Fatigue is a major obstacle metallic engineering structures face during their operational life. To not allow fatigue to become critical, structures are designed in the most generalized way based on an assumed load spectrum, a load-fatigue life (S-N) and/or crack propagation curve, a damage accumulation rule (i.e. according to Palmgren-Miner) and a safety factor. Fracture is considered to be the sign of damage and hence degradation of the material to occur. However, fracture is not the only mechanism of a material’s degradation. There are more mechanisms such as dislocation movements, phase transformation or plasticity, that may be detectable by means of non-destructive testing (NDT). In this article the fatigue degradation process of two widely used metallic materials is described with respect to the materials’ non-linear behaviour and how far this can be monitored with different NDT techniques. Compared to state-of-the-art materials’ data presentation, a 3D space diagram is used, that provides a relationship between the loading applied and the degree of degradation under constant amplitude loading similar to an S-N curve and added as a third dimension by the material response a respective NDT technique provides when being used to monitor the material along fatigue loading. The resulting topography allows the NDT technique considered to be assessed with respect to its sensitivity in monitoring the material’s fatigue degradation, including materials’ possible non-linearities. Based upon the result obtained an NDT technique might be selected for further SHM application. The 3D diagram might be used as a reference to determine a structure’s degree of degradation prior to fracture, based upon the information the resulting SHM system applied to a structure might be able to provide. The 3D diagram might further allow structures’ residual life assessments to become more precise. Furthermore, the 3D plane generated can serve as a data pool to be continuously fed with data recorded by SHM systems and can hence serve as generalized reference in the longer term. The NDT techniques considered as examples include eddy current and thermography.