Invited presentation at MRS Boston Fall 2014 Meeting:
Structure-Property Relations in Amorphous Solids

Despite significant recent advances like the Gorilla Glass© from Corning©, glasses still break, which limits their field of application. Similar challenges are also faced by the industry of cement, an ubiquitous material in our society. To design tougher materials, it is critical to understand the relationship between their structure and their ability to resist cracking. Here, we report on molecular dynamics simulations of permanently densified sodium-silicate glasses (NS2) and calcium-silicate-hydrates (CSH), the binding phase of cement. Their complex structures are described as simple mechanical trusses following the framework of the topological constraints theory. For both of these materials, we report the existence of a rigidity transition, driven by composition for CSH, and by pressure for NS2. The fracture toughness of NS2 and CSH is computed and shows maxima. We show that this toughening is achieved for materials characterized by an isostatic network, rigid but stress-free, as observed experimentally for germanium-selenium glasses. In addition, we identify a ductile to brittle transition correlated to the isostatic to stressed-rigid transition. This optimal fracture toughness arises from a reversible molecular network, allowing optimal stress relaxation and crack blunting behaviors. This opens the way to the discovery of high-performance materials, designed at the molecular scale.