Composite Materials and Structures

Historically a predominantly aerospace material, fiber-reinforced and particle-reinforced composites are increasingly used in other industries, for example, automobile, marine transport, buildings and civil infrastructure, sporting goods, medical equipment and prosthetic devices etc. With the increased use of composite materials, there is a tremendous need to develop efficient manufacturing techniques, economical and effective repair techniques, and methods to predict the short and long-term behavior of the composite materials and structures made of these materials under a variety of loading and environmental conditions.

Impact Mechanics

Fiber reinforced composites have tremendous strength and stiffness in the plane of the fibers. However their strength and stiffness in a direction perpendicular to the fibers is governed mainly by the matrix properties. Thus composite structures perform poorly under impact loads that occur due to dropped hand tools, runway debris, and hailstone. It is important to understand the threshold impact energy that will cause impact damage and also the residual strength after impact damage has occurred. Further, based on our understanding methods need to be devised to increase the energy threshold for impact damage and the impact damage tolerance.

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Fracture Mechanics

As explained in the previous section delamination is a prevalent damage mode in laminated composite structures. Fracture mechanics principles are used in determining the loads at which an existing delamination will begin to grow and also the rate at which it will grow. The loss of stiffness and strength due to delaminations can also be estimated. Translaminar reinforcements such as stitching and z-pinning are found to be effective in improving the fracture toughness of delaminated composites. We are developing experimental and analytical methods to understand the effects of these reinforcements so that laminates with optimum reinforcements can be designed.

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Micromechanics is concerned with the prediction of various properties of a composite material system from the properties of the individual constituents and the interfaces between the various phases. Some of the properties that can be predicted easily are stiffness (elastic constants), coefficient of thermal expansion, thermal conductivities etc. Prediction of strength, fracture toughness and nonlinear material properties pose a great challenge. The methods can range from simple formulas such as rule of mixtures, to sophisticated multiscale modeling and molecular dynamic simulations.

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Textile Composites

The textile technology (technology of making cloths) is one of the oldest technology known to human beings. The basic textile processes are: braiding, knitting and weaving. Stitching can also be considered as one of these processes. Using textile processes various yarns such as graphite, glass, Kevlar© can be woven into a variety of types of preforms or cloth structures. Then these preforms can be rigidized by impregnating with matrix materials in a mold and cured to form a composite structure. Textile composites, in general, have higher impact resistance and fracture toughness because of the intertwining nature of the yarns. They can also lower manufacturing costs, as they are amenable to high volume production- unlike the conventional tape laying lamination process. The challenges in textile composites are: reliable prediction of their properties (micromechanics), draping behavior of the preforms in a mold of complex shape, and modeling the resin infiltration and curing processes.

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Sandwich Construction

Although sandwich construction has been used for hundreds of years, recent developments in light weight core and adhesives have given a new impetus to the use of sandwich construction in aerospace and marine industry. Many of the problems discussed in the previous sections are relevant to sandwich construction also. One of the major issues in sandwich plates is debonding of the face sheets, which reduces the compressive load carrying capacity. Translaminar reinforcements in the form of pins are being investigated.

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