Can scratch tests give fracture toughness?

• Bažant size effect law applies to scratch tests and show fracture failure mode dominates only when cutting depth is deep.
• Cracks induced by scratch tests are shown to be affected by contact condition between cutter and rock.
• Plastic yielding data from shallow cut could be misconstrued as results of fractures.
• Mean and peak cutting forces from shallow scratch tests do not give fracture toughness.

Recently Akono and Ulm [1] and Akono et al. [2], employing the linear elastic fracture mechanics framework, proposed that scratch tests could be used for determining fracture toughness. They applied the proposal to cement paste and sandstone, among other materials. This study shows that such a claim is in general not valid for a quasibrittle material like rock using the method they proposed. Firstly, it is demonstrated that rock scratch test data are well described by Bažant’s simple size effect law, which immediately points to the fact that linear elastic fracture mechanics is only applicable asymptotically when depth of cut is fairly deep. Secondly, their proposal requires cracks induced by cuts to be horizontal, and this is shown not necessarily true both from experiments and from FEM analysis. Finally, an interpretation of the data contradicts the rationale that was the basis of the original claim of determining the fracture toughness.