Gholamhossein Liaghat

Quasi-static and dynamic compressive properties of ceramic microballoon filled syntactic foam

The compressive behaviour of epoxy based syntactic foams filled by ceramic microballoons is experimentally investigated in this study. Nine different types of syntactic foams are fabricated with three different microballoon sizes and three different microballoon fractions. All of the syntactic foam specimens are tested at various strain rates from quasi-static to high strain rates. Analysis of the results is carried out on the effect of the volume fraction, microballoon size and strain rate on the compressive behaviour of syntactic foams. Also, scanning electron microscopy is used to understand the fracture mechanisms of tested specimens. The results show that as the microballoon volume fraction increases the compressive strength, compressive modulus, failure strain and plateau stress decreases for all types of syntactic foams at all strain rates. Although, this decrease is slight for 20% and 40% volume fraction, it is considerable for 60% microballoon volume fraction syntactic foam. The results indicate that reducing the microballoon size or increasing the strain rate of testing would enhance the compressive strength.

Improving the fracture toughness properties of epoxy using graphene nanoplatelets at low filler content

Abstract This paper reports improvement in the fracture properties of epoxy nanocomposites using plasma functionalized graphene nanoplatelets (f-GNP) at low filler content. Various mechanical tests were performed on a series of f-GNP/epoxy at low nanofiller loading to assess the effect of the nanofiller on mechanical properties. Most importantly, a significant enhancement in fracture toughness is achieved without compromising the tensile and thermal properties of the nanocomposites. The fracture toughness of neat epoxy resin was increased by over 50% with the incorporation of 0.25 wt% f-GNP loading, obtaining a value of 245 J m−2, while the neat epoxy indicated a value of 162 J m−2. The glass transition temperature (Tg) and coefficient of thermal expansion (CTE) both showed a slight increase of 3% and 2%, respectively, both at 1 wt% f-GNP loading. These enhancements are competitive with current literature results on nanocomposites, but at significantly lower filler content. We therefore demonstrate that f-GNPs are capable of providing effective toughening of epoxy resins, while maintaining other tensile and thermal properties.

Investigation of the ballistic impact behavior of 2D woven glass/epoxy/nanoclay nanocomposites

In this paper, a theoretical model to investigate the ballistic impact behavior of two-dimensional woven glass/epoxy/nanoclay nanocomposites is presented, developed on the basis of dividing of the impact duration into several time intervals and calculation of the energy absorbed during each time interval. The major components of energy lost by projectile during ballistic impact were identified, namely the primary yarns tensile failure energy, the secondary yarns deformation energy, the cone kinetic energy formed on the back face of the target, the delamination of layers of nanocomposite and the matrix cracking. Ballistic tests were performed by a flat-ended projectile by a gas gun. Finally, a good correlation has been observed, comparing the theoretical model presented in this paper to the experimental results.

Compressive Properties of Nanoclay-Reinforced Syntactic Foams at Quasi-Static and High Strain Rate Loading

The compressive behaviors of nanoclay-reinforced syntactic foams filled by ceramic microballoons are investigated. Eighteen different types of nano syntactic foams are fabricated with three different microballoons volume fraction, each containing six different weight fraction of nanoclay in their matrix. Quasi-static tests are carried out using an Instron 5500 machine test and Split Hopkinson Pressure Bar (SHPB) apparatus is used to perform the high strain rate tests. Also, scanning electron microscopy (SEM) is used to understand the fracture mechanisms and the microstructure of the tested specimens.

Experimental studies of stiffness degradation and dissipated energy in glass fibre reinforced polymer composite under fatigue loading

In this work, tensile and compressive properties and fatigue performances of laminated glass fibre-reinforced polymer (GFRP) composite under constant amplitude sinusoidal load control at frequency of 5 Hz and at room temperature were investigated for three different types of loading: tension-tension at R=0.1 and 0.5, reverse loading tension-compression at R=-1 and compression-compression at R=2 and 10 in the fibre and normal-to-fibre directions. From these series of tests, the corresponding S-N diagrams were obtained. The dynamic stiffness during fatigue loading showed classical degradation of the GFRP laminates. It was observed that the dynamic modulus decreased with time, and the hysteresis loop area changed with some distortion according to the loading conditions. Finally hysteresis loops throughout fatigue testing were examined, and the variation of energy dissipated per cycle throughout the specimen lifetime was quantified. It was demonstrated that the dissipated energy during the fatigue lifetime is dependent on R-ratio and fibre orientation. However, in majority of the cases, the energy dissipated per cycle near the end of the fatigue lifetime increases as a result of an increase in the area captured by hysteresis loops.

Enhancement of the Electrical Conductivity and Interlaminar Shear Strength of CNT/GFRP Hierarchical Composite Using an Electrophoretic Deposition Technique

In this work, an electrophoretic deposition (EPD) technique has been used for deposition of carbon nanotubes (CNTs) on the surface of glass fiber textures (GTs) to increase the volume conductivity and the interlaminar shear strength (ILSS) of CNT/glass fiber-reinforced polymers (GFRPs) composites. Comprehensive experimental studies have been conducted to establish the influence of electric field strength, CNT concentration in EPD suspension, surface quality of GTs, and process duration on the quality of deposited CNT layers. CNT deposition increased remarkably when the surface of glass fibers was treated with coupling agents. Deposition of CNTs was optimized by measuring CNT’s deposition mass and process current density diagrams. The effect of optimum field strength on CNT deposition mass is around 8.5 times, and the effect of optimum suspension concentration on deposition rate is around 5.5 times. In the optimum experimental setting, the current density values of EPD were bounded between 0.5 and 1 mA/cm2. Based on the cumulative deposition diagram, it was found that the first three minutes of EPD is the effective deposition time. Applying optimized EPD in composite fabrication of treated GTs caused a drastic improvement on the order of 108 times in the volume conductivity of the nanocomposite laminate in comparison with simple GTs specimens. Optimized CNT deposition also enhanced the ILSS of hierarchical nanocomposites by 42%.

Analytical investigation of high-velocity impact on hybrid unidirectional/woven composite panels

In addition to fiber properties, the fabric structure plays an important role in determining ballistic performance of composite body armor textile. Textile structures used in ballistic protection are woven fabrics, unidirectional (UD) fabric structures, and nonwoven fabrics. In this article, an analytical model based on wave propagation and energy balance between the projectile and the target is developed to analyze hybrid fabric panels for ballistic protection. The hybrid panel consists of two types of structure: woven fabrics as the front layers and UD material as the rear layers. The model considers different cross sections of surface of the target in the woven and UD fabric of the hybrid panel. Also the model takes into account possible shear failure by using shear strength together with maximum tensile strain as the failure criteria. Reflections of deformation waves at interface between the layers and also the crimp of the yarn are modeled in the woven part of the hybrid panel. The results show greater efficiency of woven fibers in front layers (more shear resistance) and UD yarns in the rear layers (more tensile resistance), leading to better ballistic performance. Also modeling the yarn crimp results in more trauma at the backface of the panel producing data closer to the experimental results. It was found that there is an optimum ratio of woven to UD materials in the hybrid ballistic panel.

Experimental and numerical analysis of impact on curved nanocomposite panels

ABSTRACT In this work nanopolyurea is used as reinforcement for curved steel plates under low velocity impact loading. Experimental and numerical analysis are used to study the effect of plate curvature and nano particle effects. A drop weight test apparatus is utilized to apply impact loading on bi-layer panels. Experiments show that maximum improvement in energy absorption capacity of nanopolyurea is about 5%. Numerical analysis is carried out using explicit LS-DYNA solver. The results show that by increasing panel radius of curvature, impact force increases; also comparison between numerical and experimental results shows a good agreement.

Experimental and numerical analysis of penetration into Kevlar fabric impregnated with shear thickening fluid

This study presents the high-velocity impact performance of a composite material composed of woven Kevlar fabric impregnated with a colloidal shear thickening fluids (STFs). Although the precise role of the STF in the high-velocity defeat, process is not exactly known but it is suspected to be due to the increased frictional interaction between yarns in impregnated fabrics. In order to explore the mechanism of this enhanced energy absorption, high-velocity impact test was conducted on neat, impregnated fabric and also on pure STF without fabric. A finite element model has been carried out to consider the effect of STF impregnation on the ballistic performance. For this purpose, fabric was modeled using LS-DYNA by employing the experimental results of yarn pull-out tests to characterize the frictional behavior of the STF impregnated fabric. The simulation result is a proof that the increased performance for STF impregnated Kevlar fabric is due to the increased friction.

Experimental study on quasi-static penetration process of cylindrical indenters with different nose shapes into the hybrid composite panels

The main aim of the present research is to investigate the quasi-static penetration process of cylindrical indenters with different nose shapes into multilayered composite panels made of Dyneema and Glass woven fibers, and aluminum face sheets. For better understanding of the perforation mechanism of the composite panels, effects of indenter geometry, stacking sequences, and boundary conditions are studied and their effects on energy absorption, specific absorbed energy, maximum deformation, peak load, and failure modes are evaluated and discussed. Samples with different layering configurations loaded under quasi-static punch and indentation with loading rate of 5 mm/min using universal testing machine and cylindrical rigid indenters with different nose shapes geometries consist of blunt, hemispherical, conical, and ogival. Regarding the boundary condition effects, two different rigid fixtures are designed and manufactured with the same external square perimeter (250 × 250 mm) and two different internal perimeters of circular and square shapes respectively, with diameter of 15 mm and edge side of 100 mm. Results show that the hybrid composite panels composed of Dyneema sheets, exhibits significantly better load carrying capacity and specific absorbed energy under both loading conditions. Indenter nose shape significantly affects elastic load, peak load, and energy absorption and maximum deformation. Furthermore, from visual observations based on digital microscopic images, fiber breakage, fiber pull out, intralaminar delamination, and debonding between the composite layers within the damage zone were inspected and recognized as the main damage mechanisms of panels. Output data obtained from all the experimental investigation were reported, discussed, and commented upon.