**2. Characteristics of composites**

Based on the classification of composites, we are already familiar with the fact that there exists a myriad of different types of these materials. It is a common saying that different types of composites differ in their performance. Yet, composites also have some characteristics in common. Grace to their inherent beneficial characteristics, polymer matrix composites have developed to the fastest emergent and most extensively used composites. Compared with well-established materials like metals, polymer matrix composites display particular characteristics as follows:

**1.** High specific strength and high specific modulus

The most important benefits of polymer matrix composites are their high specific strength and high specific modulus. Specific strength is defined as the ratio of strength to density, while the specific modulus is the ratio of modulus to density; in both cases, length is the corresponding dimension/unit. Under the premise of equal mass, these parameters are tools to quantify the material's bearing capacity and stiffness properties, which are very significant for aerospace structural materials. **Table 1** provides an overview of values for specific strength and specific modulus of several common structural materials; it is shown that carbon fiber resin matrix composites generally show higher specific modulus and specific strength. The high specific strength and high specific modulus of composites can be explained by the high performance and low density of reinforcing fibers. As a result of relatively low modulus and high density of glass fibers, the specific modulus of the glass fiber resin matrix composites is slightly lower than measured for metallic materials.

**2.** Expedient fatigue resistance and high damage resistance

The fatigue failure of metallic materials is frequently of no apparent warning to the strikingness of damage. The fiber/matrix interface in composites can avoid crack propagation. The fatigue failure always starts from those links of fibers prone to break. Crack growth or destruction propagates gradually for a long time; hence, there is a substantial forerun before


the performance of the final product—whether it will meet the designated function and performance requirements. Inappropriate, less suitable materials can cause the following

Introductory Chapter: Background on Composite Materials

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Material selection is a multifaceted, complex process, which needs to address various factors

• The possible environmental impact of material selection, which depends on the production

The functionality of the product (based on its individual components), structure (its shape), material, and technology closely interacts and cannot be regarded independently from each other; consequently, the selection of material cannot be done independently of the

• The choice of most suitable, individual components regarding their future performance.

• Assessment of the compatibility of components—each phase of the composite material has to preserve its beneficial features; the individual components must not have a negative

• Determining an applicable geometrical form for each phase—while the stronger parts (fibers, strips, belts, etc.) need to be elongated, the weaker phase should wrap the stronger

• Composite phases should be distributed in a way which enables them to function in a

• Knowledge on the conditions in which the future composite will function in praxi, such as

one and bring individual fibers closely in contact within a single structure.

• Technological problems occurring during the production process

• Increase of production costs, consequently higher price of the final product

• Demand for energy and raw materials (material intensity of the process)

and consumer cycle (cradle-to-grave life cycle)

○ The interrelations of material, technology, and product:

○ Parameters and needs during product design/development:

temperature, humidity, pressure, abrasion, etc.

• Accessibility to material recycling:

effect/damage on each other.

synergistic way.

impairments:

such as:

• Negative ecological impact

○ Selection of material

• Material expenses

• Production cost

technology.

**Table 1.** Specific strength and specific modulus of some commonly used materials and fiber composites [10].

the onset of the final destruction. As it is visible from the S-N curve of fatigue properties, fatigue strength of the majority of metallic materials amounts to only 30–50% of tensile strength, while this value increases to 70–80% for carbon fiber/polyester composites; for glass fiber composites, the percentage is between these two examples.

**3.** Good damping characteristics

The natural vibration frequency of forced structures relates to the structure shape itself and is also proportional to the square root of the specific modulus of structural materials. Consequently, composites have a high natural frequency, and generation of a resonance in general is not easy. In parallel, the fiber/matrix interface in composites very easily absorbs vibrational energy, which results in a high vibration damping of these materials. In case vibrations occur, they can easily be stopped [10].

	- Fiber matrix and other raw materials can be selected according to the utilization conditions and performance requirements of the product; hence, tailor-made material can be designed on demand.
	- Molding processing techniques can be applied according to the size, shape, and number of the product.
	- Integrated molding can decrease the number of individual parts, which saves time and material and reduces weight.
