Kunigal Shivakumar

Carbon/Vinyl Ester Composites for Enhanced Performance in Marine Applications

The present study describes the processing and mechanical characterization of two different fibers (glass and carbon) and two different fabric architectures (woven roving and stitch bonded) made into composites with Dow Chemical’s Derakane 510A-40, a brominated vinyl ester (VE) resin. Both E-glass and T700 carbon fibers are coated with VE compatible sizing. The composite panels are fabricated by the vacuum assisted resin transfer molding (VARTM), the specimens are machined, and the mechanical tests are conducted as per the accepted test standards. Tension, compression, in-plane shear, and interlaminar shear properties are measured and their associated failure modes are compared with each other. The specific properties of the composites are compared with that of the marine steel. The carbon composites have superior properties, higher specific strength, and specific modulus than the marine steel. The glass composites have higher specific strength but lower specific modulus than marine steel. The glass composites are well suited for constructing ship hulls which are only strength critical. The topside (upper) structures of the ship are stiffness critical. The carbon composites are applicable to both the topside and the hull structures of the ship to reduce the total weight. The straightness of the fiber and the FOE sizing are the possible reasons for the superior performance of the carbon composites. The predicted elastic constants based on the simple micromechanical equations of the composites agree very well with the experimental data.

Polymer Nanofabric Interleaved Composite Laminates

The concept of electrospun polymer nanofiber fabric interleaving to enhance dynamic properties, impact damage resistance, fracture toughness and resistance, and delamination onset life was evaluated. Polymer nanofabric interleaving increased the laminate thickness and weight by an order of 1%, and its impact on in-plane mechanical properties of the composite laminate would be statistically zero. On the other hand, its influence on interlaminar fracture toughness and resistance, impact damage resistance, and damping is substantial. Results of this study showed that interleaving AS4/3501-6 composite laminate increased the damping by 13%, reduced the impact damage size to one-third, increased fracture toughness and resistance by 1.5 times and one-third, respectively, significantly increased delamination onset life, and increased the fatigue threshold energy release rate by two-thirds. These improvements are comparable to that of the commercial T800H/3900-2 composite but with no thickness increase penalty, loss of in-plane properties, or multiple glass transition temperatures.

A Re-examination of DMA Testing of Polymer Matrix Composites

Dynamic Mechanical Analysis (DMA) is one of the most powerful tools to study the behavior of plastic and polymer composite materials. Unfortunately, as observed in literature and from authors experiences, there are discrepancies in the measurement of storage modulus using DMA, particularly for high modulus materials like carbon fiber composites. This is because of lack of guidelines for DMA testing of materials. This paper systematically studied the DMA testing of composites for both rectangular and cylindrical beam type specimens, identified the problems, made corrections and established simple test guidelines. Using these guidelines a wide variety of materials were tested and the test results are presented. Some of the test results were compared with the ASTM D790 bend test results.

Effect of Thermal Fatigue on Tensile and Flexural Properties of Carbon/Cyanate Ester Pultruded Composite

The effect of thermal cycling on tensile and flexural properties of Cytec T650 carbon fiber/Lonza Primaset PT-30 cyanate ester pultruded composite rods proposed to be used for brush seals in gas turbine engines was evaluated. The thermal cycle consisted of 4min each of heating and cooling with 28min of hold at 315°C and 24min of hold at room temperature. Tension and three-point bend flexure tests were conducted after thermal cycling for 100, 200, 400, 600, and 800 cycles. Thermal cycling reduced the tensile strength and fracture strain almost linearly with the number of cycles while the tensile modulus remained unchanged. The flexural modulus did not change for the first 100 cycles and then decreased as much as 28% after 800 thermal cycles. Thermal cycling increased the Tg of the composite rods from 408 to 468°C. Optical and scanning electron microscopy analysis showed fiber/matrix interfacial separation and matrix shrinkage and oxidation due to thermal cycling.

Thermomechanical and Fracture Properties of Exfoliated Nanoclay Nanocomposites

This article investigates the dispersion of nanoclay and its effect on the properties of nanoclay/epoxy nanocomposites developed using a high-shear mixing technique. Epon 828, Nanomer 1.30 E, and a compatible amine-curing agent (Epikure W) were the materials used. The dispersion of nanoclay was studied using XRD and TEM at different levels of magnification. Even though nanoclays were found to have exfoliated based on XRD’s d-spacing data (>8 nm), they remained as tactoids based on TEM data. The effect of curing on the dispersion of nanoclay was also studied using XRD analysis, which showed polymerization of resin in-between the nanoclay layers during curing. This polymerization was only helpful for the exfoliation of nanoclay, it could not completely delaminate and disperse the individual nanoclay layers. The nanocomposites were characterized using dynamic mechanical analysis and ASTM standard fracture tests. Measured elastic modulus was compared with the predicted modulus using modified Halpin—Tsai model. The experimental value was less than the predicted value and the difference was attributed to the poor dispersion of nanoclays. However, addition of 5.3 wt% nanoclay increased the storage modulus and the fracture toughness of epoxy resin by 21 % and 16%, respectively. The fracture morphologies indicated that the major toughening mechanism was due to crack deflection around nanoclay tactoids.

Assessment of mode-II fracture tests for unidirectional fiber reinforced composite laminates

Three basic mode-II test methods (ENF, JIS, and ASTM D7905M-14) are assessed using the material system AS4/8552 carbon/epoxy unidirectional composite laminate to understand similarities and differences. The modified JIS method uses a PTFE film coated stainless steel rod instead of the PTFE strip that was proposed in JIS. The ASTM D7905M-14 test method determines FEP film crack front (NPC) and shear precrack front (PC) fracture toughnesses. Alternately, wedge precracked specimens were also tested to assess the shear versus opening mode precracking on mode-II fracture toughness. The analysis and test results revealed that the JIS method is a mixed-mode I-II test and result in lower value of mode-II fracture toughness. The GI loading is about 51 J/m2 for the material tested and GIIc measured by JIS is always less than pure mode-II fracture toughness. The GIIc measured from the ASTM D7905M-14 NPC and the ENF tests are almost identical, but the ASTM test offers a compliance equation that may be beneficial in fatigue crack growth studies. As suggested in ASTM standard, shear precracked specimen is appropriate to measure mode-II fracture toughness.

Enhancement of electrical conductivity of epoxy using graphene and determination of their thermo-mechanical properties

A three-roll mill processing technique was used to disperse graphene nanoplatelets into epon 828 epoxy system. As a first step of this research, processing of graphene/epoxy nanocomposites was explored with different weight percentages of graphene. After establishing an optimal and repeatable process to achieve good electrical properties, the materials were tested for thermal conductivity and mechanical properties. The xGnP-25 graphene nanoplatelet supplied by XG Science Inc. was used; the graphene average diameter was 25 μm and thickness was 6–10 nm. Mechanical mixing, sonication and three-roll mill dispersion techniques were investigated to disperse graphene in epon 828 epoxy. The study showed that the three-roll dispersion is effective, repeatable and potentially scalable to disperse graphene into epoxy to increase the electrical conductivity. The weight percentage of graphene used ranged from 0.5 to 5.0. Percolation threshold of graphene was found to be 1.0 wt%. Through-the-thickness or volume electrical conductivity increased by nine log cycles, thermal conductivity doubled and fracture toughness increased by one-third for 1.0 wt% addition of graphene to epon 828. However, the mechanical properties remained almost unchanged.

Shear fatigue characterization of fire resistant syntactic foam core sandwich beam

Eco-Core is syntactic foam made by high volume percent of flyash and a small amount of phenolic resin. Because of very low volatile content in the mixture, it has demonstrated to be a fire resistant material for composite sandwich structural applications. Its superior mechanical and fire safety properties were established previously. Its fatigue performance under compression, shear and flexure loading are being investigated. Objective of this paper is to establish shear stress–life relationship and failure modes of sandwich beams. The Eco-Core sandwich specimens made of FGI 1854 glass/vinyl ester face sheet were designed to fail in core shear and were tested by shear fatigue loading at a frequency of 2 Hz and load ratio of R = 0.1. The Eco-Core density was about 0.5 g/cm3. The fatigue test was conducted at maximum shear stress (τmax) values of 0.7τc to 0.9τc using four-point bend load specimen, where τc is the static shear strength of the core. Shear fatigue failure of the core material was found to be three types: shear crack on-set, crack propagation and ultimate shear failure (represented by shear crack linking to top and bottom face sheets and finally interface delamination). These failures were found to be represented by 2, 5 and 7% change in compliance. The fatigue stress–life (S–N) relationship was found to follow the power law equation, τmax/τc = AoNα. Constants of the equation were established for all three modes of failure. Based on 1 million cycles limit, the endurance limit was determined to be 0.66τc, 0.68τc and 0.69τc, respectively, for damage onset, propagation and ultimate failure. The typical shear fatigue failure was a 45° crack in the core, followed by crack propagation to face sheet and finally interfacial delamination between face sheet and core.

Impact Damage Resistance and Tolerance of Polymer Nanofiber Interleaved Composite Laminates

Polymer nanofiber interleaving is a novel technology to enhance toughness of composite laminates. This paper focuses on the comparison of low velocity impact damage resistance and tolerance of base (no interleaving) and polymer nanofiber interleaved composite laminates. A 24-ply aerospace grade AS4/3501-6 Carbon/Epoxy laminate was made in an autoclave. The interleaved laminate was made by placing a layer of Nylon-66 nanofiber between the adjacent plies and at the top and bottom of the laminate. The nanofabric was made by electrospinning 12% wt. of Nylon-66 solution made by dissolving Nylon-66 crystals in a mixture of 90% formic acid and chloroform in a weight ratio of 75/25, respectively. The average areal density of the fabric was 0.7 g/m 2 and the AS4/3501-6 composite ply was 260 g/m 2 . Impacted panels were c-scanned and the measured damage of the two laminates was compared with each other. Compression was implemented to the specimens for impact test to measure the damage tolerance. Results showed that polymer nanofiber interleaving does have a potential to improve impact damage resistance and tolerance. Specifically, interleaving increased the threshold impact force by about 12% and the compression strength by about 10%.

Electrospun Nanopaper and its Applications to Microsystems

A new method of preparing Nylon-66 nanopaper using electrospun nonwoven nanofiber and fiber fusing is presented. The fusing temperature for Nylon-66 nanofiber was found to be 190°C. Both carbon and glass fiber reinforced nanopapers were prepared. The unreinforced Nylon-66 nanopaper of areal density 4.5 g/m2 had a modulus and strength of 681 MPa and 92.8 MPa, respectively, while the unfused nanopaper had 430 MPa and 59.3 MPa, respectively. This increase was attributed to fusing of randomly oriented fibers. Several types of insect wings, namely FlyTech dragonfly and Deadalus flight system wings, were fabricated and tested for their flyability. Vibration test was conducted to measure the wing stiffness by matching the measured first natural frequency to the stiffness.