Selvum Pillay

Design and Manufacture of Woven Reinforced Glass/Polypropylene Composites for Mass Transit Floor Structure

Recent developments in hot-melt impregnation, extrusion, and thermoforming offer multiple avenues for the design and manufacture of low-cost thermoplastic composites for mass transit and automotive applications. Fiber reinforced thermoplastics such as glass/polypropylene (glass/PP) have found their applications in front-end bumper beams, under body shields, and other automotive applications. These materials also have potential usage in mass transit vehicles, such as buses. The present study focuses on the design and manufacture of a segment of the floor of a mass transit bus using glass/PP woven tape forms developed through a hot-melt impregnation process. An initial study on the existing mass transit buses found that a metal skeletal frame and plywood panels are used in the construction of the floor structure. The representative thermoplastic composite floor segment featured a glass/PP woven tape material, belt-pressed to form a flat laminate, and adhesively bonded to a vacuum thermoformed ribbed laminate. A combination of analysis software including Pro/Engineer, Hyper Mesh, and ANSYS 7.0 were used for the design and analysis. Weight savings up to 40% were realized using glass/PP woven tape thermoplastic composites as compared to the conventional metal/ plywood design

Impact Damage of Partially Foam-filled Co-injected Honeycomb Core Sandwich Composites

The present study considers filling of honeycomb type cores with foam to produce sandwich constructions. The potential benefits of this approach are enhancement of damage resistance, and ability to process honeycomb type sandwich structures through cost-effective vacuum assisted resin transfer molding (VARTM). As weight penalty is incurred in complete filling of honeycomb cells with foam, an alternative approach to reduce weight is partial filling of the cells, without losing the advantage of VARTM processing of the core. Two cores are considered, a polyurethane foam for full filling of honeycomb cells, and syntactic foam for partial filling, in conjunction with carbon–epoxy facesheets. Their impact response was investigated under low and high velocity impact (LVI and HVI respectively). For both cores, the foam filling was found to provide confinement to the cells. The resistance to penetration, energy absorbed and damage modes in LVI and HVI were a function of core stiffness, extent of filling and number of facesheet plies. The results illustrate that partial syntactic foam filled sandwich plate (with reduced weight penalty in comparison to full filling) can provide LVI response improvement in the order of 56% increase in peak load, and for HVI about 74% improvement in ballistic limit.

Processing and characterization of nanographene platelets modified phenolic resin as a precursor to carbon/carbon composites (part II)

Nanocomposites were formed by curing the dispersion of carbon nanofillers—nanographene platelets and vapor grown carbon nanofibers—in resole type phenolic resin. X-ray diffraction, fourier transform infrared spectroscopy, scanning electron microscopy, thermal expansion, and thermogravimetric and flexure testing were carried out to study the morphology and thermal and mechanical properties of the manufactured nanocomposites. The coefficient of thermal expansion decreased by 15.36% (73.83 µm/m℃) and 14.23% (74.81 µm/m℃) with 1.5 wt% nanographene platelets and 1.5 wt% vapor grown carbon nanofibers, respectively, compared with neat phenolic (87.23 µm/m℃) in the temperature range 60–80°. The flexure strength of neat phenolic resin increased by 31.62% (48.57 MPa) and flexure modulus by 42.23% (2.9 GPa) at 0.5 wt% nanographene platelets. Comparatively, vapor grown carbon nanofibers at 1.5 wt% increased the flexure strength by 14.3% and flexure modulus by 23.5%. Nanographene platelets and vapor grown carbon nanofibers increased the char content of neat phenolic resin. The char content increased by 200% at 800℃ in 5 wt% nanographene platelet nanocomposites, compared with 75% increase in 3 wt% vapor grown carbon nanofibers nanocomposite. Nanographene platelets were more effective than vapor grown carbon nanofibers in lowering the coefficient of thermal expansion of neat phenolic, in improving its flexure strength and modulus and in increasing the char yield. The results indicate that nanographene platelets can be effectively used as carbon nanofiller in the manufacture of carbon/carbon composites.

Design and Development of a Test Fixture and Method for Investigation of Impact During Pre-Stressed Compression

As composites become more integrated into large structures and vehicles, there is a need to design for laminate failure associated with realistic combined loading scenarios. Because a structural composite is typically brittle, impact damage has been extensively studied and residual strength has been evaluated post impact damage. Few studies have been conducted to determine the effects of pre-stressing a composite during impact. In the case of in-plane compression during impact, it has been found that there is a synergistic effect which causes more damage than occurs with a standard compression-after-impact (CAI) test. When compressive pre-stress greater than 70 MPa was applied to 150mm×100mm ×5.4mm glass fiber/vinyl ester laminates, an impact initiated shear crack (IISC) developed perpendicular to the applied load leading to failure. To accommodate designers concerned with compressively loaded composite structures that may endure an impact event, a test method has been established to conduct compression during impact (CDI) testing. This paper has reported the design, construction, and implementation of an innovative test fixture that enables investigation of failure mechanisms in the CDI test mode.

Fiber content measurement for carbon fiber–reinforced thermoplastic composites using carbonization-in-nitrogen method

Carbon fiber–reinforced thermoplastic composites are gaining increasing interest in various applications thanks to their combined properties of high specific stiffness, high specific strength, and superior toughness. Their mechanical properties are highly dependent on the carbon fiber content. In this study, the carbonization-in-nitrogen method (CIN) developed in previous work is used to measure the fiber content of carbon fiber thermoplastic composites. Three types of carbon fiber thermoplastic composite samples were prepared using hot-melt impregnation. The carbon fiber thermoplastic composite sample is carbonized in a nitrogen environment alongside a neat resin sample that is used for calibrating the resin carbonization percentage. A good agreement is achieved between the nominal carbon fiber content and the carbon fiber content measured using the CIN method. It is concluded that the CIN method is an accurate and efficient way to characterize the carbon fiber content for carbon fiber thermoplastic composites. This work completes the verification of the CIN method, which enables extended application to thermoplastic composites. Moreover, it has its unique merits on evaluating the carbon fiber content for high-temperature and solvent-resistant thermoplastic composites that would encounter challenges using other methods.

Colored inorganic-pigmented long-fiber thermoplastics

Long-fiber thermoplastic (LFT) composite materials are rapidly expanding in automotive, transportation, and recreational industry. Most of these materials are natural or black in color with a need for secondary painting of the manufactured products. Standard organic pigments and dyes are not stable above 250°C and degrade during processing. Alternatively, inorganic pigments are thermally stable to at least 800°C. High-performance inorganic pigments offer resistance to outdoor weathering, chemicals, and acids. However, in fiber-reinforced composites, the pigment causes fiber attrition and thereby shows reduction in strength. This work explores colored inorganic-pigmented LFT composites. The ability to integrate the color in the manufacturing steps eliminates the need for secondary painting. Pigment variables such as particle size, distribution, chemistry, and coatings have been investigated. The article presents the processing and performance envelopes of colored inorganic-pigmented LFTs in comparison with unpigmented standard LFTs.

Processing and Characterization of Continuous Fibre Tapes Co-Moulded with Long Fibre Reinforced Thermoplastics

An important advantage when designing with plastics is the ability to incorporate features such as ribs, grids and bosses in the part. Rib stiffened polymer matrix composites have been widely used in aerospace, automobile, and civil infrastructure applications due to their high impact and fatigue resistance, high strength and stiffness to weight ratio, and damage tolerance. However, for long fibre-reinforced polymer composites, the processing complexity increases for features including ribs, grids and bosses. An innovative method of replacing ribs is the use of pre-consolidated continuous fibre reinforced thermoplastic (CFRT) tapes that can be co-moulded with long fibre thermoplastics (LFT). This work focuses on processing and performance evaluation in terms of the static and dynamic properties of LFTs co-moulded with pre-consolidated CFRTs, referred to as endless long fibre thermoplastic (E-LFT). The E-LFT approach is an alternative to rib stiffened composites. The effect of the thickness and fibre type in tape reinforced LFT is compared to LFT (with and without ribs) of equivalent flexural rigidity for static flexure and low velocity impact (LVI) response. In all these conditions, E-LFT samples performed better that the LFTs with and without ribs. LFT samples with and without ribs exhibited a brittle failure, as opposed to the progressive failure exhibited by E-LFT.

Compressively Pre-stressed Navy Relevant Laminated and Sandwich Composites Subjected to Ballistic Impact

Assembled structures such as ship decks, walls, and masts are often times under different degrees of pre-stress or confinement. Structural composite integrity can be compromised when subjected to impacts from events such as wave slamming, tool drops, cargo handling, and ballistic fragments/projectiles. It has been shown by several researchers that when a highly pre-stressed structure is subjected to impact, the damaged area and impact response changes. The main focus of this study was the impact of compressively pre-stressed structures which can also be considered as compression-during-impact. The results showed that for various laminate configurations, there was a compressive pre-stress threshold above which impact damage caused more damage than witnessed in typical compression after impact (CAI) tests. Both fiberglass and carbon laminates pre-stressed to higher than 30% of ultimate compressive strength, failed from impact at 300 m/s; but the carbon laminates developed shear cracks above 10% of the ultimate compressive strength. The work is of benefit to naval and other composite designers to be able to account for failure envelopes under complex dynamic loading states, i.e. pre-stress and impact for various composite configurations.Book chapter for paper presented at Office of Naval Research (ONR) Workshop/Conference, June 23–24, 2011, Instit Clement Ader (ICA), Toulouse, France, Organized by – Serge Abrate, Bruno Castanie and Yapa D.S. Rajapakse.

Characterization of discontinuous carbon fiber liquid molded PA-6 composites via strategic placement of additional reinforcements

The flexibility of processing PA6-based discontinuous carbon fiber panels using vacuum-assisted resin transfer molding was studied. The ease of incorporating various reinforcements namely baseline, tow in the center of preform, fabric in the center of preform and fabric on the outside as skin was investigated. Mechanical characterization was conducted on all the variations made. There was an average increase of about 3%, 20% and 47% in the tensile properties of tow in the center, fabric in the center and fabric on the outside as skin, respectively, as compared to the baseline. A similar increase in properties was noticed in its flexural and impact strength. The data showed a correlation between the mechanical properties and the total surface area of additional reinforcements used. As the surface area of the reinforcement increased, the mechanical properties increased as well. It also showed that reinforcements on the surface of the preform as a skin performed the best. DMA analysis showed the effect of reinforcement on the storage modulus and tan delta across temperatures ranging from 30°C to 150°C. SEM analysis showed that the fibers and the additional reinforcements were coated with PA6 which translated into consistent mechanical performance.

Compression molding of algae fiber and epoxy composites: Modeling of elastic modulus

The demand for natural fiber composites in the automotive industry in both Europe and the United States has been forecasted to increase in the coming years. The natural fiber composites based on highly commercialized fibers such as flax, hemp, and sisal has grown to become an important sector of polymeric composites. However, little attention has been addressed to expanding natural fiber composites to include new sources of emerging natural reinforcements, such as reclaimed algae fibers, that have a multiple environmental benefits. Not only are extracted algae fibers biodegradable, the reclamation process has the added benefit of restoring health of waterways choked with algae. This study focuses on the processability of algae fiber–epoxy composites. Short fibers, chemically extracted from raw reclaimed algae, were prepared for natural fiber composite products in two ways. First, randomly oriented mats were produced using the wet-laid process to create layered, compression-molded laminates. Second, loose fibers were dispersed directly into the thermoset matrix to produce a bulk molding compound that was further compression molded into composite lamina. The effect of processing variables such as compaction pressure, temperature, and time were addressed. Moreover, the effect of fiber volume fraction (υf) and fiber form were considered. Enhanced mechanical properties were found when 56% υf algae fiber was used for the compression-molded laminates composite. This variant exhibited an improvement on the flexural and tensile modulus of 70% and 86% when compared to the neat epoxy. However, the volume of porosity on the same variant was 11% due to lack of compression in some of the fibers. The effect of porosity on the theoretical stiffness was estimated by using the Cox–Krenchel model. Furthermore, an empirical exponential model was formulated to characterize the multi-scale effect of compaction pressure on the overall fiber volume fraction, υf.