Scratching viscoelastic liquids

Viscoelastic materials are ubiquitous in our everyday life: from skin tissue to butter or bitumen, we are surrounded by materials that display both a viscous and an elastic response under external deformation. Beyond the linear deformation regime, such materials often exhibit complex flow profiles, from spatially heterogeneous ductile flow to crack and fractures. In this presentation, we focus on the latter category of materials and show that a scratch technique, which has been extensively applied for determining the resistance to fracture of solids, can be successfully applied to viscoelastic materials. Using a Rockwell diamond probe of conical shape mounted on a micro scratch tester (Anton Paar), we perform direct measurements of the fracture toughness of various type of bitumen samples at ambient temperature. We show that the fracture toughness of bitumen increases as a power-law of the horizontal scratch speed, independently of the vertical loading rate. The power-law exponent is characteristic of the sample and increases for decreasing penetration grade. Finally, we use the scratch test to quantify the impact of various additives (polymer, glass beads, etc.) on the fracture toughness of bitumen samples.