Journal of Composite Materials recent issues

Development, Dielectric, and Thermal Studies on HAF Carbon-filled Polyester Gradient Composites

Chand, N., Nigrawal, A. — Wed, 04 Nov 2009 09:16:38 PST

In this study, high abrasion furnace carbon having particle size 28—36 nm has been incorporated in unsaturated polyester resin to develop gradient composites. Dielectric measurements have been conducted on the graded composites by using a LCR meter. Effect of temperature and frequency variation on dielectric constant (^'), dielectric dissipation factor (tan ) and on a. c. conductivity (_a.c.) of the samples have been determined. Dielectric measurements have been performed in the temperature range from 30°C to 150°C and in the frequency range 1—10 kHz. Dielectric constant increases gradually with the increase of filler content in the direction of centrifugal force, which shows the existence of gradient structure in the composite. This is confirmed based on the microstructures observed at different positions in the graded composites. On increase of frequency, reduction in dielectric constant in carbon-filled polyester composites has been formed. Dielectric constant and a.c. conductivity increased with increase in temperature. ^' variation with temperature shows a plateau which shifts from 145°C to 125°C on increase of carbon content. Linear dependence of ln ₀ on E_a follows the compensation law for HAF carbon-filled polyester gradient composite, which is similar to semi conducting composite behavior. T_g obtained by DSC plots for samples 1—4 are higher as compared to polyester resin.

Wood Flour -- Recycled Polyol Based Polyurethane Lightweight Composites

Racz, I., Andersen, E., Aranguren, M. I., Marcovich, N. E. — Wed, 04 Nov 2009 09:16:38 PST

This work is focused on the production and characterization of lightweight polyurethane (PU) composites reinforced with pine wood flour (WF), which can have applications in car interior panels, construction, and acoustic insulation. A crosslinked PU formulated from a recycled polyol was used as matrix. The mechanical performance of the reinforced composites was studied through tensile, three point bending, and dynamic mechanical tests. The strength, modulus, and storage modulus increased with filler concentration.The addition of Al(OH)₃ did not reduce the flammability of the composites with low WF concentrations; however, some improvements were found in the case of composites prepared with 20 wt% WF.

Delamination Analysis in Drilling of CFRP Composites Using Response Surface Methodology

Krishnamoorthy, A., Boopathy, S. R., Palanikumar, K. — Wed, 04 Nov 2009 09:16:38 PST

Carbon fiber reinforced plastic (CFRP) composite materials are finding increased applications in many industries due to their excellent properties. Drilling of CFRP composites is needed for joining of these composite structures. During drilling, delamination is an important factor, which affects the property of the drilled hole. In this work experiments are conducted to analyze the delamination in drilling of CFRP composites. The cutting parameters considered are spindle speed, feed rate, and drill diameter. The tool material used for the present investigation is ‘‘BRAD and SPUR’’ type made of carbide. Second-order mathematical relation is established using response surface methodology for predicting delamination in drilling of CFRP composites. The results indicate that the model can be effectively used to predict the delamination in drilling of CFRP composites. The effect of different parameters in drilling of CFRP composites is analyzed and presented in detail.

Modeling VARTM Processes with Hybrid Media Incorporating Gravity Effects

Yoon, M.-K., Chen, H., Simacek, P., Heider, D., Gillespie, J. W. — Wed, 04 Nov 2009 09:16:38 PST

Vacuum-assisted resin transfer molding (VARTM) processes are increasingly used in manufacturing scale-up composite applications. Accordingly, gravity can significantly influence the flow behavior in tall-structure composite manufacturing processes. The present study developed a closed-form analytic solution incorporating gravity effects with the equivalent parameter approach in order to predict the resin flow behavior in a tall structure resin infusion. A hybrid model was used that consists of thin distribution media and a fibrous preform. An analytic solution was developed and validated with experiments as well as numerical methods in terms of the resin flow front shape, lag length, and the flow front location with time for the horizontal, upward, and downward infusion cases. The flow front locations were monitored using time domain reflectometry sensors embedded in the fabric layers. The lag length was constant in the horizontal infusion case, but decreased and increased in the upward and downward infusion cases, respectively, as resin progressed. The downward infusion case showed the fastest fill time, which corresponds to the previous results using a homogenous model. However, the flow became unstable at a certain location where the local flow front speeds increased suddenly along the distribution media. These phenomena were not observed in a model with homogenous media but were observed in a model with hybrid media, only in the downward infusion cases. The analytic solution also identified the stable flow conditions with the mold angles. The results obtained in the present study can be used to estimate stable process conditions in designing VARTM processes for manufacturing large-scale tall composite structures.

On the Sensitivity of the Nanostructural Parameters on Young's Modulus of PLSNs in Fully Intercalated Structures

Yazdi, A. Z., Bagheri, R., Kazeminezhad, M. — Wed, 04 Nov 2009 09:16:38 PST

Polymer-layered silicate nanocomposites have been observed to demonstrate enhanced mechanical properties particularly at low weight fractions of silicate. Experimental and theoretical investigations reveal that numerous structural parameters strongly influence the modulus of such nanocomposites. A multiscale micromechanical model is developed which considers a wide range of different affecting parameters including the particle aspect ratio, the number of silicate layers per stack, the d-spacing ratio between the layers, the penetration of polymer chains along silicate sheets, the intercalation feature, and the particle volume fraction. The developed model illustrates the accuracy and flexibility superiorly compared with the conventional models proposed in the literature. Moreover, good agreement is found between the experimental data and the modeling results.

Experimental Investigation of the Dynamic Response of a Symmetric Laminated Composite Beam Via Laser Vibrometry

Kiral, Z., Malgaca, L., Akdag, M., Kiral, B. G. — Wed, 04 Nov 2009 09:16:38 PST

This article reports the experimental analysis of free and forced vibrations of a cantilever symmetric laminated composite beam with different lay-up sequences. The free vibrations generated due to initial displacements are recorded by two laser displacement sensors. The damping ratios for different ply orientations are determined by using the ratios between the successive peaks in the free vibration responses and the corresponding envelope curves. The dynamic response of the beam to a moving load with constant velocity is also investigated. The pressured air is blown out onto the beam to create the load and the movement of the load is achieved by an industrial robot manipulator. The moving load mechanism is peculiar to this study. The free and moving load responses are also examined by the finite element method. A commercial finite element package ANSYS^® is used for numerical analyses. The results reported in this study show that the lay-up sequence has an important role on the dynamic response of the laminated composites and the velocity of the moving load affects the dynamic response considerably, especially for [90]_2s lay-up.

The Effects of Composite Plate Design Parameters on Linear Vibrations by Discrete Singular Convolution Method

Secgin, A., Atas, C., Sarigul, A. S. — Wed, 04 Nov 2009 09:16:38 PST

This study is devoted to some specific free vibration analysis of thin composite plates based on discrete singular convolution (DSC) approach. As the first analysis, a parametric study is performed on the basis of number of lamination, boundary condition, and orientation angle of symmetrically laminated composite plates. As the second, the effects of material type, boundary condition, and stacking sequence on the modal characteristics of laminated plates made of E-glass/epoxy, Kevlar/epoxy, and carbon/epoxy are investigated. Thirdly, linear modal characteristics of fiber metal laminates are specifically analyzed due to their common use in aircraft design. Hopefully, the results presented in the article may be practically of interest for engineers and designers. This study displays the applicability of the DSC method for real-life problems.

Compression Strength Degradation of Nanocomposites after Lightning Strike

Mall, S., Ouper, B.L., Fielding, J.C. — Wed, 04 Nov 2009 09:16:38 PST

This study investigated five conductive nanocomposites for their compressive strength degradation after subjecting them to a simulated lightning strike. These systems consisted of a ply of nickel-coated carbon woven fabric as lightning strike protection component besides the four plies of standard carbon fibers (AS4) fabric embedded in the epoxy (EPON 862). The other four systems had an additional protection system, which was nickel-nanostrand veil (NiNS), aligned buckypaper, random buckypaper, or mixed buckypaper made up of vapor-grown carbon fibers and single-walled nanotubes. All other buckypapers were made of single-walled nanotubes. Failure and damage mechanisms were also investigated. The ultimate compressive strength reduced by about 75—30% from a simulated lightning strike. This reduction as well as the damage was maximum with NiNS and minimum with random buckypaper. Damage from the lightning strike was related to the electrical conductivity and degradation in the compressive strength of the tested systems.

Expanded Waste Ground Rubber Tire Powder/Polypropylene Composites: Processing-Structure Relationships

Wed, 04 Nov 2009 09:16:38 PST

The usage of waste tire powder as dispersed phase in polypropylene matrix offers an interesting opportunity for recycling of the waste tire. In order to obtain ‘value added products’ from polypropylene (PP)/waste ground rubber tire powder (WGRT) composites, in this study, the processing of foamedPP/WGRT composites was investigated using a single-screw foam extrusion setup and chemical blowing agent. The regression models were constructed to study the relationships between the foam structure (i.e., void fraction, average cell size, and cell density) of foamed PP/WGRT composites, the processing conditions (extruder’s die temperature and screw speed), and the formulation compositions (WGRT content and blowing agent concentration) by applying a four-factor central composite design (CCD) statistical approach. The response surface plots generated using the regression models allow the rapid selection of the proper process parameters to obtain PP/WGRT composite foams with the desired density and morphology.

The Influence of the Particle Size of Silica on the Ballistic Performance of Fabrics Impregnated with Silica Colloidal Suspension

Lee, B.-W., Kim, I.-J., Kim, C.-G. — Fri, 30 Oct 2009 07:17:35 PDT

This study investigated the effect of the particle size of silica colloidal suspension (SCS) in SCS-impregnated plain woven fabrics on the ballistic performance of those fabrics. In order to examine the particle size effect, spherical silica particles with average diameters of 100 nm, 300 nm, and 500 nm were used to fabricate SCS. The SCS-impregnated fabrics were subjected to ballistic tests under various boundary conditions. The fabric impregnated with SCS produced using silica particles with an average particle diameter of 100 nm showed better impact performance than those of the SCS-impregnated fabrics containing larger particles and untreated fabrics in terms of impact energy absorption and resistance to blunt trauma, while earlier failure of their primary yarns in the impacted zone under fully clamped conditions was also observed. The results of the drop impact test and yarn pull-out tests indicated that SCS-impregnated fabric with smaller particles exhibits the largest increment of inter-yarn friction at the onset shear strain for shear thickening. It was found that impregnation with SCS affected the interfacial friction of filaments and yarns, which contributes to a larger transfer of the impact energy to interaction of individual yarns in the fabric, resulting in significantly higher energy dissipation with less transverse deformation. Further, the influence of impregnation with SCS on ballistic performance was found to be closely associated with boundary conditions. Thus, the particle size of the SCS and the boundary conditions are the dominant factors that can be manipulated to fully utilize the benefits of SCS impregnated fabrics for flexible body armor.

Thermal Fatigue Cracking of an IM7/5250-4 Cross Ply Laminate: Experimental and Analytical Observations

Dalgarno, R. W., Garnich, M. R., Andrews, E. W. — Fri, 30 Oct 2009 07:17:35 PDT

Coupons of a balanced and symmetric cross ply laminate with polished edges made from IM7/5250-4 were thermally cycled between room and liquid nitrogen temperatures to induce cracks in the matrix. Edge crack densities for each symmetric lamina pair were recorded as a function of the number of cycles. Coupons with an epoxy coating on the polished edges experienced minimal cracking in all interior plies compared to significant cracking for uncoated specimens. This suggests that edge effects play a dominant role in crack initiation. Crack counts were quite different than recorded by another research group for nearly identical tests. Thermal shock was eliminated as a possible significant contributor to the overall physics driving crack formation. Stress analysis was used to support discussions that explain some of the observed trends in crack densities in the various ply pairs. The number of thermal cycles reached 2600 without a clear indication of crack saturation in any of the plies.

The Effect of Maleic Anhydride Graft on the Interfacial Adhesion of Carbon Fiber Reinforced Thermoplastic Polystyrene Composite

Li, J., Sun, F.F. — Fri, 30 Oct 2009 07:17:35 PDT

The quality of interfacial interaction is dictated by the surface chemistry of the carbon fibers and the composition of the matrix. The composition of polystyrene was modified by the addition of maleic anhydride (MAH)-grafted polystyrene (MAH-g-polystyrene). The surface properties of the various matrix formulations were characterized by contact angle. Carbon fibers were modified by oxidation in nitric acid. The surface composition of the carbon fibers was characterized. The interaction between modified polystyrene and the carbon fibers was studied by single fiber pull-out tests. The best adhesion behavior was achieved between polystyrene containing grafted MAH and nitric acid oxidation carbon fibers. The addition of MAH-g-polystyrene to the unmodified polystyrene caused the interfacial shear strength to increase. The apparent interfacial shear strength of this fiber-matrix combination allowed for the utilization of 100% of the yield tensile strength of polystyrene.

Merchant Model Applied to Precision Orthogonal Cutting of Pa66 Polyamide with and without Glass Fiber Reinforcing

Silva, L. R., Davim, J. P., Abrao, A.M., Faria, P.E. — Fri, 30 Oct 2009 07:17:36 PDT

Polyamide composites are widely employed in various fields of engineering, such as aircraft, automotive, robots, and machinery owing to their remarkable properties. Precision machining aims the production of advanced components with high-dimensional accuracy and acceptable surface integrity. This work presents a preliminary experimental study based on Merchant theory applied to precision radial turning of PA 66 polyamide with and without 30% glass fiber reinforcing. Distinct feed rate values were tested using uncoated carbide tool (grade K15) without chip breaker. The aim of this study is to evaluate the chip compression ratio (R_c), chip deformation (), friction angle (), shear angle (), normal stress (), and shear stress (). In general, similar results were obtained when comparing the results obtained using Merchant model with the experimental findings.

Effect of Nanoparticles on Wear Resistance and Surface Hardness of a Dental Glass-ionomer Cement

Zhao, J., Xie, D. — Fri, 30 Oct 2009 07:17:36 PDT

We have studied the effect of nanoparticles on wear resistance and surface hardness of a dental glass-ionomer cement. The effects of composition, loading ratio, and surface modification of the nanoparticles as well as particle size were investigated as well. Commercially available spherical nanoparticles were incorporated into a commercial dental cement Fuji II. One of the nanoparticles was surface-modified with poly(acrylic acid) via surface-initiated atom-transfer radical polymerization. All the cements were conditioned in distilled water at 37°C for 24 h prior to testing. The results showed that all the particles incorporated (either nano-, micro-, or surface-modified) increased the resistance to attrition of Fuji II by approximately 50% but slightly decreased the resistance to abrasion. Most of the modified cements showed increased surface hardness values as compared to Fuji II. Surface modification significantly increased the loading of the nanoparticles into Fuji II from 10% to 30% as compared to the unmodified ones (only 10% at the maximum). Within the limitations of this study, it seems that incorporation of 1-3% spherical silica nanoparticles can effectively improve both resistance to attrition and surface hardness of Fuji II cement.

Studies on Morphology and Thermomechanical Behavior of Polyethylene Glycol/1,8-Octanediol-modified Epoxy-organoclay Hybrid Nanocomposites

Chozhan, C. K., Elumalai, P., Alagar, M. — Fri, 30 Oct 2009 07:17:36 PDT

Organoclay-filled polyethylene glycol (PEG)/1,8-octanediol (OCT)-modified epoxy matrices were developed. Epoxy systems modified with 10, 20, and 30 wt% of PEG/OCT were made by using diglycidyl ethers of bisphenol A epoxy resin and PEG/OCT with 4,4^'-diaminodiphenylmethane as curing agent. The PEG/OCT-modified epoxy systems were further modified with 1, 3, and 5 wt% of organoclay. The thermal behavior of organoclay-filled PEG/OCT-modified epoxy matrices were characterized using differential scanning calorimeter, thermogravimetric analysis, and dynamic mechanical analysis. Mechanical properties were studied as per ASTM standards. Three percent organoclay-filled PEG-modified epoxy possesses high storage modulus (6240 MPa) compared to 3% organoclay-filled OCT-modified epoxy (5064 MPa). The values of impact strength for 3% organoclay filled PEG and OCT modified epoxy systems are 122 and 85.5 J/m, respectively. Homogeneous and heterogeneous morphology was ascertained from scanning electron microscope. From the X-ray diffraction analysis, it was observed that the absence of d₀₀₁ reflections in organoclay-filled PEG/OCT-modified epoxy systems indicated the formation of exfoliated nanohybrids. The significant improvement in impact strength was achieved without any reduction in glass transition temperature (T_g) due to the addition of organoclay. Thermal stability with better char yield and improved impact strength were attained during the formation of nanocomposites due to the increment of organoclay in PEG/ OCT-modified epoxy systems.

Influence of Hyperbranched Polyamine-ester Modified Carbon Nanotubes on Properties of Epoxy Nanocomposites

Zheng, Y.-P., Zhang, J.-X., Yu, P.-Y., Liu, L.-L., Gao, Y. — Fri, 30 Oct 2009 07:17:36 PDT

Surface modification of the acidified multi-walled carbon nanotubes (MWNTs) was conducted by hyperbranched polyamine-ester (HBP). The sample was characterized with the Raman spectra, X-ray diffraction, and transmission electron microscopy. Nanocomposites with MWNTs/epoxy matrix were prepared by compounding the modified MWNTs and epoxy resin. As a result, the MWNTs-capped organic molecules formed core/shell structure. The grafted organic molecules solve the dispersion of the MWNTs in the epoxy resin-based nanocomposites and then enhancing the antifriction and dielectric property of such nanocomposites.

Mechanical Properties and Flame Retardancy Research of Montmorillonite Intercalate Polyamide 66 Composites

Yang, X., Li, Q., Chen, Z., Han, H., Jing, H. — Fri, 30 Oct 2009 07:17:36 PDT

A series of exfoliated nanocomposites or microcomposites have been fabricated with different organophilic montmorillonite (OMMT) loading via in situ intercalation polymerization and melt intercalation. Their structure and mechanical properties were investigated and compared with XRD and other advanced facilities systematically. Flame retardancy was evaluated by conventional limiting oxygen index (LOI) and char residue ratio. Experiment results shows that fabricated technology have unobvious influence on micro-structure, tensile strength and tensile modulus of PA66/OMMT composites. On the contrary, the alteration of OMMT loading cause dramatic change on both mechanical performance and flame retardancy. The maximum tensile strength of PA66/OMMT composites is 106 MPa with 3 wt% OMMT loading. The addition of OMMT enhances obviously the mechanical property and flame retardancy of PA66.

The Effect of Tool Geometry on the Machinability of Polyamide During Precision Turning

Silva, L. R., Davim, J. P. — Fri, 30 Oct 2009 07:17:36 PDT

The cutting tool geometry plays a significant role on the performance of conventional machining operations, irrespectively of the work material. For instance, alterations in the cutting edge preparation will result in changes in tool wear rate, cutting forces, temperature, and machined surface finish. This article compares the performance of uncoated carbide tools with standard cutting geometry and tools with modified edge preparation during precision turning of polyamide with and without 30% glass fiber reinforcing. The results indicated that, in general, the turning force components are reduced with the tool nose radius and the specific cutting force (K_s ) decreased as feed rate is elevated, presenting values comparable to metallic alloys. In addition to that, the polyamide without reinforcing presented a three-fold increase in specific cutting force (K_s) compared with the reinforced composite. Finally, the surface roughness increased as feed rate is elevated and tool nose radius is reduced.

Synthesis and Swelling Properties of Guar Gum-g-poly(sodium acrylate)/ Na-montmorillonite Superabsorbent Nanocomposite

Wang, W., Wang, A. — Fri, 30 Oct 2009 07:17:36 PDT

Novel guar gum-g-poly(sodium acrylate)/Na-montmorillonite (GG-g-PNaA/MMT) superabsorbent nanocomposites were synthesized by grafted copolymerization among natural guar gum (GG), partially neutralized acrylic acid (NaA) and Na-montmorillonite (MMT) in the aqueous solution using ammonium persulfate as initiator and N,N'-methylenebisacrylamide (MBA) as crosslinker. Fourier transform infrared spectroscopy confirmed that NaA had been grafted onto GG and the -OH groups of MMT participated in the reaction. X-ray diffraction and scanning electron microscopy analyses revealed that MMT was exfoliated and equably dispersed in GG-g-PNaA matrix. The effects of MMT and MBA on water absorbency were investigated. The results show that introducing MMT into the GG-g-PNaA network improved the swelling capability and the swelling rate of the superabsorbent nanocomposite. The nanocomposites keep water absorbency high, within a wide pH range from 4 to 11 and exhibit better reswelling capability.

Crystallinity Behavior of MDPE-Clay Nanocomposites Fabricated using Ball Milling Method

Abareshi, M., Zebarjad, S.M., Goharshadi, E.K. — Fri, 30 Oct 2009 07:17:36 PDT

Polymer-clay nanocomposites were fabricated from medium-density polyethylene and organically modified Na-montmorillonite (MMT) using the planetary ball milling as a new method. The materials were characterized using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction (XRD). The crystalline size of polyethylene (PE) filled with MMT was studied using XRD. These studies indicated that the addition of MMT decreases the crystalline size of the polymer. Moreover, the degree of crystallinity of both ball-milled and un-milled PE and PE-clay nanocomposites was measured by applying the XRD technique. The results show that both milling time and clay content decrease the degree of crystallinity of PE.

Evaluation of Dynamic Tensile Characteristics of Polypropylene Composites with Temperature Variation

Jin Sung Kim, , Huh, H., Lee, K. W. — Fri, 30 Oct 2009 07:17:36 PDT

This article suggests two useful methods to obtain true stress—strain curves with the variation of the strain rate ranging from 0.001 to 200/s using a finite element technique and optimization for two typical polymers: a polypropylene; and a polypropylene composite. The polypropylene is adopted as a core material of a vehicle’s instrument panel and the polypropylene composite sustains the passenger-side air bag module (PAB module) as a structural member. Their dynamic tensile characteristics are important to predict deformation modes of the PAB module which undergoes high speed deformation during air bag expansion. Since the polymers are temperature sensitive materials, the dynamic tensile tests were performed at the three levels of temperature: the low temperature (—30°C); the room temperature (21°C); the high temperature (85°C). During tensile tests of two polypropylene composites, the gage region in the specimen of the polypropylene composite elongates uniformly while that of the polypropylene does not elongate uniformly. In the case of the polypropylene, experimental stress—strain curves were modified by enforcing the load response calculated by finite element analysis coincident with that of the experiment. In the case of the polypropylene composite, experimental stress—strain curves were scaled by applying change rate of the gage length. Stress—strain curves of both cases provide excellent performance in their numerical response with the finite element analysis. The material properties obtained are indispensible to estimate the high speed deformation mode during air bag expansion at both normal and harsh operating conditions.

Impact Behavior of Composite Hollow Sphere Structures

Vesenjak, M., Ren, Z., Fiedler, T., Ochsner, A. — Tue, 27 Oct 2009 03:17:38 PDT

Porous composite materials constitute an innovative group of lightweight materials which combine high specific stiffness, good damping properties, and thermal insulation with the ability to absorb large amounts of energy at a low constant stress level. In the scope of this study, adhesively bonded metallic hollow sphere structures (MHSS) fully embedded within the adhesive matrix are considered with aim to determine their macroscopic behavior under uniaxial impact loading conditions by means of parametric computational simulations. The base material properties have been determined by quasi-static and dynamic experiments. Three topologies of syntactic hollow sphere structures of various dimensions are considered, namely the cubic primitive, the body centered cubic and the face centered cubic topology. Results of computational simulations show significant influence of topology and strain rate sensitivity on the composite structure behavior, while the influence of metallic hollow sphere wall thickness is less pronounced. Computational simulations show that it is possible to combine the MHSS topology, metallic hollow sphere wall thickness and strain rate sensitivity to achieve any desired dynamic response of MHSS adapted to a given engineering problem.

Permeability of Various Hybrid Composites Subjected to Extreme Thermal Cycling and Low-velocity Impacts

Mcvay, A. C., Johnson, W.S. — Tue, 27 Oct 2009 03:17:38 PDT

This experimental exploratory study investigates the effects of embedding a ‘barrier’ layer within a graphite/epoxy composite during manufacture on reducing the permeability after thermal cycling and low-velocity impact events. The baseline composite material was an eight-ply graphite/epoxy system. Barrier layer candidates include aluminized Mylar^®, aluminum foil, and two β-Ti 15-3 films. Cryogenic thermal cycling was performed on the hybrid composites and control composites to determine if the interleaved composites exhibited reduced permeability after thermal cycling as compared to the controls. Drop-weight impact tests were performed on the hybrids and controls to determine the effect of interleaving on the critical impact energy of the graphite/epoxy composites. The results of this research suggest that the addition of an embedded barrier layer can increase a graphite/epoxy composite’s resistance to low-velocity impacts that cause permeation. This research indicates that hybrid composites are promising materials for applications where extreme temperatures and moderate impact loads are experienced.

Study on the Mechanical Properties of Different Silkworm Silk Fibers

Cheung, H.-Y., Lau, K.-T., Ho, M.-P., Mosallam, A. — Tue, 27 Oct 2009 03:17:38 PDT

Mechanical properties of Bombyx mori, twisted B. mori, and Tussah silk fibers were investigated. Their ultimate tensile strength, elongation at break, and Young’s modulus were examined by performing a uniaxial tensile test on a single fiber. Scanning electron microscopy was used to observe the morphology of two different types of silk fiber, and to measure their apparent diameters from which the cross-sectional area of the silk fiber for stress-strain analysis can be determined. Based on experimental results obtained, it was found that Tussah silk fiber has a relatively high extensibility as compared to B. mori silk fiber and other natural fibers. Weibull analysis was also used to quantify tensile strength reproducibility of the silk fiber. Both single and twisted B. mori silk fibers have a better reproducibility of tensile properties than Tussah silk fiber.

3-Aminopropyltriethoxysilane Effect on Thermal and Mechanical Properties of Multi-walled Carbon Nanotubes Reinforced Epoxy Composites

Jeong Tai Kim, , Kim, H.-C., Kim, S.-K., Kathi, J., Rhee, K.-Y. — Tue, 27 Oct 2009 03:17:38 PDT

Diglycidyl ether of bisphenol A nanocomposites with unmodified multi-walled carbon nanotubes (u-MWCNTs) and silanized multi-walled carbon nanotubes (si-MWCNTs) were prepared by cast molding method. The effects of 3-aminopropyltriethoxysilane functionalization of MWCNTs on the thermal and mechanical properties of the nanocomposites were examined. The nanocomposites were characterized by thermogravimetric analysis, dynamic mechanical thermal analysis, and flexural testing. The results showed that epoxy composites based on si-MWCNTs showed better thermal stability, glass transition temperature, and flexural properties than the composites based on u-MWCNTs. These results prove the effect of silane functionalization on the interfacial adhesion between epoxy and MWCNTs. This was further confirmed by morphology study of fractured surfaces of nanocomposites by scanning electron microscopy.

Effect of Thermomechanical Degradation of Polypropylene on Mechanical Properties of Wood-Polypropylene Composites

Najafi, S. K., Mostafazadeh-Marznaki, M., Chaharmahali, M., Tajvidi, M. — Tue, 27 Oct 2009 03:17:38 PDT

In this research, the influence of thermomechanically degraded polypropylene (PP) on mechanical properties of beech sawdust-PP composites was studied. For this purpose, a virgin PP (VPP) was thermomechanically degraded by two times extrusion under controlled conditions in a twin-screw extruder at a rotor speed of 100 rpm and a temperature of 190^°C. The results showed that melt flow index, flexural modulus, and hardness of PP were significantly increased by extrusion and re-extrusion of VPP. The PP (virgin and recycled PP in each stage) and beech sawdust were compounded at 60% weight sawdust loading in a counter-rotating twin-screw extruder in the presence or absence of maleated polypropylene (MAPP) to produce sawdust-PP composites. The nominal cross section and density of the manufactured composites were 70 x 10 mm² and 1 g/cm³, respectively. From the results, the composites containing recycled PP exhibited higher flexural properties and hardness and lower impact strength. In the presence of MAPP, all mechanical properties increased.

Strength Enhancement of Advanced Grid Stiffened FRP Tube Confined Concrete Cylinders under Fire

Ji, G., Li, G., Pang, S.-S., Alaywan, W. — Tue, 27 Oct 2009 03:17:39 PDT

Recently, a new confining device - a lattice of interlaced fiber-reinforced plastic (FRP) ribs that were wrapped by an FRP skin, has been developed to provide better confinement to a concrete core through interfacial mechanical interlocking [1,2]. A systematic study has validated the enhanced structural capacities over traditional FRP tube encased concrete counterparts. However, like traditional FRPs, advanced grid stiffened FRP tube confined concrete cylinders (AGS-FRPC) are vulnerable to fire hazards. The purpose of this study was to investigate the enhancement of fire tolerance as a result of incorporating organically modified montmorillonite (MMT) and a traditional fire retardant additive (TSWB^®) into a vinyl ester (VE) matrix. Two series of samples were prepared. The first series consisted of groups A, B, C, and D, in which A was regular AGS-FRPC, B was enhanced with MMT, C was enhanced with TSWB^® fire retardant additive, and D was enhanced with both MMT and TSWB^®. The second series of samples was identical to the first series as a control, and it consisted of groups of E, F, G, and H, respectively. The only difference was that the first series was exposed to a jet fire of 982^°C for 5 min while the second series was not. It was found that fire exposure had a significant impact on reducing the residual strength of the confined concrete cylinders. The introduction of MMT increased both the fire tolerance and compressive strength of AGS-FRPC. The TSWB additive was effective in minimizing the effect of the fire hazard, but it reduced the compressive strength.

Mechanical Performance of Low Shrinkage Engineered Cementitious Composite in Tension and Compression

Zhang, J., Gong, C., Guo, Z., Ju, X. — Tue, 27 Oct 2009 03:17:39 PDT

An experimental study was carried out to assess the mechanical performance of engineered fiber-reinforced composites (ECC) with characteristic of low drying shrinkage. Research emphasis is placed on the influence of water to binder ratio (w/c) on tensile and compressive properties, such as stress-strain curves, strength, and strain values as well as elastic modulus. Four mixtures with w/c of 0.55, 0.45, 0.35, and 0.25 were designed in the experimental program. Experimental results show that composite with compressive strength ranging from 20 to 60 MPa can be obtained by varying w/c from 0.55 to 0.25, while retaining the tensile strain hardening and multiple cracking performances. A significant plasticity, like yielding of metal, is found under compressive load after peak stress, except for the similar behavior under tensile load.

Characterization of Quasi-static Mechanical Properties of Polymer Nanocomposites Using a New Combinatorial Approach

Gershon, A. L., Kota, A. K., Bruck, H. A. — Tue, 27 Oct 2009 03:17:39 PDT

Recently, it has become very important to rapidly characterize the processing-structure-property relationships in polymer nanocomposites using minimal quantities of expensive nanoscale fillers. To address this issue, we present a new combinatorial approach developed for characterizing the variation in mechanical properties as a function of filler composition in polymer nanocomposites. The fundamental basis for the combinatorial approach is the generation of compositional gradients through transient operation of a twin-screw extruder (TSE). The compositional variation in the specimens could be rapidly predicted a priori using a convolution process model and was verified a posteriori using pycnometry measurements and thermogravimetric analysis. To characterize the quasi-static mechanical properties along the compositional gradient, sub-scale specimens that are proportional in size to ASTM type I specimens but with a gage section that is a factor of 10 smaller, were tested using a microtensile tester. The properties of the sub-scale specimens processed in the combinatorial approach correlated well with those of sub-scale specimens of similar composition processed in steady-state, thereby indicating that the properties were unaffected by the transient operation of the TSE. Furthermore, the quasi-static mechanical properties of the steady-state ASTM type I standard specimens were compared with those of the sub-scale specimens to determine the effect of specimen size. The results were nearly identical, except the increased size of the ASTM type I standard specimens resulted in substantial reductions in ductility that are most likely due to an increase in the number of processing-related defects.

Thermal Properties of Composites Made of Heat-treated Wood and Polypropylene

Kaboorani, A. — Tue, 27 Oct 2009 03:17:39 PDT

Exposure of wood to high temperatures improves thermal stability of wood and it has been used as a means to boost dimensional stability of wood for centuries. Inclusion of wood in thermoplastic matrix composites reduces the thermal stability of the composites considerably because of poor adhesion between wood and the matrix, and lower thermal stability of wood in comparison to the matrix. In order to invest natural fiber/thermoplastic composites with thermal stability, wood flour was heat treated under different temperatures and time. Thermal stability measurements conducted by thermogravimetric analysis (TGA) indicated that heat-treatment of wood increased thermal stability, ash content, and DTG_max degradation temperature of wood. As the heat-treated wood flour used as filler for PP composites, thermal stability, ash content, and DTG_max degradation temperatures of the composites were affected markedly as well. The higher the temperature and longer the time employed in heat-treatment, the more was the improvement gained. The melting point (T_m) of composites measured by differential scanning calorimetry (DSC) showed that wood content and preheat treatment had no effect on melting points (T_m).

Effect of Interlayer Delamination on Mechanical Behavior of Carbon/Epoxy Laminates

Reis, P.N.B., Ferreira, J.A.M., Antunes, F.V., Richardson, M.O.W. — Tue, 27 Oct 2009 03:17:39 PDT

This article presents the results of a current study on the influence of interlayer delaminations on the static and fatigue behavior of composite laminates. The composite was manufactured by a vacuum molding method using 12 balanced bi-directional carbon fiber layers and epoxy resin. Delaminations with different length were artificially introduced. The specimens with dog bone shape were cut from the original plates having 3 mm thickness and fiber weight fraction of 0.66. Static tests were performed in order to study the influence of delamination size on the laminate stiffness and strength. Complementary finite element analysis was carried out showing the influence of angular misalignments of fiber/matrix delaminations on the laminate stiffness. Fatigue tests were performed in load control for R = 0.05 and R = —1, with a loading frequency of 10 Hz, at room temperature. The artificial interlayer delaminations have a negligible influence on the fatigue strength for tensile cycle loadings, but produce significant decreases in strength for R = —1 fatigue loadings.

Mechanical Behavior and Failure Mode of Unidirectional Fiber Composites at Low Strain Rate Level

Sun, L., Jia, Y., Ma, F., Sun, S., Han, C. C. — Tue, 27 Oct 2009 03:17:39 PDT

The rate responses of unidirectional fiber composites at low strain rate level (range from 2E-5 to 2E-2 s^-1), including tensile mechanical and stress relaxation behaviors, were investigated in this article. The results obtained showed that the ultimate tensile stress increases as strain rate increases, and the stress relaxation time decreases as pre-tensile strain rate increases. And besides, the macro-failure modes of fiber composites changed from ductile fracture feature to brittle fracture feature as strain rate increasing. Furthermore, the micro-failure modes of multi-fiber composite were investigated originally, with the results indicating that fiber-fiber interactions decreased while increasing tensile strain rate.

Modeling the Dimensional Variations of Composites Using Effective Coefficients of Thermal Expansion

Dong, C. — Tue, 27 Oct 2009 03:17:39 PDT

With the increasing demands for energy efficiency and environmental protection, composite materials have become an important alternative to traditional materials. Composite materials offer many advantages over traditional materials including low density, high strength, high stiffness to weight ratio, excellent durability, and design flexibility. Despite all these advantages, composite materials have not been as widely used as expected because of the complexity and cost of the manufacturing process. One of the main causes is associated with poor dimensional control. ‘Spring-in’ is a common dimensional variation of composite angled parts. Due to the layerwise structure of composites, traditionally the spring-in was calculated numerically by defining each lamina as an element along the through-thickness direction and assigning its material properties and orientation. This method is usually too time-consuming for practical applications. In this article, the effective coefficients of thermal expansion of symmetric laminates were derived. Using the effective CTE, spring-in can be calculated by a simple mathematical formula. This approach was validated by finite element analysis and experiments. The effective CTE are very useful in 3-D finite element analysis. The effective CTE provide a feasible approach to the 3-D finite element analysis. Compared with the layerwise approach, the number of elements is significantly reduced. This was illustrated by an example.

Strength and Damage of Elementary Flax Fibers Extracted from Tow and Long Line Flax

Andersons, J., Sparnins, E., Porike, E. — Tue, 27 Oct 2009 03:17:39 PDT

Flax fibers possess high specific strength and stiffness, and thus are competitive in terms of mechanical properties with traditional reinforcing fibers used in polymer-matrix composite materials. For environmental and economical benefit, it would be preferable to apply nontextile grade fibers in composites provided their mechanical characteristics are acceptable. Elementary fibers have been extracted from long line flax, used as high-quality raw material for textile industry, and flax tow, and their strength distribution and damage level determined. It is shown that the elementary flax fibers coming from short flax fiber are not inferior to those of textile-quality flax in terms of strength and damage.

Abrasive Wear Behavior of Al-4.5 wt% Cu/(Zircon Sand + Silicon Carbide) Hybrid Composite

Das, S., Das, K., Das, S. — Tue, 27 Oct 2009 03:17:39 PDT

In the present investigation the abrasive wear behavior of Al-4.5 wt% Cu alloy-based hybrid composites reinforced with zircon sand and silicon carbide particles has been studied. The abrasive wear resistance of these hybrid composites has been found to be better than that of Al-4.5 wt% Cu alloy without reinforcement. It has been also found that the size and morphology of the reinforcement particles has a significant effect on the abrasive wear resistance of the hybrid composites. The worn surfaces of the hybrid composites have been studied under a scanning electron microscope and it has been found that the wear is mainly due to the ploughing of the surface by abrading silicon carbide particles.

A Method to Estimate the Accuracy of Radial Flow--Based Permeability Measuring Devices

Tan, H., Pillai, K. M. — Wed, 23 Sep 2009 02:22:35 PDT

Radial flow experiment is often used to measure the permeability of fibrous porous media by the composites processing community. However, very little work has been done to calibrate the radial flow set-up with media of known permeability. This article presents a new method for calibrating the permeability measuring set-ups based on radial flows. A reference medium in the form of two concentric annular slits is created for the radial flow mold whose permeability is estimated analytically and numerically. It is observed that the experimentally measured permeability deviates from the theoretical and numerical permeabilities with an increase in the flow rate for an injection setup using a gear pump as well as a piston-cylinder pump. The deviation can be attributed to two possible reasons: one is the leakage between the calibration device and mold plate surfaces because of a slight mold deformation at higher flow rates, the other one is the increase in the gap clearance of the reference medium because of the higher gap pressures at higher flow rates. No such deviation is observed while measuring the permeability of a fiber mat at different flow rates. Hence, the proposed reference medium can calibrate a conventional radial flow-based permeability measuring setups at low flow rates; the accuracy thus obtained can be extended to higher flow rates as well if permeability of a fiber mat can be shown to remain constant with the flow rate.

A Metamodeling Approach to Mechanical Characterization of Anisotropic Plates

Setiawan, R., Syngellakis, S., Hill, M. — Wed, 23 Sep 2009 02:22:35 PDT

The proposed characterization method is based on the knowledge of free vibration characteristics of plate specimens. Effective laminate properties are estimated through minimization of the error between known and predicted natural frequencies. Metamodeling is used for approximating the relationship between the material properties and the corresponding natural frequencies. This results in an economical characterization process, applicable to a large number of specimens of various shapes and dimensions. The proposed strategy is first tested on a range of hypothetical composite materials with numerically simulated test results, then applied to reliable published experimental data. It is thus shown to have the potential of a fast and accurate, in-situ characterization tool.

Scattering of Thermal Waves and Unsteady Effective Thermal Conductivity of Particular Composites with Functionally Graded Interface

Fang, X.-Q., Liu, J.-X., Zhang, T., Wang, X.-H. — Wed, 23 Sep 2009 02:22:35 PDT

In this study, thermal wave method is applied to investigate the unsteady effective thermal conductivity of particular composites with a functionally graded interface, and the analytical solution of the problem is obtained. The Fourier heat conduction law is applied to analyze the propagation of thermal waves in the particular composite. The scattering and refraction of thermal waves by a spherical particle with an inhomogeneous interface layer in the matrix are analyzed, and the results of the single scattering problem are applied to the composite medium. The wave fields in different material layers are expressed by employing spherical wave function expansion method, and the expanded mode coefficients are determined by satisfying the boundary conditions of the layers. The theory of Waterman and Truell is applied to obtain the effective propagating wave number and the unsteady effective thermal conductivity of composites. Through the numerical examples, it is found that in different region of wave frequency, the effect of the thickness of the interface on the unsteady effective thermal conductivity of composites shows great difference. The effect of the thermal conductivity ratio of the particles and matrix on the unsteady effective thermal conductivity is closely related to the incident frequency of thermal waves. The variation of the interface properties between the particles and matrix also expresses great effect on the unsteady effective thermal conductivity of composites. Finally, comparison with the results obtained from other methods is made.

Modeling of Strength Degradation for Fiber-reinforced Polymer Composites in Fire

Bai, Y., Keller, T. — Wed, 23 Sep 2009 02:22:35 PDT

A model for predicting composite material strength degradation under elevated and high temperatures is proposed. This model is based on the morphology of the mixture of materials in different states. The degradation of resin-dominated shear strength can be well described by the rule of mixture while the degradation of nominal compressive strength tends to follow the lower bound of strength defined by the inverse rule of mixture. Composite materials under tension may exhibit fiber- or resin-dominated behavior. In a lower temperature range, strength is dominated by the fiber tensile strength, while at higher temperatures, tensile components may exhibit resin-dominated failure in joint regions. The parameters required in the model can be obtained on the basis of kinetic analysis of dynamic mechanical analysis results. The fitting of experimental curves of material strength degradation is not necessary. The proposed modeling scheme can easily be incorporated into structural theory to predict mechanical responses and time-to-failure.

Mechanical and Abrasive Wear Behavior of Coleus Spent Filled Unsaturated Polyester/Polymethyl Methacrylate Semi Interpenetrating Polymer Network Composites

Syed, M. A., Siddaramaiah, , Suresha, B., Syed, A. A. — Wed, 23 Sep 2009 02:22:35 PDT

Mechanical properties and three-body abrasive wear behavior of 5 and 10% w/w bio-based coleus spent (CS) filled and unfilled semi interpenetrating polymer network composites of unsaturated polyester/polymethyl methacrylate (80/20) have been studied. The tensile strength and elongation at break has been evaluated using 4302 Hounsfield Universal testing machine. The effect of abrading distances viz., 150, 300, 450, and 600 m and different loads of 22 and 32 N at 200 rpm on the abrasive wear behavior have been studied using dry sand/rubber wheel abrasive test rig. The CS filler lowered the mechanical properties and improved abrasion resistance of USP/PMMA SIPN. The tensile strength of the composites lies in the range of 25.0-33.1 MPa. The wear volume loss and specific wear rate as a function of abrading distance and load were determined. The wear volume loss increases with increased abrading distance/load for all composites tested. However, the specific wear rate decreased with an increase in abrading distance/load. CS filled USP/PMMA SIPN composites showed better abrasion wear resistance as compared to unfilled USP/PMMA SIPN. The worn surface features have been examined using scanning electron microscope. Also, the wear volume loss was correlated with the product of tensile strength and elongation at break.

Effects of Modified Vermiculite on Water Absorbency and Swelling Behavior of Chitosan-g-Poly(Acrylic Acid)/Vermiculite Superabsorbent Composite

Xie, Y., Wang, A. — Wed, 23 Sep 2009 02:22:35 PDT

In this study, a series of chitosan-g-poly(acrylic acid)/vermiculite (CTS-g-PAA/VMT) superabsorbent composites containing VMT, acid-activated, ion-exchanged, and organic modified VMT were synthesized by free-radical graft polymerization in aqueous solution, using N,N’-methylenebisacrylamide as a crosslinker and ammonium persulfate as an initiator. The effects of VMT and modified VMT on equilibrium water absorbency, swelling rate, and swelling behavior in various cationic saline solutions (NaCl, CaCl₂, and FeCl₃) and different pH value solution of superabsorbent composites were systematically investigated. FTIR spectroscopy confirmed that acrylic acid had been grafted onto CTS and the -OH groups of VMT participated reaction. The introduction of acid-activated, ion-exchanged, and organic modified VMT into chitosan-g-poly(acrylic acid) polymeric network could improve water absorbency and swelling rate compared with that of VMT. The water-absorbing capacity of superabsorbent composites decrease gradually with the increasing of the concentration of NaCl, CaCl₂, and FeCl₃ solutions from 0.01 to 10 mM. The all prepared samples have similar swelling behavior in different pH value solution and the equilibrium water absorbencies of samples keep roughly constant in the pH range from 4 to 12.

Thermal Conductivity Properties and Heat Transfer Analysis of Multi-re-entrant Auxetic Honeycomb Structures

Innocenti, P., Scarpa, F. — Wed, 23 Sep 2009 02:22:35 PDT

In this work, the static and transient conductivity properties of a novel centersymmetric honeycomb structure are evaluated using analytical and finite element (FE) models. The honeycomb structure features a unit cell geometry allowing in-plane auxetic (negative Poisson’s ratio) deformations, and geometry parameters to be used to design the honeycomb configurations for multifunctional applications. The equivalent thermal conductivity along the two principal directions of the multi-re-entrant unit cell is calculated using a theoretical approach based on Fourier’s law and electric-thermal analogy. To validate the theoretical models, a FE analysis has been carried out on periodic unit cells under thermal steady-state conditions, showing an excellent agreement with the theoretical results. A transient thermal analysis, considering convection and radiation, has been performed numerically to evaluate the possible use of this honeycomb configuration for sandwich panels in thermal protection systems. Parametric analysis on the thermal conductivity and maximum temperatures acquired during convection-radiation is performed vs. the nondimensional geometry parameters of the honeycomb cell. The auxetic honeycomb configurations show higher out-of-plane conductivity, strong in-plane thermal anisotropy, and the lowest peak temperatures during heat transfer between the bottom and top faces of honeycomb panels.

Analysis of Underlying Organization in Randomized Fiber Arrays

Brennan, K. P., Walrath, D. E. — Wed, 23 Sep 2009 02:22:35 PDT

This work quantifies geometric effects of randomness in unidirectional fiber packing arrays over a 0.1-0.9 fiber volume fraction range. Investigated parameters are equivalent symmetry angle, nearest neighbor distance, number of touches per fiber, and specific fiber volume fraction. Ten-thousand different fiber array configurations were analyzed to generate statistical results. These results are presented as plots and least squares approximating functions. It is shown that, for fiber volume fractions exceeding 0.6, truly random fiber packing is unlikely to occur. A sample problem is also included to illustrate benefits of accounting for random behavior in simple models.

Design Analysis of the Outwater-Murphy Single-Fiber Specimen

Farooq, M.U., Carlsson, L.A., Acha, B.A. — Wed, 23 Sep 2009 02:22:35 PDT

A design analysis of the Outwater-Murphy (OM) compression test specimen is presented. The objective is to size the dimensions of the OM specimen for interface fracture toughness testing of glass and carbon fibers in a polymer matrix. A simple fracture mechanics analysis is developed based on mechanics of materials principles assuming linear-elastic behavior of the fiber and matrix. A successful test demands fiber/matrix debonding prior to any other failure mechanisms such as net section yielding, fiber microbuckling and global buckling of the specimen. The analysis of the results indicates that the test should be viable for OM specimens with a small cross section and a small central hole. Tests on E-glass/vinylester specimens were successfully conducted providing a fiber/matrix interface toughness of 38 J/m², in reasonable agreement with interface toughness data for similar systems determined by other test methods.

Time Dependence of Material Properties of FRP Composites in Fire

Bai, Y., Keller, T. — Wed, 23 Sep 2009 02:22:35 PDT

Temperature dependence of physical and mechanical properties is considered in most thermal and mechanical response models for composite materials. Based on thermogravimetry analysis, differential scanning calorimetry, and dynamic mechanical analysis conducted at different heating rates, it is demonstrated that thermophysical and thermomechanical properties are not univariate functions of temperature, but also functions of time. Temperature- and time-dependent models for physical and mechanical properties at elevated and high temperatures are proposed, which show good accordance with experimental results. Based on a finite difference method, complex realistic thermal loading programs such as the ISO fire curve can be taken into account.