•The paper provides safe default values of the Theory of Critical Distances parameters.•The paper analyses the structural integrity of different materials.•The analysis combines Failure Assessment Diagrams and the Theory of Critical Distances. When the structural integrity of notched components is analysed, it is generally assumed that notches behave as cracks, something which generally provides overconservative results. Thus, it is necessary to derive models that take into account the higher fracture resistance developed by structural materials when notches (and not cracks) are present. In this sense, the use of the Theory of Critical Distances (TCD) for the estimation of the apparent fracture toughness (KNc) observed in notched components has been validated for different types of materials, such as ceramics, polymers, composites and metals. The estimations, for U-shaped notches, arise from the combination of the TCD with the Creager-Paris stress distribution ahead of the notch tip, and apply a notch correction factor to the material fracture toughness observed in cracked conditions (Kc). Such correction only depends on the geometry (notch radius) and the material critical distance (L). The latter is the critical issue when applying the TCD, given that it generally requires calibration through experimental results and simple statistics (best fitting), or through a combination of experimental results with finite elements modelling. This paper provides some default safe values of the material inherent strength that may be used to derive safe estimates of the corresponding value of L, without any further calibration, to be finally used in the apparent fracture toughness predictions and subsequent structural integrity assessments.