Module 1: Introduction to Composites
  Lecture 3: Reinforcement: Materials
 


Cross Sectional Shapes of Fibres

The cross sectional shapes of fibre of various types we have studied above are different. The cross sectional shape of the fibres, although is assumed to be circular, is not circular in general. The various cross sectional shapes of the fibre are shown in Figure 1.10.  

Figure 1.10: Cross sectional shapes of fibres

Fiber Properties

The following are the important points regarding the fibre properties.

  • Density, axial modulus, axial Poisson’s ratio, axial tensile strength and coefficient of thermal expansion are some of the important properties.
  • Advanced fibers exhibit a broad range of properties.
  • Properties of carbon fiber can vary significantly depending upon fabrication process.
  • For the advanced fibres studied above one can attain either high modulus (> 700 GPa) or high strength (> 5 GPa) but not both attainable simultaneously.
  •  SCS-6, IM8, boron and sapphire fibers offer the best combination of stiffness and strength but have large diameters and thus limited flexibility. However, IM8 fibers are exception for flexibility.
  • The specific stiffness of some of these fibres is almost 13 times of structural metals.
  • Similarly, the specific strength of some of these fibres is almost 16 times of structural metals.
  • Weight saving, when the composites of these fibres are used, is tremendous due to high specific stiffness and strength.
  • Actual properties of composite (fiber + matrix) are reduced.
  • Specific properties are reduced even further when the loading is in a direction other than the length direction of fibers.
  • Tailorable properties.
  • One can get the desired heat transfer or electrical conductivity with proper designing.
  • The increased fatigue resistance is attainable with the use of these fibre composites.
  • Aging effect can be significantly lowered.

Note: The fibres are classified based on their values of modulus as follows:

  1. Ultra-high-modulus, type UHM (modulus > 450 GPa)
  2. High-modulus, type HM (modulus between 350-450 GPa)
  3. Intermediate-modulus, type IM (modulus between 200-350 GPa)
  4. Low modulus and high-tensile, type HT (modulus < 100 GPa, tensile strength > 3.0 GPa)