Search

This study investigates the effect of viscoelastic properties in the numerical modeling of foam-filled sandwich panels when their composite facesheets are exposed to low-velocity impact loading. A finite element (FE) model is developed using the software package of ABAQUS and used to investigate the effect of both considering and ignoring the time-dependent behavior of composite laminate facesheets as well as the foam core. Material constitutive equations, damage characteristics, and failure modes are defined in a FORTRAN user-subroutine VUMAT. To characterize the material behavior of both polyurethane foam and glass/epoxy composite material, a set of experimental tests are conducted under compression, tension, and stress relaxation modes. To validate the numerical results, low-velocity impact tests at two energy levels of 9.81 and 17 joules are carried out on sandwich panels with foam core. The results of numerical simulations are found to be in good agreement with the experimental test results. It is shown that ignoring the viscoelastic properties in the composite laminate and the foam core can lead to deviations of up to 7% and 25%, respectively, from experimental results. The analysis reveals that, ignoring the viscoelastic behavior along the fiber-direction in the composite facesheets does no change the results considerably. The viscoelastic properties perpendicular to the fibers, however, has a more noticeable effect on the results due to the prevalence of the properties of the polymeric resin in that direction.

Road safety barriers prevent cars from deviating from the roads. Guardrails as common roadside safety barriers protect cars from dangers alongside the roads, such as steep slopes and trees, and redirect traffic from road hazards. Conventional guardrails are generally made of metal or concrete. Although steel guardrails absorb a high amount of crash energy by deformation, these types of guardrails have short lifecycles of less than 15 years. Due to their unique properties, composite materials are potential candidates to use as guardrails. The present study considers the three types of guardrails (steel, composite, and fiber metal laminate (FML)), and the crash model is developed according to standards. LS-DYNA software is employed to simulate and conduct the crash test In the first stage, impact simulation is performed on a carbon/epoxy composite plate. In the next step, impact simulation is performed on a metal fiber multilayer (FML) plate with carbon fiber and aluminum metal. Then, the crash test on the steel guardrail is completely simulated. These three stages are performed to verify the proposed model. In the next stage, the crash test for a carbon/epoxy composite guardrail is simulated, and the effect of different parameters is investigated. Finally, a crash test on the FML guardrail with carbon fibers and steel metal is simulated. Two FML samples with [CE/Steel/CE] layups are considered. The absorbed energy, specific energy, and acceleration severity index (ASI) for all different guardrails are obtained and compared to determine the optimum guardrail regarding energy absorption, mass, and ASI parameters. Results reveal that sample FML-B guardrail is the best of the three guardrail types, with the highest specific energy absorption and lower ASI parameter.

Implementation of natural fibers like kenaf in composite laminates has been the focus of many researchers. The considerable price of Low-Velocity Impact (LVI) testing, as well as the time constraints imposed by the short duration of this experiment, and prior reports of the obedience of Quasi-Static Indentation (QSI) test findings to LVI test results, have encouraged researchers to employ QSI instead of LVI. This work compares the mechanical characteristics of composite laminates consisting of chopped glass and kenaf fibers as a core and investigates the effect of hybridizing the mentioned chopped fibers as the core of a laminate. Three types of laminates with different core materials were fabricated to do this research. The results showed that among the laminates with the same weight, kenaf core laminate has the highest resistance against indentation and has the best performance in terms of absorbed energy, peak load, and damaged region. Using chopped kenaf fiber instead of chopped glass fiber can increase the absorbed energy and the peak load by 36% and 17%, respectively. Also, hybridizing the mentioned chopped fibers has not fully maximized the properties of the composite laminate during the QSI test. Finally, finite element modelling of the laminates was performed, and the numerical and experimental results were pretty aligned.

To harvest energy from environment vibration, it is a good way to use piezoelectric material as a generator that converts mechanical strain to electrical charge. This paper aims to develop a numeri...

This paper presents an experimental and numerical study of the low-velocity impact on foam core sandwich panels reinforced using hybrid short fibers. The foam cores were reinforced with carbon, aramid and carbon-aramid hybrid short fibers. The face-sheets were made of two layers of glass/epoxy, and foam cores were made of two-part polyurethane. In order to acquire the appropriate weight ratio between foam and short fibers, the weight percentage of 10% was chosen for short fibers. Comparing the experimental results proved that carbon, aramid, and carbon-aramid respectively had a better effect on increasing Young modulus by around 100 to 180 per cent. Before performing impact tests, indentation tests were conducted and based on the results for the parameter of impact energy, the value of 6 J was chosen. According to the results of impact tests and the maximum contact force, hybrid reinforced foam, aramid short fiber reinforced foam and carbon short fiber reinforced foam improved the properties respectively by 18 to 30 per cent in comparison to non-reinforced foam. Furthermore, numerical simulations were conducted via ABAQUS. After modeling face-sheet and foam separately, and verifying the results with experiments, the sandwich panel was modeled entirely while the simulation difference of 9.1% on average with the experiment results was concluded.

Topologically interlocked materials (TIMs) are an emerging class of architectured materials that each of block is supported by their neighboring blocks without any connector, which is inspired by m...

The application of composites in various industries has grown considerably. The relatively high cost of Low-Velocity Impact (LVI) tests and the limitation in data collection due to the short time o...

Considering failures during machinery processes such as drilling, a precautionary analysis involving delamination and the corresponding dissipated energy is required, especially for composite structures. In this context, because of the complexity of both the analysis procedure and experimental test setup, most studies prefer to represent mode I and III interlaminar crack propagation instead of that involving mode II. Therefore, in this study, the effect of mode II delamination and corresponding interlaminar crack propagation was considered during the drilling process of multilayered glass/polyester composites using both numerical and experimental approaches. In the experimental procedure, the mechanical properties of the glass/polyester specimens were obtained according to ASTM D3039. In addition, the interlaminar mixed-mode (I/II) loadings were determined using an ARCAN test fixture so that the fracture toughness of glass/polyester could then be identified. The mode II critical strain energy release rate (CSERR) was then obtained using an experimental test performed using an ARCAN fixture and the virtual crack closure technique (VCCT). It was determined that the numerical approach was in accordance with the experiments, and more than 95% of crack propagation could be attributed to mode II compared to the two other modes.

Honeycomb sandwich panels are widely used in marine, aerospace, automotive and shipbuilding industries. High strength to weight and excellent energy absorption are features that make these structures unique. Foam filling the honeycomb core enhances the mechanical properties of sandwich panels considerably. In the present study, the buckling behavior of Nomex honeycomb core/glass-epoxy face sheet sandwich panel for both bare and foam-filled honeycomb core is investigated numerically and experimentally, considering the viscoelastic properties of the sandwich panel. Indeed, the viscoelastic properties of the composite face sheet and foam are determined by relaxation test and are implemented in ABAQUS using VUmat code. The finite element method is also performed using ABAQUS to model the buckling behavior of the sandwich panel incorporating both elastic and viscoelastic material behaviour. The effects of composite face sheet lay-up, core thickness, core cell size, and foam filling are also evaluated. The experimental and numerical results show that the foam increases the critical buckling load and energy absorption.

Nanocomposites including engineering materials that absorbed a great deal of scientist’s attention due to their excellent properties such as high strength and corrosion resistance. Optimization of development process and careful application of reinforcing nanoparticles are some approaches to improve mechanical properties. Thus, stir casting is considered as a dispersion method of reinforcement in melt. According to the properties of carbon-based particles, graphene nanoplates and carbon nanotubes were used as reinforcing particles in three different percentages including 0.01, 0.05, and 0.1 wt% in the initial alloy A356. Optimum conditions were determined by graphite stirrer with rotational speed of 500 rpm, in a continuous mode at 740 °C for 1 min. Elemental and phase analysis confirmed formation and distribution of reinforcing nanoparticles in a composite matrix. Tensile and compression strengths raised as a result of increment in reinforcement amounts. So that maximum increases of 28% and 55% were respectively reported for tensile and compression strengths using 0.1 wt% of graphene, compared with the sample containing minimum amount of reinforcement. Also 0.1 wt% carbon nanotube led to increase in nanocomposite hardness up to 88.4, indicating an improvement of 33%, compared with the reinforcement-free alloy. Increase of impact energy by 51% in the presence of 0.1 wt% carbon nanotubes has been the maximum enhancement in impact energy in the Charpy impact test, compared with the minimum amount of application in the same reinforcement.

An appropriate finite element model is developed to simulate the high velocity impact of projectiles on polyurethane foam filled aluminum honeycomb. First, the numerical model of the high velocity ...

Due to the high strength to weight ratio of composites, their applications are extending rapidly in many industries. Composites can be fabricated by using different constituent (including fiber and...

In this study, numerical modeling is used to investigate the performance of a single-sided composite patch with different scale fillers, as reinforcement of a cracked aluminum plate under static tension. The main concerns of previous studies are about the geometry of patches, composite layups, and failure of adhesive. In this research, the effect of patch properties such as size and fiber volume fraction, the thickness of patch, and thickness of adhesive on the overall performance of the cracked aluminum plate are investigated numerically. Indeed, first, a 3 D representative volume element (RVE) is adopted to calculate the mechanical properties of carbon nanotube (CNT)/epoxy and carbon fiber (CF)/epoxy composite patch at each specified volume fraction for investigating the effect of patch properties on the performance of a single-sided patch for crack repairing. In this regard, the cohesive zone model is adopted to analyze the debonding between the epoxy matrix and reinforcement to characterize the mechanical properties of composite patches. Finally, a linear 3 D finite element analysis is performed to calculate the stress intensity factor (SIF) for cracked aluminum plate repaired by a single-sided composite patch at each specified reinforcement volume fraction for different thickness of patch and different thickness of adhesive. The results demonstrated that the stress intensity factor highly depends on the patch properties (patch stiffness) in addition to patch thickness and adhesive thickness.

The effect of adding carbon nanofibers (CNFs) on thermal residual stresses in polymer nanocomposites was conducted semi-analytically considering aligned and random distribution of CNFs. The Halpin-...

In this study, the fatigue behavior of composite reinforced cracked aluminum 1050 plates is investigated experimentally and numerically. The tests are conducted in four different stress ratios betw...

Delamination is one of the most common defects caused by drilling, which can have negative effect on the joint performance. This study investigates the effect of delamination on the bearing strength of [0/90]2s, [15/−75]2s, [30/−60]2s and [45/−45]2s GFRP layers numerically. Cohesive zone method and virtual crack closure technique have been used for delamination modeling and the results of these two methods have been compared. FEM results show good agreement with available experimental data. Results demonstrated that delamination reduces the bearing strength. Among four different stacking sequences, delamination has the most effect on the laminate with the stacking sequence of [0/90]2s. In both delaminated and non-delaminated models, [0/90]2s and [45/−45]2s stacking sequences have the most and the least bearing strength, respectively. By increasing the radius of delaminated zone from 3 mm to 15 mm, bearing strength does not change a lot. As the delaminated zone reaches the edge of the specimen, bearing strength reduces strongly because the layers separate completely and the load-carrying capacity reduces. A parametric study was also conducted to examine the effects of different factors. The results of parametric study showed that by increasing the volume fraction of the fiber as well as the use of carbon fiber instead of glass fiber, the bearing strength increases.

Recently, an increasing interest has been emerged on nontraditional drilling methods of FRP materials. This paper aims to study two critical quality criteria pertaining to both surface quality and mechanical properties of laser drilled GFRP laminates. The effect of major laser drilling parameters was investigated on the HAZ and bearing strength through utilizing the combined GA-ANN approach. At first, the experimental design was performed based on a full factorial design with different process parameters, later on the experimental results were used to develop a GA-ANN model. The GA was integrated to the ANN model to determine the optimal ANN architecture through optimizing the number of hidden layer neurons, momentum coefficient and learning rate. The effectiveness of the models was ultimately evaluated on the basis of experimental data and statistical analysis. It was found that the GA-ANN method could predict the HAZ and bearing strength more accurately than trial-and-error ANN. Furthermore, the SEM micrographs from the contact area of the bearing tests were discussed and compared with mechanical drilling method. The results drawn from this study helps to better understanding of the laser drilling process of FRP materials and for predicting the mechanical properties of laser drilled FRP laminates applicable in aircraft part manufacturing.

This chapter presents a complete literature review of thermal residual stresses from different points of view such as their formation and effects as well as methods of measurement and prediction of them. In order to understand the effects of residual stresses, the main factors for the residual stress build-up must be understood. Consequently, the first part of this chapter introduces the important parameters for residual stress occurrence. The second part of this chapter investigates the influence of residual stresses on composites as well as the material characteristics. In the following, experimental methods for the prediction of residual stresses are presented, including destructive and nondestructive. Finally, analytical and numerical methods for the prediction of thermal residual stresses are studied. In the conclusion section, a comparison of experimental and theoretical methods is made in the last section of this chapter.

The present paper aims to study different laser drilling/cutting parameters including laser intensity, cutting speed, and gas pressure in order to achieve minimum surface roughness (Ra), heat-affected zone (HAZ), taper angle (TA), and maximum tensile strength (TS) of the laser-drilled glass fiber-reinforced plastic (GFRP) laminate. Full factorial design of experiment method and analysis of variance were adopted to investigate the effect of each parameter on the responses, and in addition, the surface roughness and tensile strength were compared with that of conventional drilling. The morphology of the laser-drilled holes was studied through the scanning electron microscopy (SEM). It was found that optimum laser drilling parameters can yield higher tensile strength and lower surface roughness in drilled GFRP laminate compared to the conventional drilling. Moreover, multi-response optimization was carried out on the results in order to obtain maximum tensile strength, minimum HAZ, minimum surface roughness, and minimum taper angle. The results revealed that the laser intensity, cutting speed, and assist gas pressure should be set at 2.04 W/cm2, 8 mm/s, and 4 bar, respectively, to obtain higher tensile strength and surface integrity. The results drawn from this study can be applied to improve the surface quality and mechanical properties of laser drilling of FRP materials especially in aircraft part manufacturing.

In this paper, a numerical model is developed to simulate the ballistic impact of a projectile on a sandwich panel with honeycomb core and composite skin. To this end, a suitable material model for the aluminum honeycomb core is used taking the strain-rate dependent properties into account. To validate the ballistic impact of the projectile on the honeycomb core, numerical results are compared with the experimental results available in literature and ballistic limit velocities are predicted with good accuracy. Moreover, to achieve composite skin material model, a VUMAT subroutine including damage initiation based on Hashin’s seven failure criteria and damage evolution based on MLT approach modulus degradation is used. To validate the composite material model VUMAT subroutine, the ballistic limit velocities of the projectile impact on the composite laminates are predicted similar to the numerical results presented by other researchers. Next, the numerical model of the sandwich panel ballistic impact at different velocities is compared with the available experimental results in literature, and energy absorption capacity of the sandwich panel is predicted accurately. In addition, the numerical model simulated the sandwich panel damage mechanisms in different stages similar to empirical observations. Also, the composite skin damages are investigated based on different criteria damage contours.

Polymer-matrix composites have a variety of uses in industry. However, their failure behavior is not well characterized yet. Specially, the bearing response of mechanical composite joints is very complex and needs further study. One of the important parameters influencing the bearing strength is loading speed that has not been investigated thoroughly and lacks sufficient data. In this research, effect of loading speed on bearing strength has been investigated empirically on glass-epoxy composites. Samples have been tested in different loading speeds in batches of five. The finite element model has been developed for this experiment in Abaqus . Considering that there is no specific amount to show the mechanical behavior of joint, using statistical analysis is essential. Empirical results based on ASTM D 5961 Standard have been fitted to Weibull distribution . The effect of loading rate on the probability of failure and survival have been analyzed. The results demonstrate that by increasing the loading rate, survival reliability and bearing strength increases, absorbed energy increased to a specific loading rate, then decreases and also the sigma of statistical charts is increases. In comparison between experimental results and finite element result , by increasing the loading speed the error of FE result will increase.

In this paper, the uniaxial load capacity of two coaxial steel tubes joined by glass fiber reinforced plastic (GFRP) was investigated. The axial connection was made by hoop winding of textile woven fabrics on circular hollow steel tubes. A simple efficient theory was selected to estimate the load capacity of sleeve joint. To define novel parameters in load capacity and failure, tensile tests were conducted on several specimens. Based on fracture mechanics experimental observations showed that three kinds of failure modes occurred including GFRP rupture, adhesive failure and mixed mode. The wetted cross-ply textile of woven glass was used in order to study the effect of the number of plies. One novel parameter named lamination cohesion rate was introduced to explore the GFRP rupture. The results revealed that increasing the number of plies leads to maximize the allowable axial tensile load. A second novel parameter was introduced for Adhesive failure. By employing more plies, fracture mode changes from GFRP rupture to adhesive failure resulting in a considerable increase in the load capacity. However, in this fracture mode, by increasing the number of plies the joint average shear stress increases but the maximum shear stress approximated from theories decreases. In the case of load capacity, a good agreement was found between theoretical and experimental results. By using this joining method, steel tubes can be toughly connected and withstand stresses up to steel ultimate tensile stress.

This study attempts to analyze the behavior of different types of woven composite preforms (i.e. plain, twill, and satin) under distinct loading conditions. To this end, fabric elements are modeled...

Nowadays, composite pipes, due to their unique properties such as high stiffness to weight ratio and excellent corrosion resistance, play a significant role in the industry. Composite pipes are usually loaded by internal hydrostatic pressure for a long time. Therefore, their long-term creep behavior has been considered as a severe issue. Although the experimental method is a reliable and standard studying method, it is costly and time-consuming, especially in creep studies. Therefore, researchers have been looking for alternative methods. In this regard, the primary purpose of this study is to model the long-term creep behavior of composite pipes considering moisture and impurities, such as short fibers and sand layers (which are commonly used in composite pipes). In this research, a comprehensive approach is provided, relying on available short-term creep experimental data on the pure resin, to predict the long-term behavior of each type of composite, particularly composite pipes. By employing Python script, the high-level algorithm is developed to model the creep behavior of lamina by using the micromechanics model considering the impurity. The effect of stress on the viscoelastic behavior of lamina is modeled. Then creep behavior of a lamina is developed to composite laminates by employing a finite element model. Finally, the failure in the laminated composite is examined. The results predicted by the present study have a good agreement with available experimental data. Results reveal that moisture reduces the lifetime of composite pipes. Furthermore, results are extended for 50 years, and the failure pressure is estimated, which the percent error is 2.6% approximately compared to available experimental data.

Generally, in thin laminates, residual stresses are studied in two scales; micro mechanics and macro-mechanics. So far, no study has included both the micro- and macro-scale analysis for predicting the thermal residual stresses of composite laminates. In other words, no literature model is proper for both the micro and macro scale at the same time. This paper presents a procedure to integrate micro and macro thermal residual stresses in composite laminates. The combination of micromechanics-based analytical results and hole-drilling calibration factors leads to predict the macro thermal residual stresses of a unidirectional laminate. Classical lamination theory (CLT) is then employed to predict macroscopic thermal residual stress of each layer in laminated composites (with different orientations). Comparison of longitudinal, transverse and in-plane shear stresses between the proposed approach and published experimental results is made for glass/epoxy laminates in cross-ply and quasi-isotropic configurations. Available experimental data from incremental hole-drilling method show that the present combination procedure yields more precise residual stresses predictions in comparison with the state that the CLT is used only. In other words, the presented procedure which adds the micro residual scale results to the CLT, gives rise to a better prediction of residual stress fields. Consequently, ignoring the micro-mechanical effects may result in some uncertainties in thermal residual stress evaluation of thin composite laminates. Furthermore, while the CLT is not able to take into account the in-plane shear residual stresses for the cross-ply composites, the present procedure can provide an estimation of shear residual stress field.

The vibration behavior of the rotating viscoelastic nanobeam embedded in the visco-Pasternak foundation is studied. The governing equation is extracted by using the surface elasticity and the nonlocal elasticity theory. The influence of the humidity on the vibration frequencies of the viscoelastic nanobeam is investigated in the thermal environment. The effects of the linear and the nonlinear thermal stress cases on the vibration frequencies of the viscoelastic nanobeam are studied. The vibration frequencies are obtained based on the differential quadrature method. Also, the numerical results are compared with those which are reported in the literature, and a good correlation is obtained. The results are presented for the different boundary conditions as well as the effect of the torsion spring on the viscoelastic nanobeam ends is investigated. This study focuses on the combined effects of the angular velocity, the internal and external damping, the humidity change, the temperature change, the surface effects (surface density, surface elasticity and surface stress), the nonlocal parameter, the boundary conditions, the visco-Pasternak foundation, the cross section geometry, and the torsion spring. The results of this study could be used to design and manufacture nanosensors, biosensors, atomic force microscope and the NEMS/MEMS devices.

In the present research, the effects of using steel wires on energy absorption of polyurethane foam core glass/epoxy skin sandwich panels have been investigated experimentally and numerically. Panels with different layouts of stainless steels between layers of upper skin were subjected to low-velocity impact tests with hemispherical steel impactor. The histories of energy absorption of reinforced and non-reinforced panels at the peak load of upper skin, and contact force and penetration of projectile during the impact test are obtained by experiment. Moreover, A Finite Element Analysis is performed using Abaqus/Explicit package. There is a good agreement between numerical and experimental results. The polyurethane foam core was modeled using VUMAT Subroutine and Ductile Criteria used for the evaluation of stainless steel wires. Besides, the impact behavior of these reinforced specimens compared by non-reinforced specimen, and “Star layout” specimen is observed to be more effective in enhancing the low-velocity impact behavior of sandwich panels compared with others. This layout can enhance the energy absorption of sandwich panels more than twice the non-reinforced samples by only adding 10% of the weight of pure specimens. Consequently, the stainless steel wire reinforcement improves the impact resistance of these sandwich panels and could be cost-effective.

An experimental and semi-analytical study of distortion of asymmetric composite laminates with different cooling rates and lay-ups has been presented. In this study, thermomechanical constitutive equations of thin composite laminates are developed using basic viscoelastic constitutive law considering chemical and thermal effects with time-temperature dependent material properties. To solve a fully scouple problem, both the thermochemical and thermomechanical constitutive equations are formulated. The general heat conduction equation known as the Fourier-Biot equation, viscoelastic laws, Boltzmann superposition principle and composite equations are utilized to formulate thin composite laminates. A static model with constant properties in ambient temperature and a transient model by obtaining constitutive equations are simulated. Results are compared with experimental data. Changing lay-up from cross-ply to angle-ply and then quasi isotropic will increase the value of maximum distortion. Results indicated that the increasing cooling rate will increase the value of the maximum distortion. The differences between FEM results with static analysis of different lay-ups and experimental specimens that cooled in the oven, environment and refrigerator is about 3%, 35% and 55% respectively. The differences between FEM simulation with transient analysis of different lay-ups and experimental specimens that cooled in the environment and the refrigerator is less than 9%.

Predicting the final shape of composite laminates after the curing cycles is crucial. An experimental and two numerical (with constant and temperature dependent material properties) studies have been presented. The purpose of this study is to investigate the effects of different cooling rates on the distortion of thin asymmetric composite laminates with different lay-ups after the curing cycle. Properties of a UD layer in different temperature have been obtained using TMA, DMA and micromechanical formulation. The samples were subjected to three different cooling stages. Results indicated that the distortion of the asymmetric composite laminate was significantly depending on the cooling rate and lay-up configurations. Accelerating the cooling rate can significantly enhance the out of plane deformation of the asymmetric laminates. In the slowest cooling rate, the effects of thermal residual stresses due to temperature change are minimum and consequently the results of both numerical analyses are closer to experimental observations. The analysis with temperature dependent material can predict the deformation of specimens with slowest cooling rate better than the analysis with constant material properties. Because of the sudden change in temperature, for prediction of distortion after the higher cooling rates , a transient analysis will be needed.

Article history: Received October 6, 2014 Accepted 2 February 2015 Available online 2 February 2015 In this research, the influences of modal properties of a micro cubic satellite on equivalent static loads (due to combination of quasi-static and dynamic loads during launch time) have been studied. The study shows that the magnitude of equivalent static loads can be affected by satellite effective modes and effective mass distribution along natural frequencies. Besides, when to distinct launchers with different dynamic environments candidate for launch, analytical results illustrate that sometimes the equivalent static combined load value can be higher for the launcher with smaller quasi-static loads. This phenomenon is due to effective modal mass distribution of the satellite. Consequently, a higher combined load may be occurred during launch for smaller quasi-static conditions. Furthermore, in separation phase of satellite from the launcher, the satellite modal parameters influence the magnitude of applied shock load in both axial (launch) and lateral directions, importantly. Thus the present study yields reliable input values for separation shock test which considers dynamic properties of the satellite in comparison with some standards and launcher manuals, suggest a rough estimation of shock load, neglecting the satellite structure dynamic behavior. © 2015 Growing Science Ltd. All rights reserved.

Smart materials are being utilized in many fields and different external stimuli are used to change specific properties of these materials. In this research, a novel method was developed to design a structure with the desired nonlinear effective Young’s modulus. This method is geometric based where the structures are designed with a gap between them. These structures exhibit nonlinear elastic response. Wide range of structures with desired stress–strain curve can be generated using this approach. First, a unit cell was designed and later used to create a periodic structure. Numerical simulations have been exploited to prove the efficiency of the method. A prototype was manufactured by the Fused Deposition Modeling (FDM) 3D printing method. The compression test was performed on the structure. Both simulations and experimental results proved that the effective Young’s modulus of the structure can be increased up to 142%. Second, the designed unit cell was optimized using Genetic Algorithm (GA) to achieve a cell with desired nonlinear stress–strain curve. This cell was optimized considering five effective geometric parameters to alter the effective Young’s modulus of the cell. Finally, a periodic structure was created by repeating a cell with two different gap’s distances. A structure with a desired stress–strain curve was designed using the same method.

This paper investigates the effects of hybrid interphase region on the calibration factors of the hole-drilling method for orthotropic composites. For this purpose, a three-phase composite has been considered which includes fiber, matrix as well as the interphase between them. By employing the available analytical predictions for elastic properties of the interphase and based on micromechanical equations, the mechanical properties of the three-phase orthotropic composite can be obtained. These properties are applied to the available exact solution in order to determine the calibration factors for the three-phase orthotropic plate. Four different composites have been considered in order to study the interphase influences on the calibration factors matrix. Analytical results show that for carbon/epoxy composite, bonding conditions affect all of the calibration factors importantly, while for boron/epoxy, glass/epoxy, and aramid/epoxy composites, some of these factors are not sensitive to interphase thickness. Sometimes, bonding conditions change some of the calibration factors considerably between 30 and 50%. Consequently, when a central hole-drilling experiment is performed in an orthotropic layer, the precision of residual stresses measurement is identically dependent on the elastic properties and the thickness of the hybrid interphase.

In this paper, the effects of carbon nanofiber (CNF) additives with random and oriented distribution, in different weight fractions (0.1%, 0.5%, 1% and 2%) on thermo-mechanical properties and residual stresses of polymeric nanocomposites is investigated numerically. For distribution of CNFs in the models, a Python script was used to develop models in conjunction with ABAQUS finite element analysis (FEA) software. Young's modulus and coefficient of thermal expansion (CTE) were acquired for random distribution and aligned CNFs, and they were compared with experimental and analytical results, which were well correlated. Residual stresses were compared for carbon fiber (CF)/CNF/epoxy and glass fiber (GF)/CNF/epoxy nanocomposites for different lay-ups using classical lamination theory (CLT) for three cases of random, longitudinal and transverse aligned distribution of CNFs. Using 2 wt.% of CNFs in unsymmetrical nanocomposites, transverse aligned CNFs caused 15% (using carbon fiber) and 29% (using glass fiber) further reduction of the residual stresses in comparison with random distribution of CNFs. For symmetric CF/CNF/epoxy nanocomposites, transverse aligned CNFs have no advantage in reducing the residual stresses compared to a random distribution, but in GF/CNF/epoxy nanocomposites, the reduction of residual stresses contain 2 wt.% transverse aligned CNFs is 17% higher than their random distribution.

Article history: Received January 20, 2014 Received in Revised form April, 10, 2014 Accepted 27 April 2014 Available online 29 April 2014 Nano composite materials based on polymers are widely used in restoration martials like dental. Poly methyl methacrylate (PMMA) is one of the most used polymers as dental material. PMMA has disadvantages such as low flexural strength properties and impact strength. In this paper, the influences of additive aluminum oxide and hydroxyapatite nanoparticles on the mechanical and strength properties of PMMA (including flexural strength, impact strength, surface hardness and shrinkage behavior) is studied experimentally. For this purpose, nine standard mechanical testing samples of pure PMMA, PMMA/5HA and PMMA/10HA with various amounts of Nano aluminum oxide (3,6,8 wt.%) were prepared. The results showed that the mechanical properties of hybrid Nano-composites were significantly improved in comparison with the pure samples in a way that optimal Nano-composite with convenience flexural properties and impact strength is obtained. Moreover, the use of nanoparticles results in shrinkage reduction of hybrid Nano-composites in comparison with pure PMMA.

Based on the energy method and the principle of minimum total complementary potential energy, a new three-dimensional analytical model is developed. A three-dimensional analysis is performed for a unit-cell representative volume element (RVE) consisting of a monofiber, a matrix, and an interphase region. The model is evaluated using the finite-element technique. In order to study the effects of neighboring fibers on the residual stresses, several microstructural finite-element models are considered. It is proved that the RVE used in the analytical model is suitable for predicting the thermal microresidual stresses in polymer composites. In comparison with the analytical model, the finite-element analysis presents a little higher residual stresses. According to the results of the analytical model, the interfacial shear stress reaches a maximum in the vicinity of fiber ends, while the finite-element analysis gives a maximum shear stress at composite ends. This is due to the edge singularity, which is not considered in the finite-element model. Although the interphase region affects the distribution of interfacial radial stresses only slightly, it has a significant effect on the maximum interfacial radial stresses at fiber ends.

In polymer composites, thermal residual stresses arise due to the mismatch of the thermal expansion coefficients of the fiber and matrix. Polymer composites generally consist of three phases: fiber, matrix, and the interphase. In this article, to study the effects of the interphase on residual stresses, a three-dimensional energy-based closed-form solution is developed. Residual stresses are assumed variable along longitudinal and radial directions, and two Airy stress functions are introduced. By considering the interphase region, in comparison with the perfect bonding conditions, thermal residual stresses decrease. The longitudinal residual stress distribution is sensitive to the interphase thickness. The maximum axial stress occurs at the fiber mid-length and decreases by increasing the interphase thickness. The maximum fiber–interphase and interphase–matrix interfacial shear stresses vary by increasing the interphase thickness, whereas their position is approximately unchanged. An increase in the interphase thickness leads to a decrease of the shear stress concentration factor near the fiber ends. For different magnitudes of bonding efficiency, the interfacial radial stresses are zero at the maximum fiber length. Although, the interphase region has a minor effect on the interfacial radial stresses, it has a significant effect on the maximum interfacial radial stresses at the fiber ends.

This paper investigates the influences of imperfect bonding between the fiber and matrix on thermal micro-residual stress fields in polymer matrix composites. For this purpose, a representative volume element consisting of a three-phase composite material subjected to a uniform temperature change is considered. Based on the energy method, a three-dimensional closed-form solution for micro-residual stresses is obtained. Besides, a finite element model is developed and the results are compared with the analytical solution. Both the energy method and finite element analysis show similar trend for thermal stress distribution along the fiber length, while due to the stress singularity, the interfacial shear stress from the finite element solution cannot satisfy the stress-free condition at the fiber end. The analysis shows that the magnitude of thermal stresses and their distribution mainly depend on the bonding efficiency parameter. An increase in thermal and elastic properties bonding efficiencies leads to a considerable decrease in composite axial and shear residual stresses, while the Poisson's ratio bonding efficiency does not affect the thermal stress fields. The interfacial radial residual stress distribution is approximately independent of the bonding conditions. Inefficient bonding may result in higher residual stresses in comparison with the perfect bonding condition. It means that in cases of low bonding efficiency conditions, the ability of composites to sustain and transmit load decreases drastically. Thermal stress concentration occurs at the vicinity of the fiber ends, although peak values depend on the bonding efficiency value.

In this paper, the influence of transversely isotropic behaviour of the fibre on micro-residual stress fields is investigated. For this purpose, the energy method is utilized to predict the micro-structural stresses in polymer matrix composites under uniform thermal loads. The representative volume element (RVE) considered here includes a transversely isotropic fibre embedded in an isotropic polymer matrix. Based on the energy method, a three-dimensional closed-form solution for micro-residual stresses is obtained. The analytical results are evaluated with a finite element solution. The analysis shows that the transversely isotropic behaviour of the fibre greatly increases the fibre and matrix axial residual stresses in comparison with the isotropic fibre assumption. Fibre anisotropy has a significant effect on the interfacial shear stress distribution, in contrast with the interfacial radial stress field. Both the finite element solution and analytical method result in a similar behaviour for micro-residual stresses distribution along the fibre length. However, the finite element method cannot satisfy the stress-free condition for the residual shear stress at the fibre end due to the stress singularity.