|Title||An improved technique for characterizing the fracture toughness via scratch test experiments|
|Publication Type||Journal Article|
|Year of Publication||2014|
|Authors||Akono A-T, Ulm F-J|
|Pagination||117 - 124|
The scratch test consists in pushing a tool across the surface of a weaker material at a given penetration depth; and it has several applications in Science and Engineering including strength testing of rocks and ceramics, damage of polymers and metals and quality control of thin films and coatings. Despite numerous attempts in the scientific literature, the application of scratch tests to the characterization of fracture properties remains a challenge and a heavily controversial topic. Therefore, this investigation aims at articulating a rigorous theoretical and experimental framework in order to assess the fracture toughness at both the macroscopic and the microscopic length scales, using scratch tests. First, we apply optical microscopy and scanning electron microscopy to investigate the physical evidence of crack initiation, crack propagation and material removal mechanisms during scratch tests. Then, we employ Finite Element simulations of crack growth during macroscopic scratch tests to assess the influence of the blade back-rake angle, the friction coefficient between the blade and the material and the wear flat of the blade on the scratching forces, thus testing the robustness of our Linear Elastic Fracture Mechanics scratch model. Finally, at the microscopic scale, a meticulous scratch probe calibration procedure is described to improve the accuracy of the fracture properties determination by addressing important issues such as moisture content, specimen surface cleanliness and choice of reference material. In summary, we bring forward a robust, convenient and accurate method that is applied to polymers, ceramics and metals and can be further applied to the multi-scale study of fracture processes in complex and challenging materials such as gas shale, cement paste and cortical bone.