Fibrous structures are not classical continuum, but rather are discrete because of the existence of the macro-pores. Micromechanics is often used to study the mechanical behavior of discrete media from microstructural considerations and is based on the properties of its constituents. However, the inherent random nature of the physical and geometrical features of discrete media is fundamentally different from the macroscopic level of the assumed continuum, when the method of combined microanalysis and continuum theory is used. Therefore, the connections between the formulations from the microstructural analysis and the macroscopic performance have to be established as the premises for the discrete media study. Axelradhas proposed that, in the for mulation of the mechanics of a discrete medium, three measuring scales should be used to define such a system. The smallest scale is called a “microelement” of the structure. It is a typical representative element of the microstructure of the system on which all of the continuum concepts are applicable, as it is a continuum by definition. Then, an intermediate scale named “mesodomain” containing a statistical ensemble of the microelements follows. The physical and geometrical parameters of the mesodomain are independent of the positions, and have to be derived statistically based on the parameters of its constituent microelements. In fact, the mesodomain is defined as a portion of, or as the representative of, the whole system on which the continuum approach is once again valid, provided that only the effects over distances appreciably greater than the distance between the microelements are concerned.
Finally, a finite number of nonintersect ing mesodomains form the macroscopic material body. These three divisions clearly illustrate the relationships between the different structural (from microscopic to macroscopic) levels, and thus, actually provide the natural sequence of the micromechanical analysis.field. More recently, Panhas considered the steric hindrance effect, i.e., the interference of existing fiber contacts on the successive new contact to be made. Komori and Itohsubsequently published a study to treat the same problem but to allow the system volume to change so that there become two competing factors affecting fiber contact. On the one hand, an existing fiber contact reduces the effective contact length of a fiber and hence diminishes the chance for new contacts. On the other hand, the existing fiber contact point will also abate the free volume of the fiber mass, and consequently increase the chance for successive fibers to make new contacts. Some of the research results in this area have been applied to study the compressionaland shearbehavior of general fiber assemblies, as well as the prediction of nonwoven products,leading to considerable progress in those areas. Nevertheless, research on this problem is stillvery elementary. To understand the behavior of fibrous structures, one must examine the microstructure or the discrete nature of the structure. However, a thorough study of a structure formed by individual fibers is an extremely challenging problem. It is worth mentioning that the problem of the microgeometry in a fiber assembly can be categorized into a branch of complex problems inmathematics called packing problems. Consider, for example, the sphere-packing problem, also known as the Kepler problem, which has been an active area of research for mathematicians since it was first posed some 300 years ago, and remains unsolved.Yet, it seems that the sphere packing would be the simplest packing case, for one only needs to consider one characteristic size, i.e., the diameter of perfect spheres, and ignore the deformation caused by packing. Therefore, it does not seem that the fiber-packing problem can be solved completely anytime soon.