The Fiber role
The fibers in a composite system are the main load carrying member and are responsible for such structural properties as tensile strength and stiffness. Almost all fibers in use are solid and have a circular cross section, generally, the smaller diameter the greater strength of the fiber.
Raw reinforcements have the appearance of simple fabrics or textiles, these textile forms associated with resin systems that develop greater strength than materials such as metal or wood, but are much lighter. The mechanical properties of the fiber/resin composite are therefore dominated by the contribution of the fiber to the composite.
The four main factors that govern the fiber’s contribution are:
- The basic mechanical properties of the fiber itself
- The surface interaction of fiber and resin
- The amount of fiber in the composite
- The orientation of the fibers in the composite
Basic properties of commonly used fibers and other engineering materials:
Material (Std)
Tensile Strength
(GPa)
Tensile Modulus
(g/cc)
Typical density
Modulus
Elongation to break
(%)
Carbon fiber
3500
400
1,8
1,5
Aramid / Kevlar
3100
120
1,5
2,5
Glass fiber
2400
85
2,5
4,8
Aluminium
400
1069
2,7
na
Steel
450
200
7,8
na
Stainless steel
800
196
7,8
na
Note: Properties of common materials; all data to be used as a guide only; not a specification.
Simple Fiber comparison:
Carbon
Aramid / Kevlar
Glass
Tensile strength
A
A
C
Tensile modulus
A
B
C
Compressive strength
A
C
B
Compressive modulus
A
B
C
Impact strength
C
A
B
Interlaminar shear
A
B
A
Density
B
A
C
Tension fatigue
A
B
C
A = Best B = Average C = Worst
The Resin role
The main purpose of the resins in fiber reinforced composites, sometimes referred as a “polymers”, is to bind the reinforcement (fibers) together enabling the transfer of loads between fibers, and to protect the flaw- or notch-sensitive fibers from self-abrasion and externally induced scratches. They can be classified under two types, thermoplastics and thermosetting according to the effect of heat on their properties.
Other properties of the resins:
- Protects fibers from environmental moisture and chemical corrosion or oxidation
- Keeps the reinforcing fibers in proper orientation to carry the intended loads
- Distributes loads though the laminate
- Provides resistance to crack propagation
In composites the most common resins used are polyester, vinylester and epoxy. These thermoset resins cannot be melted or reshaped again after the cure by the application of heat and pressure.
Simple Resin comparison:
Polyester
Vinylester
Epoxy
Mechanical properties
C
B
A
Delamination resistance
C
B
A
Price
A
B
C
A = Best B = Average C = Worst