Role of Constituents in Advanced Fiber Reinforced Polymer Composites

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Advanced composites are made of the fiber and matrix materials. The boundary where fiber and matrix come in contact with each other and form a bond is called the interface. These three constituents contribute to the overall behavior of composites. In this article, we be discussing the role of each constituent in the manufacturing of advance polymer composites. Fiber Constituent The fibers are mainly responsible for the improved properties of advanced composites. Typical fiber materials used in the manufacturing of polymer composites are glass, carbon and aramid. These are all synthetic fibers and these days the natural fibers of various types are also getting attention in various applications. Pros and cons of using the fiber form of reinforcement constituent are listed below: Pros: Superior strength of the fiber form Generally, materials are available in their bulk form that refers to their existence in a relevantly large volumes. Such finish forms of these materials are already avail...
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How to characterize composite a manufacturing process?

Keywords

Dry Spots, Fiber Orientation, Fiber Volume Fraction, Resin Rich Areas, Curing, Solidification, Dimensional Control, Coefficient of Thermal Expansion (CTE)

Introduction

This article examines the general characteristics of various manufacturing processes which involves mixing of fiber reinforcement and polymer matrix constituents to produce the composite products. These characteristics affect the quality of composite products if are not taken care of during manufacturing, as shown in Figure 1.

Figure 1. Cross-sectional view of unidirectional composite: Idealized vs real manufacturing

A brief discussion on the implications of general characteristics of composite manufacturing process is as follows:

Bonding between fibers and matrix

In order to improve the properties of composites, a proper bonding between fiber and matrix constituents is necessary. If there is no bonding in some locations, called dry spots, no load transfer between fiber and matrix will occur. Consequently, such localized weak regions may serve as nuclei for cracks to form, grow and propagate to other regions and finally leading to failure if not arrested. It is ironic that in certain circumstances such dry spots contribute positively by absorbing the impact energy.

Orientation of fibers

Mechanical properties of composites of very sensitive to the fiber orientations. If fiber orientation deviate from the intended one during layup stacking or due to the resin flow, the stiffness and strength of the product may suffer drastically. For example, a 10 degrees change in the fiber direction from 0 degrees in in unidirectional composites can lower their stiffness by roughly 30 percent.

Fiber volume fraction

In composites, the amount of fiber in the matrix is expressed in terms of fiber volume fraction. Since fibers are responsible for the improved properties of composites, therefore, higher the value of fiber volume fraction, the better will be the properties of composites. If fiber volume fraction is zero, it means that the matrix is without fiber i.e. pure matrix, and if fiber volume fraction is one, it means the dry fibers without any matrix.

Distribution of fibers in matrix

For better load transfer, the fibers should be uniformly distributed within the matrix, otherwise, there will be some regions, called resin rich areas, where there is more matrix material, and in other some regions fibers might be physically touching each other due to their higher distribution density and lack of matrix. Such weak regions can also serve as nuclei to form cracks.

Curing or solidification of matrix

In polymer matrix composites, the resin is generally a low viscosity liquid so that it can flow and wet out fibers. After the completion of wetting, the resin needs to solidify and harden. This process is called curing for thermoset resins, and called solidification for thermoplastic resins. This curing or solidification needs proper care so that the reinforcement effect can take place. Otherwise, there will be weak regions having improperly cured or solidified resins, which may serve as nuclei to form cracks.

 Void content

During manufacturing of composites, the voids and defects can be formed due to lack of compaction between layers, low pressure caused by resin flow, or trapped air etc. It should be keep in mind that it is not possible to make composite 100 percent free of any void or defect. Therefore, the amount of voids needs to be within an acceptable value. Generally, for a good manufacturing process, the void content should be less than 1 percent.

Dimensional control of final product

After completion of wetting the fibers, the liquid polymer resins solidify which introduces 5 to 8 percent shrinkage depending on the type of polymer matrix material. This shrinkage produces residual stresses which may lead to warping i.e. out-of-plane distortion, of composite products. Warping in composite products can also occur due different coefficient of thermal expansion (CTE) of fiber and matrix materials.

For small size products, this effect might be hard to notice, but, large size components like wind turbine blade, a visible deflection of structure can occur. Shrinkage can be reduced by adding special additives in polymer resins, while, warping due different CTEs be controlled by optimally designing the layup in such a way that the warping in each layer may conceal out the effect of other, consequently, leading to the dimensional stable composite product.

Summary

No composite manufacturing process is free of defects like dry spots, fiber disorientation, non-uniform distribution of fibers, resin rich areas, improper curing or solidification, delamination between layers, voids, or shrinkage etc. However, if these defects are not within an acceptable limits, then, the quality of composite product is compromised drastically. The defects will be the epicenter of stress concentrations and crack formation in composites, which may lead to the premature failure.

About the Author: 
Dr. Khazar Hayat is a professional engineer with almost 15+ year of experience in research, design, analysis and development of products made of fiber reinforced plastics composites (FRPCs). Currently, he is working as an Associate Professor at Mechanical Engineering Department, The University of Lahore, Pakistan, can be reaching by emailing at khazarhayat@gmail.com.

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