نشریه علوم و فناوری کامپوزیت
سال ششم شماره 4 (پیاپی 22، زمستان 1398)

Fiber metal laminates (FMLs) are a new group of composite materials that consist of alternating thin laminates of metals and fiber reinforced polymeric composites. FMLs combine the advantages of metals and polymeric composites. In order to enhance the interfacial bonding between the alternating laminates, various surface treatment methods can be used on the metal surfaces. In this research, FMLs of alternating laminates of aluminum/epoxy-basalt fibers were fabricated using hand lay-up method. The effect of various surface treatment methods of mechanical and chemical (consist of alkaline and sulfochromic etching) on the flexural properties of the fabricated FMLs were investigated with three-point bend tests. SEM were utilized to investigate surface morphology of aluminum. Results showed that the sulfochromic etch method brought about a significant improvement in the flexural strength, strain to failure and absorbed energy in comparison to the mechanical and alkaline methods. Difference in flexural behavior of samples is attributed to their aluminum surface morphology. SEM observations showed that sulfochromic etch method created a porous layer on the aluminum surfaces. This porous layer provided channels for the epoxy resin to fill in, creating better adhesion and a much stronger mechanical interlocking between the laminates.

The occurrence of damage is an unavoidable fact in polymer composites. Damage modes in polymer composites are delamination, surface cracking, polymer cracking, etc. The presence of self-healing could extend the service life time of structure via preventing of damage growth. In this study, a self-healing E-glass fibers/epoxy composite based on micro-vascular channels has been fabricated and focused on the repair of the structure through the delivery of self-healing agents. The specimens were fabricated by hand lay-up route, while the fabrication of microvascular channels was conducted through creating solid preforms and then removing them. Since an important factor for effective healing of this structures after damage creation is high fluidity and suitable miscibility in the damage area, so anhydride resin-hardener system was used because of the higher fluidity in comparison to the amine resin-hardener. The aim of this study is to investigate the role of healing time for achieve of system optimum healing efficiency. To do so, microvascular channels with a constant volume fraction (4%) were incorporated in the composites. The flexural and tensile behavior of the specimens were assessed during the different times (0, 4, 7 and 11 days) from the primary damage creation. After damage creation and break, healing agents present in the microvascular channels flowed in the damage area and over a time span local polymerization and restoring of structure were completed. The results showed that, the highest flexural and tensile strength recovery was obtained 59.07% and 68.05% for the specimen after 7 days from initial damage creation.

In this study, the effect of mechanical reinforcement of glass fiber sizing on the transverse mechanical properties of the glass/epoxy composite, due to the significant impact of this region on the overall mechanical properties of reinforced composites with fibers, has been investigated. To predict the transverse elastic modulus and tensile strength of the glass/epoxy composite, a representative volume element (RVE) and a three-dimensional shell model are simulated respectively, in ABAQUS commercial software. Sizing mechanical properties are held non-homogeneous and non-uniform along its thickness in the simulation. Furthermore, sizing reinforcement is done by a Random-Distribution method using carbon nanotubes (CNTs). Different lengths, diameters and, volume fractions are considered for the CNTs in sizing reinforcement in this simulation, and then the results are compared. The comparison between the results obtained from simulation and available experimental data illustrates that the sizing simulated by non-uniform mechanical properties provides more precise results than the sizing assumed by constant mechanical properties. Also, it is shown that increasing in CNTs length or decreasing in their diameter, which are distributed in sizing, would lead to improving the transverse elastic modulus and tensile strength of the glass/epoxy composite.

In this investigation, consequence of introducing nanoclay particles on the flexural response of basalt fibers reinforced epoxy composites was examined. To improve the dispersion of nano reinforcements into the polymer matrix, their surface was modified with 3-glycidoxypropyltrimethoxysilane coupling agent (3-GPTS). Accordingly, the surface functionalization was vouched using Fourier transform infrared spectroscopy (FT-IR). Results of the three-point bending test suggested that the highest enhancement of flexural properties was achieved via 5 wt. % of nanoclay particles. In this case, with the addition of 5 wt. % of nanofillers, flexural strength, flexural modulus, failure strain and energy absorption were increased by 30, 38, 15 and 40 percent, respectively. Microscopic investigations demonstrated that presence of nanoclay particles within the structure of basalt fibers reinforced epoxy composites enhances the stress transfer between epoxy matrix and basalt fibers which, in turn, causes a significant improvement in the mechanical properties of basalt fibers reinforced polymer composites.

In this research, the effect of adding multi-wall carbon nanotubes on high velocity impact behavior of epoxy matrix composites with kevlar fibers and ultra-high molecular weight polyethylene fibers layered arrangement was investigated. Initially, hybrid composite specimens were made in 0.1, 0.3, 0.5 and 0.9 wt.% cabon nanotube by hand lay-up method, in temperature of 200 °C and 50 minutes. Therefore, high velocity impact test with a sharp projectile with velocity 84 m/s was performed on hybrid composites. Also, to investigate the failure mechanism of hybrid nanocomposites, a field emission scanning electron microscope (FESEM) was used. The results showed that energy adsorption increased for the specimen containing 0.1 wt.% of carbon nanotubes by 13.56% in comparison with the without nano specimen. Proper distribution of carbon nanotubes and stress transfer between fibers and matrix, increased delamination and has been more energy absorbed. Also, microscopic results showed that in 0.1 wt.% of carbon nanotube, the cracking bridging phenomenon and nanotube pulling out, energy absorption in the hybrid composite was increased.

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.

One of the major problems of composite parts during their services is the creating and growing microcracks into them. The growth of microcracks and incorporation of them together, can lead to the catastrophic failure of composite structure. To solve this problem, the researchers especially during the last years ago have made many efforts to fabricate self-healable materials to heal the microcracks and prevent the failure of whole part. The self-healable matter is the substance which is caused to heal the microcracks automatically and without any external intervention. According to self-healing methods, these smart materials are divided into two major group of intrinsic and extrinsic. In intrinsic healing system, the healing was carried out by physical, chemical and super molecular interactions. In contrast in extrinsic healing system, the healing agent is stored into the container such as hollow fibers, micro vascular and microcapsule. The present work tries to investigate the most recent breakthroughs in the various extrinsic healing systems with emphasis on using them into the polymeric matrix composites, especially in period time of 2009 up to now. In this regard and in the review work, the necessity composite healing, self-healing concept, different extrinsic self-healing system, and healing performance evaluation in the different mechanical exams, as well as the related statistical reports and development to the self-healing are exhibited.

In this paper, the low velocity impact of woven carbon-fiber-epoxy composites have been investigated experimentally using a number of impact tests. The woven laminates are twill and made by vacuum infusion technique (VARIM). The low velocity impact tests were carried out with different impact energies of 20, 30, 50, 60 and 80 J to find the penetration and perforation threshold energies using profile energy method. Then the impact behavior of the samples was studied using drawn diagrams of contact force-deflection, contact force-time, deflection-time, and energy-time to investigate the effect of the energy of impact and its' variations on the maximum contact force, absorbed energy and deflection in the woven Carbon-Fiber-Epoxy Composites. The results show that the contact force, absorbed energy and deflection increases when the applied impact energy increases up to 60 J. It is worth mentioning that the observed enhancement trends of the contact force, absorbed energy and deflection are different from each other.

In the present study, for the first time the Young's modulus, anisotropy coefficient and elastic and plastic parameters of multi-layered Al/Br/Al composite produced by cold roll bonding process were assessed by digital image correlation method. The digital image correlation (DIC) is a relatively new method used to measure strain fields to determine various parameters such as anisotropy and Young's modulus for many materials and alloys. Structure, mechanical properties, elastic and plastic parameters are determined by optical microscopy (OM), micro-hardness measurements and tensile tests equipped by 2D DIC system. Using longitudinal and transverse strains from DIC and plasticity theory thickness strain, anisotropy coefficient and other elastic and plastic parameters were calculated. The results showed that mechanical properties were improved compared to the primary samples, so that the yield strength and ultimate tensile strength of the composite were more than five times the original aluminum, and microhardness of both layers of aluminum and brass improved more than two times due to cold working caused by increasing the dislocation density during rolling. The value of the calculated elastic modulus was obtained 77.8GPa by digital image correlation method, which are little difference from the values obtained from theoretical relationships based on the volume of the composite materials. Also, the anisotropy coefficient during the tensile test, after the initial oscillation increased to the necking point, then decreased and a little before the failure point fixed.

The replacement of damaged components is not affordable in important structures such as aircraft, ship or gas pipelines. So, the repair of a defective structure is an acceptable process. Composite patches are used to repair the damaged metal and composite structures in different industries, especially the aerospace industry. Assessment of the repaired structure is a challenging topic in order to ensure the restoration. The thermography technique is one of the most powerful non-destructive testing methods that is used to survey the repaired structures. In the present study, the defects of the de-bonding type between the based aluminum structure and the carbon/epoxy patch made by 4 layers with layup configuration [04] have been investigated by step heating thermography method. Defects locate close to the patch edges because it is more likely that debond onset in a repaired structure at edges in practice. Furthermore, detection of the edge defects is more difficult than the middle defects because of edge effects. The step heating thermography results have been processed by using pulse phase thermography (PPT) approach. Also, the simulation of thermography testing procedure carried out by finite element modeling (FEM). Finally, the results of the experiment and finite element modeling have been compared and good accuracy has been obtained in step heating thermography and PPT algorithm.

Despite previous research on the corrosion behavior of glass fiber reinforced polymer composites, the relationship between the degree of weakening of the resin/fiber interface bond and their mechanical properties is still necessary. In this research, empirical research is conducted to evaluate this issue. For this purpose, the polyester/glass samples were immersed in 10% HCl with three different temperatures of 25 °C, 50 °C and 70 °C, and changes in the mechanical properties of the samples and the apparent variations of the solution at time intervals of one up to four weeks are examined. The results showed that the ultimate tensile strength and Young modulus of composite specimens were reduced when placed in a solution with higher temperature or immersion time. Cracking caused by corrosion was shown in the resin/fiber interface using SEM photographs, and the degradation of the polyester resin was confirmed by observing the increase in surface cracks and changes in the solution color. Furthermore, visual inspection of sample failure surfaces after a tensile test showed that the failure occurred as DGM type. Finally, atomic absorption spectroscopy (AAS) analysis was performed to prove the occurrence of ion exchange mechanism. The results of this study indicate the occurrence of corrosion mechanisms in the interface area of composite specimens.

In this study, develop accurate finite element models of plain woven fabrics to determine their mechanical properties and progressive damage analysis. In addition, in order to describe the internal geometry from actual measurements of tow geometry made on photomicrographs of sectioned laminates. In the first step, a unit-cell of composite was created in CATIA software in meso scale. After that, unit-cell was imported in ABAQUS software to finite element (FE) analysis. For each architecture, the yarns are assumed to be transversely isotropic and the resin assumed isotropic. After that a stress averaging technique based on an iso-strain assumption is used to determine the effective moduli of the unit cells. Afterwards, The damage model is implemented in the FE code by a user-defined (USDFLD). In this damage model, in order to predict damage initiation of composite plates, 3D Hashin’s failure criterion is chosen. And in order to evolution damage instantaneous Material Property Degradation Method is applied. The good agreement between the theoretical results and experimental data, introduces the ability of the applied model and provided subroutine.

Composite sandwich panels with grid stiffened core, are composed of composite face sheets and kagome type lattice core. These structures can be used as alternative to the structures reinforced with stringer, sandwich panels with honeycomb core and aluminum grid structures. In this study, experimental tests and finite element analysis using ABAQUS software are applied for low-velocity impact on grid stiffened sandwich panels. In the experimental method, two sandwich panels with grid stiffened core are manufactured and undergo drop weight impact with a hemispherical steel impactor. Also, in the numerical method, the results are compared with the three-dimensional elements and progressive damage model is applied by employing user defined material subroutines in finite element method using ABAQUS software. Making comparison between the present numerical results with experimental results, shows that the finite element method is an efficient way to reduce the time and cost for understanding the behavior of this type of structure against impact loads. The energy absorption occurs in the structures mainly due to the induced damage in the impact region of the structure. This damage may affect the top face sheets or the ribs within the core of the sandwich panel and the rigidity of the impact position, reduces the visible damaged area in the structure. Also, the impact on the points such as the ribs’ intersections, which are more rigid than the space between the ribs, causes the contact time to be decreased and the maximum contact force to be increased.

Reliability analysis of composite structures has gained increased attention due to the growing use of composite materials in many industries such as aerospace, automotive and construction in recent decades. Uncertainty analysis approaches are effective tools in order to probabilistically assess the behavior and evaluate the reliability of composite structures with variabilities in material properties. In this study, a computationally efficient surrogate model based on the polynomial chaos expansion for reliability analysis of composite structures with a large number of uncertain parameters is presented. The uncertain input parameters including composite material properties, geometry and loads, are assumed as random variables with a normal distribution and are taken into account for reliability evaluation. A sparse grid collocation strategy is used to determine the sample points for constructing the surrogate model relating the uncertain variables to the structural response. In the end, a numerical example is performed to demonstrate the accuracy and efficiency of this methodology for a higher number of uncertain variables by comparing the results with the direct Monte Carlo simulation method.

In this research, the nonlinear free vibration analysis of functionally graded (FG) rectangular plate is investigated analytically using first order shear deformation theory (FSDT) for the first time. For this purpose, firstly, using Hamilton principle, the partial differential equations of motion are developed based on first order shear deformation theory (FSDT) and von Karman nonlinearity strain displacement relations. Afterward, by applying Galerkin method, the nonlinear partial differential equations are transformed into nonlinear ordinary differential equations. Then, using the modified Lindstedt-Poincare method, the nonlinear equation of transverse motion of the FG plate is solved analytically to determine nonlinear frequency ratio. The material properties of the plate are assumed to be graded continuously according to power law distribution in the thickness direction. The effects of some key system parameters such as vibration amplitude, volume fraction index and aspect ratio on the nonlinear natural frequency ratio to linear natural frequency are discussed. To validate the analysis, the results of this study are compared with the results of previously published papers and numerical solution and good agreement has been observed.

Polytetrafluoroethylene (Teflon) is one of the most widely used materials as the matrix in polymer-matrix composites due to its unique properties. In this study, the effect of zirconia particles addition on the wear rate and friction coefficient of pure Teflon has been investigated. For this, composite samples were manufactured by adding of 5 to 30wt.% zirconia into the PTFE matrix containing 10wt% graphite. Production of the samples were done by cold press and sintering method according ASTM D4894 and 4745 standards. To evaluate the wear resistance of the samples, pin on disk test was performed according to ASTM G99 standard. Finally, the hardness of the samples was measured using ASTM D2240 (Shore D) standard. The scanning electron microscope was applied to investigate the distribution of second phase particles and worn surfaces of the samples. The lowest wear rate and friction coefficient and the highest hardness were obtained for the sample containing 20wt.% zirconia. This sample had approximately 3 times lower wear rate and 12% higher hardness with respect to pure PTFE. This improvement is due to the higher load carrying capability of composite samples as a result of zirconia particles incorporation in the polymer matrix.