Role of Constituents in Advanced Fiber Reinforced Polymer Composites

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 available in market. The rule is that higher the volume of a material during its manufacturing, the more likely is that it may contains defects. These defects weakens the material, so lowering its strength.

On the contrary, if the same material is manufactured in the fiber form which is of relatively small volume, then, there is relatively lower chance of defects, as a results, the fiber form demonstrates superior strength properties to that of its bulk form. For example, the moduli of glass in its bulk form i.e. plate (70 GPa) and its fiber form Eglass fiber (72 GPa) is almost the same, but, the strength of Eglass is roughly 3500 MPa while that of the glass plate is 70 MPa only, a huge improvement in the strength.

Reinforcement effect

The properties of composites depend mainly on the type of reinforcement material that can be in the form of continuous fibers, short fibers or particulates. Each form contributes the reinforcement effect of various degrees in the composites. The reinforcement effect is governed by the aspect ratio of a reinforcement, which is a ratio of its length to diameter dimensions. In case of continuous fibers, the aspect ratio is relatively high, as the length of fibers ranges from few millimeters to a few meters, while, their diameter is roughly 10 micrometer. Due to the highest aspect ratio, the fiber form of reinforcement is used in advanced composites. Then, comes the aspect ratio for short-fibers, and finally the particulates which have the aspect ratio varying from 2 to 3.

If aspect ratio of fibers is higher than a critical value, then, the failure will be due to the fiber breakage, otherwise, there will be slippage between the fibers and the matrix which usually happens in the short-fiber composites. For particulates composites, owning to relatively very low aspect ratio, no significant improvement in mechanical properties occurs. The particulates are merely used as fillers or additives to enhance non-mechanical properties of composites.

Ease in forming

Fibers possess high stiffness and strength, and complex shapes can be manufactured from them with ease which might be challenging to fabricate with the bulk form of materials otherwise. Fibers can be bent elastically around short edges and corners without breakage. For example, if a glass fiber having 10 um diameter is bent around at 2.54 mm sharp radius, the axial strain produced can be estimated as following relation based on elastic assumption:

The computed axial strain is 1.97x10-3 which is still far less than its typical failure strain value of 0.05 or 5%.

Use of advanced fabrication techniques

The fiber forms permit various fiber processing steps e.g. vapor deposition, oxidation etc. which are difficult to apply on materials in their bulk form. Consequently, we are able to achieve unique properties of the same materials which were not possible previously.

Cons:

Need of large number of fibers

Fibers possess small diameters. For example, the glass fiber has a diameter of 10 micrometer. It means that if we want to manufacture a part having 1 mm thickness, then, we need millions of fibers to achieve that thickness. Besides, fibers tend to curl and get entangled easily. So we need to align them by using a certain amount of tension in order to realize their properties effectively. Special manufacturing methods and skillsets are needed to accomplish this.

Variation in Fiber Strength

There is a significant variation in the strength properties of fibers. The strength of a fiber is sensitive to its material microstructure, the amount of defects incurred, the fiber handing and alignment during manufacturing.

Fiber Volume Fraction

Fibers is responsible for the improved mechanical properties of advanced composites. So, more and more fibers should be added in the matrix as possible. The amount of fiber is a composite is expressed in terms of fiber volume fraction, which is a ratio of the volume of fiber constituent to the volume of composite. The modulus of the composite along the fiber direction directly proportional to the fiber volume fraction as following:

For simple estimation of fiber volume fraction, a simple square array of fibers in the matrix is assumed to replicate the distribution of fibers in the matrix, as shown in Figure 1.

Figure 1. A square array of fibers in the matrix.

Some may assume another type of fiber packing e.g. hexagonal array as shown in Figure 2 that may better represent the fiber distribution. However, the idealized fiber volume fraction can be estimated in a similar fashion as done for the square array. The highest fiber volume fractions of value 0.875 and 0.907 are achieved for the hexagonal arrays with open packing and close packing.

Figure 2. Hexagonal arrays of fibers with (a) open packing and (b) close packing.

In actuality, the fibers are randomly distribution inside the matrix, as shown below Figure 3, where the white dots represent the fibers and the dark dots represent the matrix. The fiber distribution in different regions may follow different kind of packing arrangement.

 Figure 3. The cross section of a unidirectional composite.

Due to manufacturing constraints, the attainment of very high fiber volume fraction is not possible. Generally, a fiber volume fraction of 0.68 in case of hand-layup with autoclave molding, and of 0.70 in case of pultrusion can be reached.

Interfiber spacing is another related factor to the fiber volume fraction, which represents the gap between advancement fibers. For square packing of fibers, it can be estimated with relation shown in Figure 1. For example, for a glass fiber with diameter of 10 micrometer and with a fiber volume fraction of 0.60, the computed interfiber spacing is 0.74 micrometer. In real manufacturing, its values varies, and that is why it is expressed in an average value. A low average value of interfiber spacing means that many fibers are touching each other which can cause the following:

• The stress concentration occurs due to direct touching of fibers. It influence mainly the matrix dominated properties.
• It reduced the pressure on the resin needed during its proper curing leading to potential voids as shown in Figure 4a.
• It affects the drape, a term used to define the ease with which a fiber fabric can easily conform to the shape of mold. This occurs due to increase the shear resistance as packed fibers are in contact with each other.
• It lowers the permeability a term used to describe the resin penetration into the fiber bed. The permeability of resin to the fiber bed is directly proportional to the square of interfere spacing. Similar to anisotropic properties of fibers, the anisotropic behavior of liquid resin in terms of permeability into interfiber spacing is evident during manufacturing. Generally, the ratio of axial to transverse permeability (i.e. S11/S22), as shown in Figure 4b, is of same order as that of axial to transverse moduli of fibers (i.e. E11/E22).

Figure 4.(a) Fibers touching each other during manufacturing, (b) Anisotropic permeability of liquid resin into fiber bundle.

Matrix Constituent

Align Fiber

Fibers get entangled easily during handling. Besides, to manufacture a component with a certain dimension we need to lay fibers in certain orientations. We need the matrix material which not only binds these individual fibers but also keep them align in the require orientations.

Load transfer between fibers

Apart from binding the fibers, the matrix also transfers the load from one fiber to adjacent fibers. During this load distribution among fibers, the shear strength of matrix material plays a key role. For example, the tensile strength of epoxy matrix is 35-130 MPa, while, its shear strength is 20 MPa only.

Assist fiber subjected to compressive loads

Fibers are long and slender. It act like a string. The individual fiber can withstand tensile load, but, do not resist the compressive load and buckle easily. The matrix sounding the fibers, keep them align to bear the compressive load and contribute to the compressive properties of composites.

Enhance Shear Properties

In the absence of matrix material, the dry fibers can relatively slide, thus, offering no shear resistance. The matrix material not only provide shear load transfer between fibers but also contribute to shear properties of composites.

Environment protection

Fibers are prone to environment effects. For example, the moisture can adsorb to the surface of glass fibers, and a long term exposure causes cracks leading to their breakage. The matrix can provide the protection to fibers from hygro-thermal effects which significantly enhances the durability of composite products.

Interface constituent

The bond between the fibers and matrix constitutes the interface between them. In order to build this interface, there are two main requirements from manufacturing perspective.

Wetness of fiber surface

For good interface bonding, the liquid resin have to wet out completely the surface of individual fiber. Several methods has been designed to do so, which we know as various manufacturing processes. The bulk flow of liquid resin under the influence of pressure is affected the fiber architecture i.e. woven fabric or braided fabric etc., however, the final flow through the fiber bundle to wet out the surface each fiber is like flow through an anisotropic porous medium can be modelled using Darcy’s law. The flow of liquid resin into fiber bundle is governed by the viscosity of liquid matrix, permeability of fiber bed and amount of pressure applied to move the liquid resin through some distance.

Compatibility of fiber and matrix materials

There should exist the thermodynamic compatibility between the solid fiber and matrix materials. In simple terms, low surface energy liquids wet out high surface energy solids. This principle limits the possible combinations of material systems that can be used to form composites. Since, the liquid polymers have the lowest surface energy than that of most of solid fibers, therefore, we find more polymer composites than that of the ceramic or metal composites.

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.