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The aim of this research is to extend the KIvM fracture toughness measurement method to non perfectly brittle metallic materials. Many experiments on tough Steels, Aluminium and Titanium alloys showed that the measured KIvM is strongly dependent on specimen size. Increasing specimen size measured KIvM also increases. This phenomenon is coherent and may be explained by the rising R-curves of these materials. In fact, in a bigger specimen, unstable crack propagation takes place after a larger stable crack propagation. In the cases of materials with rising R-curves, measured KI values corresponding to larger crack propagations are higher. In order to obtain in each material an experimental KIvM value not higher than the valid KIC (ASTM E399), the “chevron notch” specimen size must not exceed a certain value. A validity condition about the maximum size of “chevron notch” specimens to obtain measured KIvM values not higher than the valid KIC in Steels, Aluminium and Titanium alloys is proposed. This validity condition is based on the concept that, in order to obtain a KIvM KIC, the stable crack propagation in KIvM measurement must not exceed the admissible stable crack propagation in the valid KIC measurement.