرست مطالب مهدی عبدالهی آذغان

Due to their cellular structure, metal foams have properties such as low density, strength to weight and high energy absorption under compressive stresses. Among the various metals used for foaming, aluminum and its alloys are the most widely used due to their relatively low density, availability, and ease of foam formation. In this research, A356 aluminum and composite foams with different amounts of silicon carbide (SiC) particles as a reinforcing and stabilizing agent and calcium carbonate powder (CaCO3) as a foaming agent were produced using direct foaming method from the melt. The density of the foam sample was measured between 0.493 and 0.863 g/cubic centimeter. Then, the microstructure of the foams was examined by OM and SEM microscopes and the compressive and flexural properties of the foams in both conventional and sandwich panels. The relationship between flat stress and density with the weight percentage of CaCO3 and SiC particles was investigated, which shows that the compressive behavior of foam products is not uniform. On the other hand, it was shown that at 10% by weight of the SiC boosting agent, plateau stress increases with decreasing CaCO3 powder content from 6.89 to 11.62 MPa.

Reactive composites are a new group of composite materials consisting of two or more materials that cannot ignite or explode in the Environmental conditions, but can release a lot of energy due to shock and severe impact loads. This study aimed to investigate the effect of milling time on the microstructure, thermal and mechanical properties of the Al-Ni composite. For this purpose, the Al-Ni compound with a 2:1 Al:Ni molar ratio was milled for 0.5, 1, 2, 4 and 6 hours in attrition mill and mixed. Then the samples were cold press and sintered at 400 ˚C under argon atmosphere for one hour. The microstructure of samples was analyzed by field emission scanning electron microscope (FESEM) and XRD. To investigation of thermal properties DSC and DTA analysis and for mechanical properties compression test and Hopkinson test were used. The DSC analysis results showed that by increasing the milling time, the reaction start temperature decreased from 650.34 °C in the sample milled to 0.5 hour to 645.84 °C the sample milled to 6 hour and the reaction heat (enthalpy) decreased from 26.87 J/g to 14.84 J/g. The results of compression and Hopkinson tests of samples after 6 hours milling time showed 21 and 42 percent increase in compressive strength, respectively compared to samples after 0.5 hours milling time. Also, the results showed that the compressive strength increased by changing the strain rate from 0.01 s-1 (in the pressure test) to 1000 s-1 (in the Hopkinson test).

This study was conducted to investigate the effect of adding titanium diboride (TiB2) nanoparticles on the microstructure and tensile properties of the Al5083 matrix composite. Al5083/TiB2 metal matrix composites (with 5 and 10 wt % reinforcement) along with zirconium (Zr) and cerium oxide (CeO2) additives with different wt % were fabricated by in situ-stir casting at 1000 °C. The samples were then subjected to hot extrusion for uniform distribution of reinforcements in the matrix. TiB2 nanoparticles were in-situ processed in molten aluminum using the precursors such as cryolite (Na3AlF6), titanium oxide (TiO2), and potassium tetrafluoroborate (KBF4). The microstructure, surfaces, and failure mechanism of the samples were investigated using X-ray diffraction (XRD), optical microscopy (OM), and scanning electron microscopy (SEM). Tensile test results showed that the addition of 10 wt % TiB2 particles increased the ultimate tensile strength by 17.7 % and decreased the strain by 19.2 % compared to the sample without reinforcement. Besides, the addition of Zr and CeO2 increased the strength by 35.8 % and the strain of the sample containing 78 % by 10 % reinforcement compared to the sample without reinforcement due to the removal of intermetallic compound Al3Ti and the incompatibility between coefficients of thermal expansion (CTE) with the matrix. Also, post-extrusion annealing in the sample with 10 wt % TiB2 reduced the tensile strength.

In this study, the effect of functionalized graphene nanoplatelets (GNPs) on the mechanical properties of vinyl ester/glass fiber composites was investigated. In order to properly distribute the graphene in the matrix and create a stronger bond between surface modified GNPs, vinyl ester and glass fibers, surface treatment of GNPs was performed using (3-aminopropyl) trimethoxysilane. The nanocomposites with different weight percentages of treated GNPs (0, 0.2, 0.4 and 0.6) were fabricated by hand lay-up method. The nanocomposite with 0.4 wt.% of modified GNPs showed the best mechanical properties. The results of bending test of 0.4 wt.% showed 63 and 28 percent increase in flexural strength and modulus, respetivily compared to composite without GNPs. Also nanocomposite containing 0.4 wt.% surface modified GNPs exhibited 26% and 11% improvement in tensile strength and modulus of elstisty compared to the FMLs containing 0.0 wt.% GNPs, respectively. According to the SEM images, the increase in mechanical properties could be related to the improvement in the adhesion between glass fibers and vinyl ester resin, and also the toughening mechanism of treated GNPs.

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 study, the effects of adhesive type (Epon 828 and Araldite EA 1103), the overlapping area (12.5 and 25 mm), and the surface treatment of the polymer composite on the shear behavior of the sandwich panels with a foam core of polyvinyl chloride (PVC) C7075 and a polymer composite skin (glass fibers-vinyl ester) were investigated by finite element simulation and experiment. The results revealed that the sample with Epon adhesive exhibited higher shear strength and lower strain percentage value compared to the sample containing the Araldite adhesive. The samples with an overlapping area of 12.5 mm showed higher shear properties compared to those with a 25 mm overlapping area which is due to the presence of more bubbles and imperfections on the overlapping surface. Due to increased contact area and penetration of the adhesive into the porosities, surface-modified polymer composites with 100-grit sandpapers exhibited higher shear properties in comparison with those surface modified with 60-grit sandpapers. Also, the investigation of the fracture mechanism showed that the two mechanisms of the adhesive fracture and the rupture of the PVC foam led to the destruction of the sandwich panel. Since the foam rupture mechanism overcame the adhesive fracture, the shear properties of the sandwich panel were increased. The results showed that there was a good agreement between the experimental and simulation results.

In this study, effect of fiber arrangement and cryogenic cycles on the flexural behavior of fiber metal laminates (FMLs) were investigated. The FMLs were prepared from two aluminum 2024-T3 plates and basalt-glass fibers/epoxy composite in the central part of it. These composites were prepared in 5 different fiber arrangement by hand Lay-up technique. In order to enhancement of adhesion between aluminum and composite, aluminum surfaces were treated by electrochemical treatment (anodizing). Each cycle was carried out in 3.5 min between -100 and 25 °C. The laminates were cycled for 40 times and their flexural performance were studied before and after cryogenic cycle. In order to characterization of failure mechanism of specimens, scanning electron microscopy and optical microscopy were used. The highest and lowest flexural strength and modulus consequently belonged to the specimen with layers of basalt fibers (BFML) and the specimen consisted of glass fibers (GFML) because of weak properties of glass fibers in comparison to basalt fibers. After carrying out cryogenic cycles, BFML had the highest percentage of difference with respect to this specimen before carrying out the cryogenic cycle. These difference in flexural strength and modulus were 6% and 4.9% respectively. But in the case of GFML, difference percentage in flexural strength and modulus were 2.5% and 1.6% respectively that was the lowest percentage of difference after cycling. When the hybrid FMLs were carried out by cryogenic cycles, due to overcoming the pressure stress mechanism on delamination between composite components, results showed enhancement in flexural behavior.

In this research, the effect of graphene on the impact behavior of fiber metal laminates (FMLs) reinforced with hybrid of glass-Kevlar fibers, was investigated. FMLs including hybrid of glass-Kevlar fibers/ aluminum 2024 with different weight percent of graphene were made by hand lay-up method and subjected to impact charpy test. Also, before making composites, the surface of aluminum is modified using a chemical method to improve the bond between it and composite. The use of graphene reduced impact strength in the FMLs. Thus, by adding 0.25, 0.5 and 1 wt.% graphene to epoxy, 34.2, 41.8 and 58.86%, respectively, reduction were observed in the impact strength compared to the graphene-free sample. Increasing the amount of graphene in the composite, the degree and the probability of separation between the aluminum and the composite increased, and the FMLs showed a more brittle behavior. Thus, in the sample without graphene, the separation did not occurred, but with increasing graphene, the separation become more and finally for samples containing 1 wt.% graphene despite the separation between the aluminum and composite, brittle fracture in the composite and the subsequent failure of aluminum was observed. According to microscopic images, the presence of graphene and its agglomerate at the interface between aluminum and composite causes initiation and crack growth. Graphene also fills the cavities formed on the aluminum surface, which are formed to better interact with the resin by surface modification, and prevent the penetration of the resin and the proper bonding between the epoxy and the aluminum.

In this article, the effect of cryogenic cycling on flexural properties of fiber metal laminate (FML) has been investigated. This laminates were composed by two aluminum 2024-T3 plates and an epoxy matrix composite ply formed by four layers of basalt fibers by hand lay- up technique. Various surface treatments include mechanical (grinding), chemical (etching) and electrochemical (anodizing) were used for improving the adhesion between polymer composite layer and aluminum sheet. Each cryogenic cycle was carried out in 4 min and temperature range was between -40 °C and 25 °C. Flexural properties were evaluated on samples after 15 and 30 cycles and compared to non-exposed samples. While the cryogenic cycling decreased the flexural strength of the FML with etched aluminum, increasing at first, and then decreasing after a while was observed in the FML with anodized aluminum. About samples that the FML with grinded aluminum, both of flexural modulus and fracture energy increased at first and then decreased; while the flexural strength decreased continuously. Evaluation of optical microscope showed that the reason of decreasing in flexural properties about samples with etched aluminum and also decreasing in flexural strength for samples with grinded aluminum after cycles is changing in mechanism of failure from failure of composite to separation between layers of FML; while about FMLs with anodized aluminum, the only observed mechanism was failure of composite and it has not changed.

In this work, effect of thermal cycling on the flexural properties of fiber-metal laminate (FML) has been evaluated. FML plates were composed by two aluminium 2024-T3 and a epoxy polymer-matrix composites ply formed by four layers of basalt fibers. For FML samples the thermal cycle times were about 6 min for temperature cycles from 25 °C to 115 °C. Flexural properties were evaluated on samples after 20, 35 and 55 thermal cycles, and compared to non-exposed samples. While the thermal cycling decreased the flexural strength of chemical treated FML (etched aluminium), increasing at first, and then decreasing after a while was observed in electrochemical treated FML (anodized aluminium). The flexural modulus of FML showed irregularly changes for both of FML with anodized aluminium and FML with etched aluminium. The energy absorption of FML with etched aluminium showed a sharp decline with increasing thermal cycling while the energy absorption of FML with anodized aluminium showed a Low and irregular changes. Evaluation optical microscope showed that the mechanism of failure for the FML with etched aluminium after thermal cycling changed from failure of FML to separation between layers of FML, while for the FML with anodized aluminium before and after thermal cycling it was failure of FML and it has not changed.