نشریه علوم و فناوری کامپوزیت
سال هشتم شماره 1 (پیاپی 27، بهار 1400)

The fiber-reinforced polymer (FRP) composites under different dynamic loads experienced various strain rates. Since mechanical behaviors of fiber-reinforced polymer composites vary with the strain rate, the transient response of most of the composite structures will be dependent on the strain rate. In the present research, a comprehensive review of the previously published studies on the topic of strain-rate dependent properties of glass fiber and its fiber-reinforced composites, as the most common FRP composite, under dynamic loading was presented. At first, studies that presented the mechanical properties of the long glass fibers at various strain rates were extensively investigated. Furthermore, experimental studies on the effects of strain rate on different types of glass fiber reinforced polymer composites were categorized and presented. Also, the strain-rate dependent behavior of the thermoset polymers was investigated. The various analytical and numerical models of macro-mechanics and micromechanics presented for this type of composites were reviewed comprehensively.

Most natural materials are composites that can exhibit a wide range of material properties, such as the elastic modulus. Insect cuticle is one of these natural materials. The material gradient of the cuticle drastically varies in different insect body parts. Considering the small size of cuticular samples, conducting experimental tests is very challenging, expensive, and time-consuming. Understanding how cuticular structures work and their graded properties can help to design and develop engineering materials with enhanced properties. In this paper, the finite element (FE) method is used to investigate the function of the cuticle in the membrane of dragonfly wings. In this regard, first, the distribution of materials on the wing membrane is investigated using images obtained by confocal laser scanning microscope (CLSM). Then, in order to estimate the stress and strain values subjected to displacement, multiple stiffness functions are applied for a geometric model of a cantilever beam, which represents the membrane. The results show that the presence of a graded stiffness has a significant effect on the mechanical behavior of the cantilever beam. A comparison of the results obtained from the analysis of different stiffness functions showed that The quadratic function is introduced as a more suitable distribution for stiffness, in terms of having the least stress and strain in this structure, compared to other stiffness functions. This study can provide a proper and applied platform for further interdisciplinary research in this area.

In the last few years, researchers have been interested in the fabrication of laminated composite sheets. In this research, mechanical properties and fracture toughness of three-layer aluminum-magnesium-aluminum composite sheet produced by cold-rolling bonding method were investigated. Mechanical properties and fracture cross-sections of the specimens were studied by uniaxial tensile test, hardness test and scanning electron microscopy, respectively. It was observed that the tensile strength of composite specimens was higher than that of aluminum and the microhardness and fracture toughness values were increased compared to the initial specimens. High applied cold work can be the main reason for the increase in hardness and fracture toughness, and the reason for the decrease in composite strength over magnesium is the creation of plastic instability in the magnesium reinforcer due to the high strain.The results of scanning electron microscopy also show that the fracture mechanism is for the soft aluminum layer and for the crisp magnesium and the surface of the fracture cross section in the aluminum layer has a shallower and smaller micropores, indicating shear ductile fracture.

Non-homogeneous mixture and unintentional flaws during the production stage have always given rise to uncertainties in the structural response of composite materials. This is intensified in the composites reinforced with natural fibers due to the natural origin of the fibers. In this study, an uncertainty analysis of the frequency response of a unidirectional flax/epoxy composite, as one of the common natural fiber composites, were carried out. The variability of the fiber elastic properties, fiber density, fiber volume fraction, and the misalignment of ply orientations were considered as the uncertainty sources. The uncertain input variables were divided into normal and uniform variables through the Anderson-Darling test, based on the various experimental data acquired from the literature. Due to the high number of uncertain input variables, a computationally efficient response surface approach based on the polynomial chaos expansion was adopted and modified accordingly to take the multi-type stochasticity of the input parameters into account. Moreover, the uncertainty analysis results obtained from the response surface method were validated by the direct Monte Carlo simulation, and the accuracy and efficiency of the surrogate model were demonstrated.

The improvement level of mechanical properties of aluminum matrix composites reinforced to carbonaceous materials is lower than expected. The intrinsic characteristics and production methods are effective parameters on microstructure and efficiency of these composites. In this study, AA7075/CNTs+GNPs hybrid composites were fabricated by flake powder metallurgy process combined to solution method, semi-solid casting and high temperature extrusion. After determination of optimum VGNP/VCNT ratio in hybrid carbonaceous reinforcement (= 0.167), its effect on microstructure and mechanical properties of AA7075 alloy was investigated. The semi-solid casting process led to destroy the network structure of carbonaceous reinforcements and to entrap them mechanically into the aluminum matrix and to segregate alloy elements on their surface. In optimum temperature of extrusion process (= 400 °C), the uniform distribution of carbonaceous reinforcement and the decrease in the average grain size (~39%) were obtained. Hardness, tensile and compressive strengths of the hybrid composite were improved by 17%, 51% and 28% compared to AA7075 matrix alloy. The extraordinary efficiency of tensile strength (~3952%) was obtained in the hybrid composite as a result of effective load transfer due to effect of the pulling out and the bridging of carbonaceous reinforcements.

In this paper, the impact behavior of polyurethane foam-based composites reinforced with warp-knitted spacer fabric is investigated, experimentally. For this purpose, warp-knitted spacer fabric with different structures such as two different thickness, small and large mesh sizes and position of the meshes facing each other and not facing each other were produced. Then composite samples were fabricated using warp-knitted spacer fabrics as reinforcement, and polyurethane foam as matrix. The physical properties of samples like weight per unit area and fiber volume fraction of composite were measured. The failure energy of prepared samples was measured during quasi-static impact test, and finally low velocity impact with an initial energy of 5 J was carried out on composite samples. The results showed that the impact energy of samples is increased at least tripled by using the reinforcement. The maximum energy absorption is 2.858 J which is related to the samples reinforced with fabric with large mesh, high thickness and not facing of the meshes relative to each other. Generally, the energy absorption on average has been increased 21.2% by increasing the thickness, 9.5% by increasing the size of the meshes from small to big, and 47.3% by changing the position of the meshes from facing to non-facing.

Composite materials became a great interest of researchers on light weight structures during the last decades due to their high specific strength and high specific stiffness. Lattice and grid stiffened structures are one of these efficient composite structures especially for axial compressive loads. In this research, the following main objectives are followed: (1) The buckling strength analysis of the lattice cylinders subjected to axial compressive force, (2) The impact response and damage analysis of the lattice cylinders subjected to the transverse impact of a falling object, (3) The buckling strength analysis of the lattice cylinders subjected to axial compressive force after applying transverse impact to the structure. In order to achieve the above purposes, the finite element and the experimental methods are used. In the finite element method, ABAQUS software is used to find maximum axial strength of the structure and the impact results of the structure due to its energy absorption and damage properties. In the experimental method, first, two samples of the lattice composite cylinders are made of Kevlar/Epoxy material and then they are subjected to impact test and also buckling strength tests before and after applying transverse impact when damage occurs in the lattice structure. Finally, the results of these two methods, including impact force, impact time and damage area have been compared. Results show that the damage of the structure due to the impact test, do not causes the maximum buckling strength of the structure to be reduced significantly.

In this article, effects of the loading rate on low-cycle fatigue behaviors in carbon fiber reinforced composites have been investigated. Firstly, open-hole composite specimens were exposed to two different tensile loading rates to achieve displacements in low-cycle fatigue tests. Then, samples were subjected to cyclic loading, in three displacements and five loading frequencies. Fracture surfaces and scanning electron microscopy images were also shown. Obtained results indicated that increasing the rate enhanced the maximum stress due to the material tend to the brittle behavior. The maximum stress in different frequencies had a small enhancement; however, it enhanced at various displacements. The lifetime had a decreasing trend in a constant displacement, with increasing the frequency and increased in a constant frequency, with increasing the displacement. By analyzing the sensitivity, the change in the loading frequency (at constant displacements) and the displacement (frequencies: 1-5 Hz) were not effective on the maximum stress. However, under 10-20 Hz, it was effective on the maximum stress. The material cyclic behavior demonstrated that the maximum stress decreased during loading cycles due to the damage accumulation and the composite stiffness reduced. The failure mechanisms changed by changing the frequency and the percentage of the fiber breakage increased by increasing the loading frequency.

In this paper, bending analysis of a hyperelastic multi-layer square plate with clamped, simply supported, and free boundary conditions are studied. The right Cauchy-Green tensor and neo-Hookean strain energy function utilized to define the plate's physical nonlinear behaviour. The nonlinear strains formulated using first-order shear deformation plate theory, and the Euler-Lagrange equations employed to derive the strong form of the governing equations. The meshless collocation method based on radial point interpolation method used to solve the nonlinear governing equations. The nonlinear boundary conditions imposed directly on the plate in meshless collocation method. The logarithm basis function utilized for defining shape functions, and the nonlinear system of equations solved using the arc-length algorithm. The results of the meshless method compared to those of ABAQUS finite element software. The results show that the meshless collocation method based on radial basis functions are efficient in nonlinear bending analysis of the multi-layer hyperelastic plate with various boundary conditions such that the less difference between meshless method and finite element method is 0.93% for clamped and the most difference is 8.72% with free boundary conditions.

Unlike other materials, composite laminates have more uncertainty sources, such as uncertainty in the material properties; hence it is very essential to predict the probability of the occurrence of various types of damage modes in composite laminates due to random behaviors of composite structures. Matrix cracking induced delamination (MCID) is one of the catastrophic modes that can occur in composite laminates. In this paper, a new algorithm for probabilistic prediction of MCID based on the concept of energy release rate and its critical value was developed and by applying this proposed framework, the reliability analysis of MCID damage was performed. The limit state function was formulated using a general failure criterion which was developed by the authors of this article, previously. To represent the performance of the proposed algorithm, the probability of the occurrence and growth of MCID in a quasi-isotropic laminate including 45°, 90°, -45° and 0° plies under different loading conditions and various stacking sequences was extracted by using first and second order reliability methods (FORM and SORM),. The verification of the obtained probabilities was performed using Monte Carlo simulation (MCS). In addition, some significant results were validated using several experimental data, qualitatively. The effect of variables such as the ply thickness, the level of longitudinal uniaxial stress and the presence of general in-plane stresses on the probability of occurrence and growth of MCID was investigated.

Drilling process of Fibers Glass-epoxy composites is always associated with defects such as interlayer delamination phenomenon. This phenomenon reduces the strength of composite panels. In this study, to minimize this defect, the effective parameters including feed rate, tool diameter, thickness, edge tool angle, rotational speed, orientation fibers and coolant were studied. The number of experiments was specified based on Taguchi's design of experiments method and Experimental study was performed on the drilling process of the composite to evaluate the amount of delamination. Then holes were scanned by a Video Measuring machine(VMM) and amount delamination in them has been calculated by considering hole diameter. The collected data were used to statistically analyze and optimize the drilling parameters using variance analysis (ANOVA). The results showed that feed rate with a 29.6% and rotational speed with 24% had greatest influence on the delamination, so that with increasing the feed rate delamination was increased and with increase in rotational speed, delamination is first increased and then decreased. Also to investigation of the effect of delamination on the tensile strength of composite and stress concentration coefficient, specimens were subjected to tensile test. The results showed that with increasing delamination, the average tensile strength and stress concentration coefficient of perforated parts compared to healthy specimen( without hole), 34% decreased and 51.66% increased respectively. Also with 1.22 times the delamination compared to the delamination of initial sample, tensile strength 6.5% decreased and stress concentration coefficient 6.9 increased.

In this paper, nanocomposites based on polyamide 6 (PA6)/acrylonitrile-butadiene rubber (NBR)/Perlite were prepared by melt mixing technique in an internal mixer. Response surface methodology (RSM) and central composite design (CCD) were used to study the influence of two material variables including perlite content and NBR content on tensile strength and impact strength of PA6/NBR/Perlite nanocomposites. The microstructure of nanocomposites samples was also examined to confirm the result obtained by scanning electron microscopy images. Based on the results obtained from the response surface methodology, when NBR phase content is 20%wt., with increasing perlite nanoparticles from 3% to 7% wt., the value of tensile strength increased by 12.9% and on the other hand, the value of impact strength decreased by 47.7%. Under optimal conditions of perlite content of 4.37 wt. and NBR content of 34.83 wt., the simultaneous maximization of the tensile strength (58.4 MPa) and impact strength (66.3 J/m) could be obtained. Observations of scanning electron microscopy images showed that the difference in mechanical results was due to the different sizes of the elastomeric phase in different compounds.

In this paper, a new category of lattice structures called auxetic structures has been studied, which due to low weight, high stiffness and shear strength, have various applications including energy absorption. The unique features of these lattice structures can be related to their special geometry and the Negative Poisson's ratio. In this study, three elastomeric auxetic structures made of TPU with geometries of Anti-tetra chiral, Arrowhead and Reentrant were investigated experimentally at quasi-static and impact loading and compared with a non-auxetic Honeycomb structure. The specimens were fabricated by additive manufacturing method and evaluated using the parameters such as the absorbed energy and the energy per unit length of compaction. Impact loading was performed at two level to investigate the energy absorption capability and deformation mechanisms of the structures in different level of loading. The results show that the auxetic structures absorb much more energy than non-auxetic conventional ones in quasi-static loading and absorb more energy per unit length compaction in impact loading.

Due to the sensitivity of the results in the delamination area of composite laminates, it is necessary to use accurate theories that include all components of stress to study this area. For this purpose, in the present study, the virtual crack closure technique based on layer wise theory has been developed to analyze the propagation of delamination in a composite beam. The strain energy release rate in mode I is determined based on the properties of the material and an algorithm is proposed to implement the method. The present method has been implemented numerically in MATLAB software on a 2D and 3D Double Cantilever Beam (DCB). In order to validate the method, it has been compared with the results of previous works based on Finite Element method. An analytical solution to the problem is also presented and compared with the results of the present work. The force-displacement behavior of a DCB composite beam is analyzed, which indicates the suitable capability of this method in the analysis of propagation of delamination, and the computations are reduced relative to the three-dimensional Finite Element.

In this paper, the mechanical properties of nanocomposites based on a matrix of two phases of polyamide 6 polymer and ethylene propylene diene monomer (EPDM) reinforced with carbon nanotubes are investigated. The compounds include 0%, 1%, 2% and 3% by weight of carbon nanotubes as well as 5 and 10% by weight of EPDM-G-MA as compatible and 10 and 20% by weight of EPDM prepared by an internal mixer. Samples were prepared for mechanical tests by a hot press machine. Mechanical tests were performed to determine the impact and tensile strengths, modulus of elasticity and elongation at failure. It was observed that adding 1% by weight of carbon nanotubes increases the impact strength by 12%, tensile strength by 25% and elastic modulus by 43%, but decreases the failure length by 13%. Also, with the presence of 10% EPDM, impact strength increased by 27% and elongation at fracture by 34%, but the tensile strength of polyamide decreased by 5% and its elastic modulus by 19%. To improve the mentioned cases, by adding 5% EPDM-G-MA, the impact strength increased by 14%, the tensile strength by 5%, the elastic modulus by 20% and the elongation at break by 13%. Finally, by adding carbon nanotubes and combining EPDM and EPDM-G-MA, all the mechanical properties mentioned in polyamide 6 were improved

Nowadays, low-velocity impact resistance and damage tolerance have become very important in the design of composite structures, especially in the aerospace industry, and have led researchers to evaluate the low velocity impact response in the design of composites. In this research, a numerical model for low velocity impact (LVI) of carbon /epoxy composite laminate is proposed. This 3D damage model is based on continuum damage mechanics with subroutine development (UMAT) in Abaqus/Explicit. In this model, the interlaminar and intralaminar damage is considered and the nonlinear shear behavior is applied to consider the plasticity in the matrix. To evaluate the results obtained from the numerical model, impact testing was performed at three levels of impact energy (10 J, 15 J and 20 J). In addition, in order to verify the failure mechanisms and the amount of damage caused by the impact, delamination area was measured using radiology technique. By comparing the response of the impact behavior, a good correlation was obtained between the experimental test results and the simulation, which shows that the failure model used is well able to predict the behavior of the composite in this process.

The authors regretted the typographical error and requested a change in the second author's academic rank from "Associate Professor" to "Assistant Professor", which was applied in this amendment. It should be noted that the Journal of Science and Technology of Composites has no responsibility for the subsequent consequences of this amendment, and this has been done based on the written request of the authors, and they are responsible for any subsequent problems.