Exploration of natural and polymer based composites for advanced engineering design

Abstract

Light weighting is an important requirement in design today to save energy. For example, reduction in the weight of a vehicle can lead to improved fuel economy. Concomitantly, there is a great need for choosing lightweight materials such as plastics, particulate and fiber-reinforced materials, etc., which are easily recyclable or disposable. It needs to be pointed out that components made of lightweight materials may be subject to static and dynamic loads in operation, and hence, the materials used should have adequate stiffness and strength, and perhaps special attributes such as energy absorption capability when impact safety is also a design criterion. It is apparent, therefore, that for selection of a suitable material in design, its mechanical characterization, under both static and dynamic loads, should be carried out. Keeping in mind issues such as eco-friendliness, lightweight, and cost, various composite materials such as wood, wood-polypropylene composites, jute-polyester laminates and their hybrids, and nanoclay-polypropylene composites have been studied. In addition to being able to select a material for a given application, its constitutive behavior through the nonlinear range should be known so that numerical modeling, which is a powerful tool in design, can be effectively deployed.

With the broad objectives stated above, various engineering properties of the composites mentioned are determined through extensive testing. It needs to be highlighted that adequately defining a failure model such as the Tsai-Wu criterion for anisotropic materials poses significant challenges and requires information on tensile, compressive, and shear strengths, including off-axis properties. Wood, a highly anisotropic and a natural composite, is studied. Mechanical properties such as tensile, compressive, and in-plane shear properties of Acacia auriculiformis were determined. A modified coupon testing procedure has been developed for directly determining shear strength of a material. The Tsai-Wu failure criterion was used to predict the normal and shear strengths in various off-axis directions and compared it with experimental results.

Environmental inputs can improve the level of innovation by interconnecting them with traditional inputs regarding the properties of materials and processes as a strategic eco-design procedure. Advanced engineered polymer composites are needed to meet the diverse needs of users for high-performance automotive, construction, and commodity products that simultaneously maximize the sustainability of forest resources. In this context, wood polymer composites (WPCs) are studied to promote long-term resource sustainability and to decrease environmental impacts relative to those of existing products.

Extensive studies have been conducted on jute-polyester laminates fabricated in-house with biodegradable jute mats, and the dependence of mechanical properties on volume fraction of fibers/matrix has been identified. Although jute-polyester composites have been studied in the past, the effect of adding steel wire mesh (SWM) to such composites on mechanical properties of the resulting hybrid composites is being reported for the first time in the present work. With the stated objective, a comparative study has been carried out between jute-polyester and jute-SWM-polyester composite laminates with similar volume fractions of the polyester resin. Key mechanical properties such as stiffness modulus and strength (tensile, flexural, and compressive) were determined experimentally. It has been found that jute-SWM-polyester hybrid composites have superior properties compared to jute-polyester composites and hence, can be a cost-effective enhancement to jute-polyester composites for design applications.

The effect of varying the proportion of modified nanoclay in a nanocomposite with a thermoplastic polypropylene (PP) matrix with and without a compatibilizer is shown. Nanocomposites were prepared using equipment such as a torque rheometer, twin-screw extruder, and injection molding machine. Quasi-static (tensile, flexural, and compressive) tests and viscoelastic tests are conducted on specimens of nanocomposites. Dynamic Mechanical Analysis (DMA) tests were carried out to investigate the viscoelastic behavior of virgin polypropylene and nanocomposites. The dynamic mechanical properties such as storage modulus (E'), loss modulus (E"), and damping coefficient (tan ) of PP and nanocomposites were investigated with and without compatibilizer with varying temperature and frequencies. The experimental data obtained here indicate that the compatibilizer used leads to an under-performing nanocomposite compared to the nanocomposites without the compatibilizer.

Finite element modeling can be a useful tool for predicting the behavior of composite materials and arriving at a desirable filler content for maximizing mechanical performance. Micromechanical probabilistic finite simulation is carried out to corroborate the quantitative information on the effect of reinforcing polypropylene (PP) with various proportions of nanoclay (in the range of 3-9% by weight). Micromechanical finite element analysis combined with Monte Carlo simulation has been carried out to establish the validity of the modeling procedure and accuracy of prediction by comparing against experimentally determined stiffness moduli of nanocomposites. In the same context, predictions of Young’s modulus yielded by theoretical micromechanics-based models are compared with experimental results.

Advanced finite element modeling of nanocomposites is carried out, as it is difficult to predict the complete stress-strain behavior of any material, including the current nanocomposite, purely based on micromechanics. A macromechanical approach in which the nanocomposite is assumed as grossly homogeneous is employed using the Von Mises yield condition to reproduce the average true stress versus true strain behavior obtained in uniaxial tensile tests of nanocomposite samples of a given nanoclay content. The explicit FEA solver LS-DYNA has been used for the latter purpose. It is recognized that such an approach can be useful for performance prediction in design applications such as energy-absorption under impact loading in which material failure can be expected. Finite element modeling was done to capture the nonlinear stress-strain behavior including failure observed in experiments, as this is deemed to be a more viable tool for analyzing products made of nanocomposites, including applications of dynamics. Based on a novel combination of compression tests at low strain rates, a relation between composite strength and strain rate has been proposed. A new methodology for predicting the stress-strain behavior at a higher strain rate has been explored by using empirical relations. The effect of strain rate on strength is captured in a finite element model for analysis using an explicit code with contact simulation capabilities. Additionally, the enhanced performance of a vehicle body trim made of a nanocomposite covering a conceptual A-pillar in head impact safety protection is demonstrated using LS-DYNA.

It is necessary to choose the various materials available today and the new materials, for a particular application on a definite criterion. These criteria may include eco-friendliness, lightweight, energy-absorbing capabilities, strength to weight ratio, or strength to stiffness ratio. Stiffness to density ratio and stiffness to strength ratio of the various studied composites are used to compare among themselves and as well as with other known materials. A comparative study of nanocomposites, wood polymer composites, and jute-based composites have been carried out in terms of their strength and stiffness relative to density and using a proposed non-dimensional quantity characterizing resistance to impact perforation.