Theoretical work on NASC is usually based on the assumption that fibers are stiff and straight in matrix. The real fibers, however, bend and wind together. With the reduce of fiber contents or the increase of fiber lengths, fiber bending and winding are enhanced. Due to the forming process, the bending, winding and uneven distribution of fibers always exist randomly and are very difficult to be characterized.
1.4 Thickness of interface
For most fiber reinforced composites, the thickness of interface between a fiber and the matrix is very small and hence is usually not considered. For NASC, however, to enhance
the reinforcing effect of fibers and to optimize the bonding capability of fiber surface, short fibers should be pretreated using surface conditioning agents before mixing, and the interface has great influence on mechanical properties of NASC. The interface thickness can be easily determined by calculating the change in the fiber radius before and after pretreatment.
Some pores will occur during the preparation process of NASC, especially in the sulfuration process. Most of these pores exist in the interface and they will change the stress transfer mode. The porosity p, which is the ratio of the total volume of all pores to the volume of the composite, can be used to evaluate the volume fraction of pores in the composite.