## 5 STEPs for Enhanced Evaluation of Fatigue Data  
The desire to obtain fatigue data such as S-N curves short term has a fairly long tradition. However, the first substantial work was introduced with the PhyBaL approach [8] according to which a complete S-N curve can be determined with three fatigue tests on unnotched samples only. The first test is a LIT, where the specimen is loaded under constant amplitude fatigue loading, starting at a low level and cycling over a defined number of cycles to allow the material's behavior to stabilize before a next higher loading step is achieved. The material response obtained, being usually strain as the non-controlled measure in a load-controlled test, is plotted versus the controlled stress. This relationship is then corrected by the results obtained from two conventional load-controlled CAT usually performed at the stress level where the LIT fractured and at a stress level around the endurance limit respectively. This approach has been further extended and differentiated, being called the StressLife [9], StrainLife [10] and SteBLife [11] approach respectively.  
Figure 2 shows the calculation procedure for the StrainLife approach schematically, where a SIT and two CATs are needed. When the material response of the SIT, taken at the half of each step is plotted vs. the controlled value \(\varepsilon_{a,t}\) in a double-logarithmic scale, the relationship is divided into a part where either elastic strain or plastic strain is dominating. From this relationship \(n_{e^{\prime}}\) and \(n_{p^{\prime}}\) can be obtained, and using the empirical relationships established by [PERSON] [12], \(b\) and \(c\) can be calculated, representing the slopes of the \(\varepsilon_{a,e}-N\) and \(\varepsilon_{a,p}-N\) relationships respectively. Furthermore, with the \(\varepsilon_{a,e}\), \(\varepsilon_{a,p}\), \(M\) values taken at the half of the lifetime from the two CATs, and its \(N_{f}\), the coefficients \(B\) and \(C\) can be calculated using Basquin [13] and Manson-Coffin [14] relationships.  
Figure 1: **(a)** Unnotched specimen’s geometry, **(b)** Notched specimen’s geometry and **(c)** Light microscopic image of the material as delivered.  
To determine the resulting \(\varepsilon_{a,t}-N\) curve, the assumption is made, that total strain is a summation of elastic and plastic strain formulated as:  
\[\varepsilon_{a,t}=\varepsilon_{a,e}+\varepsilon_{a,p} \tag{1}\]  
and merging the Basquin and Manson-Coffin relationships into (1), Equation (2) results:  
\[\varepsilon_{a,t}=B\cdot(2N_{B})^{\frac{-n_{e}t^{\prime}}{5n_{e}^{\prime}+1}}+ C\cdot(2N_{B})^{\frac{-1}{5n_{e}^{\prime}+1}} \tag{2}\]  
allowing the \(\varepsilon_{a,t}-N\) curve to be described.  
Results of such an approach for the AISI 347 (X6 CrNiNb18-10) handled in MibaLeb as an example compared to a series of independent CATs performed can be found in [10], showing that STEP is a means well to be considered when the availability of raw material and cost is limited and results are expected to be provided very short term.  
Fatigue data of this character have been generated within MibaLeb on AISI 347 (X6 CrNiNb18-10) for the condition of air and room temperature (RT) and elevated temperatures of 240 and 300 \({}^{\circ}\)C under a pristine and an artificially aged condition, as well as under distilled water at room temperature and boiling water reactor conditions at 240 \({}^{\circ}\)C respectively.