Reinforcing fibers for structural composites
Principal fibers in commercial use for production of civil engineering applications, including composite-reinforced concrete, are glass, carbon, and aramid.
The most common form of fiber-reinforced composites used in structural applications is called a laminate.
Laminates are made by stacking a number of thin layers (laminate) of fibers and matrix and consolidating them into the desired thickness.
Fiber orientation in each layer as well as the stacking sequence of the various layers can be controlled to generate a range of physical and mechanical properties.
A composite can be any combination of two or more materials so long as there are distinct, recognizable regions of each material. The materials are intermingled.
There is an interface between the materials, and often an interphase region such as the surface treatment used on fibers to improve matrix adhesion and other performance parameters via the coupling agent.
Performance of the composite depends upon the materials, of which the composite is constructed, the arrangement of the primary load-bearing reinforcing fiber portion of the composite, and the interaction between these materials.
The major factors affecting performance of the fiber matrix composite are; fiber orientation, length, shape and composition of the fibers, the mechanical properties of the resin matrix, and the adhesion or bond between the fibers and the matrix.
A unidirectional or one-dimensional fiber arrangement is anisotropic.
This fiber orientation results in a maximum strength and modulus in the direction of the fiber axis.
A planar arrangement of fibers is two-dimensional and has different strengths at all angles of fiber orientation.
A three dimensional array is isotropic but has substantially reduced strengths over the one-dimensional arrangement.
Mechanical properties in any one direction are proportional to the amount of fiber by volume
oriented in that direction.
Fiber considerations--The properties of a fiber-reinforced composite depend strongly on the direction of measurement in relationship to the direction of the fibers.
Tensile strength and modulus of a unidirectionally reinforced laminate are maximum when these properties are measured in the longitudinal direction of the fibers.
At other angles, properties are reduced.
Similar angular dependence is observed for other physical and mechanical properties. Metals exhibit yielding and plastic deformation or ductility under load.
Most fiber-reinforced composites are elastic in their tensile stress-strain characteristics. The heterogeneous nature of fiber/polymer composite materials provides mechanisms for high-energy absorption on a micro-scale comparable to the metallic yielding process.
Depending on the type and severity of external loads, a composite laminate may exhibit gradual deterioration of properties.
Many fiber-reinforced composites exhibit high internal damping properties.
This leads to better vibrational energy absorption within the material and reduces transmission to adjacent structures.
This aspect of composite behavior may be relevant in civil engineering structures (bridges, highways, etc.) that are subject to loads that are more transitory and of shorter duration than sustained excessive loadings.