This chapter is dedicated to composites in which the reinforcement is a stochastic fiber network. Many network materials are reinforced by the addition of fillers of various geometry. However, in most current applications, filler dimensions are orders of magnitude larger than the characteristic length scales of the network. The focus of this chapter is on the properties of composites with matched filler-network length scales. The four sections of the chapter present the mechanics of networks reinforced with particles of dimensions comparable with the network pores, networks reinforced with stiff fibers, interpenetrating networks in which reinforcement is provided by the interaction with another network which spans the same spatial domain, and of networks embedded in a continuum matrix. It is shown that exceptional properties may be achieved due to the emergence of interphases in thermal and athermal networks with rigid fillers, and in interpenetrating network systems. The results and concepts presented are aimed to stimulate the future development of reinforced network materials.

This chapter presents a comprehensive overview of the mechanical behavior of Network materials, with emphasis on the structure–properties relation. Crosslinked and non-crosslinked Network materials are discussed in separate sections. The behavior of crosslinked networks in tension, shear, compression, and multiaxial loading is described. The effects of fiber tortuosity, fiber alignment, crosslink compliance, network connectivity, and variability of fiber properties on network stiffness and nonlinear behavior are discussed in detail. The size effect on linear and nonlinear material properties is evaluated in relation with network parameters. Three types of nonlinear behavior are identified, corresponding to networks that stiffen or soften continuously during deformation, and networks with an approximately linear response. Numerous examples of each type are presented, including collagen networks, fibrin and actin gels, elastomers, paper, and nonwovens. The response of non-crosslinked athermal networks, such as fiber wads, is studied in compression and tension. The effect of entanglements in athermal networks is analyzed and a parallel drawn with the mechanics of thermoplastics.