Gowthaman Swaminathan

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.

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.

An Improved Method of Compression Testing of Foam Materials

This paper describes an improved method for compression testing of materials like carbon foams. In this method, the end surfaces of cylindrical specimens were coated (endpotted) with Epoxy DP460 adhesive to avoid end failure, and the test results of end-potted and not-potted specimens were compared. The proposed method prevented local crushing of cells at the loading faces and the specimens failed in a typical shear mode. The high speed photography showed a clear failure initiation, propagation along shear planes, and ultimate failure. SEM analysis showed shear-like deformation of cells along the shear plane and no deformation of cells away from the shear plane, which is typical of a brittle material. The data generated had minimal scatter.

Mechanical Characterization of Nanosilica/Epoxy Nanocomposites

Poor compression strength and interlaminar properties, which are matrix related properties, are the limitations of fiber reinforced composites. One approach to enhance mechanical as well as multifunctional properties of fiber reinforced composites is through modifying the polymer matrix by nano-sized fillers. This paper focuses on improving the mechanical properties of the matrix by reinforcing with nanosilica, while the reinforced matrix will be later used in fiber reinforced composites. The matrix chosen was SC79 epoxy and the nanofiller chosen was Nanopox F400 supplied by Nanoresins AG, Germany. Nanocomposites were made by mixing SC79 epoxy resin, Nanopox F400, and curing agent using a mechanical stirrer, followed by curing. TEM analysis showed uniform dispersion of silica nanoparticles with the size of the majority of the nanoparticles in the range of 14 to 21 nm. Dynamic mechanical analysis and ASTM standard tension, compression, shear and plane-strain fracture toughness tests were conducted. Storage, tensile, compressive, and shear modulus as well as the fracture toughness of the nanocomposites increased monotonically with the addition of nanosilica. For 20% wt. nanosilica, the improvement in tensile, compressive and shear modulus was 20%, 20% and 17%, respectively, and fracture toughness increased by 30%. The 0.2% offset tensile and compressive strengths as well as Poisson’s ratio did not change with the addition of nanosilica. The comparison of experimental tensile and compression moduli agreed well with the predicted data from Halpin-Tsai equation and also with the literature. The results presented in this paper are encouraging to use the material in fiber reinforced composites.

Experimental Investigation and Development of Guidelines for DMA Testing of Polymeric 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.

Mechanical Performance of Glass and Carbon/Vinyl Ester Composites for Marine Structures

This paper describes the processing and mechanical characterization of two different fibers (glass and carbon) and two different fabric architectures (woven roving and stitch bonded) that were made into composites using Dow Chemicals Derakane 510A-40, a brominated vinyl ester resin. Both E-glass and T700 carbon fibers were coated with vinyl ester compatible sizing. Composite panels were fabricated by vacuum assisted resin transfer molding (VARTM), specimens were machined, and mechanical tests were conducted as per the accepted test standards. Tension, compression, in-plane shear and inter-laminar shear properties were measured and their associated failure modes were compared with each other. The specific properties of the composites were compared with that of steel.