Hi Vladimir. Thank you for your question. We assume that you are familiar with plane stress and plane strain conditions. We know that thinner steels can show brittle fracture, especially in the presence of stress concentrations, or when exposed to low temperatures. So the intention was not to write that we cannot test for brittle fracture on thin specimens. The intention was rather to explain that specimen thickness (in steels) can influence test results, so thickness need to be considered. It also has an influence on the cost and the schedule. We do hope this answers your question.

@@forcetechnology Thank you for your explanation. Since thinner steels show a brittle fracture, why do we still use a thin one for the test? And for brittle material, for example, ceramic, what the influence of using a thinner specimen for the test (decrease/increase the toughness and why)?

@@haozhezhang5287 Thank you for your question. Just to clarify: thinner specimens are prone to ductile fracture. When thickness or specimen size is large enough, plane strain conditions become dominant and the plastic zone near the crack tip becomes very small. In this case you will be able to obtain brittle fracture. Lower temperature makes steel more brittle, as mentioned in the previous answer. It is usually a limitation in the available material that limits the size of the test specimens. For plate materials, the test specimens might be thinner than required for obtaining a brittle fracture in the test (Valid KIC) due to plate thickness. In this case, you test the specimens that are possible to manufacture from the available material, and obtain a fracture toughness value that is valid for this material and thickness. If you obtain brittle fracture and all clauses in the standard are fulfilled, you get a geometry independent fracture toughness which is valid through all thicknesses. In general, a fracture toughness obtained in plane strain conditions is lower than a fracture toughness obtained in plane stress conditions. We have not tested ceramic materials and cannot provide an answer on how they behave at different material thicknesses.

@@forcetechnology Thank you so much for the very detailed explaination! You have completely answered my question.