Mechanical properties and fracture mechanisms of low-density fiber preforms reinforced phenolic aerogel composites

Three kinds of needled fiber preforms reinforced phenolic aerogel composites (NF/PA) were prepared using lightweight needled preforms of glass fibers, carbon fibers and quartz fibers as reinforcements, respectively. The resultant fiber-dependent mechanical properties and fracture behavior were studied systematically. It is found that the NF/PA has a low density of ~0.45 g/cm³ and good insulation with room-temperature thermal conductivity as low as 0.046-0.067 W/(m·K). The phenolic aerogel has typical 3D porous structure with the overlapped and interconnected phenolic aerogel nanoparticles filling in fiber preforms, which has excellent interface structure with fiber. The NF/PA have obvious plastic deformation in the process of tension and compression. Furthermore, this work analyze the energy absorption of NF/PA during the crack propagate, and conclude that the type of fiber significantly affects the interface characteristics and thereby the fracture and failure mechanisms of composites. The interfacial bonding strength of carbon fiber is less than the ultimate shear stress of phenolic aerogel, and thereafter fibers are initially deboned with phenolic nanoparticles during the fracture process, responding to a “slip interface” feature. On the other hand, the bonding strengths of glass fiber and quartz fiber are both greater than the limit of shear stress of phenolic aerogel. Therefore, the phenolic aerogel is destroyed initially during the fracture process, which manifests as a “sticky interface” feature. Compared with glass fiber and quartz fiber, the carbon fiber shows more toughening and reinforcing effect on NF/PA.