## 2 Material and Testing Methods
### Fracture Toughness and Fatigue Resistance Testing Procedures  
The fracture toughness of the steel was determined on the basis of the ASTM E1304 [24] procedure, which involves the use of a short-bar specimen (SBS) with a geometrical configuration compatible with the dimensions of the studied bars, presented in Figure 2a. Since ASTM E1304 [24] requires a minimum of three tests to be performed, three SBS specimens were prepared for fracture toughness testing. In order to assure the fracture test's validity, conditioned by the crack growth in the small-scale yielding regime, the chevron notches of the tested SBSs were fatigue precracked. This involved the use of the same servo hydraulic equipment as for the tensile tests, with the load cell operating in dynamic mode. An average of 12,000 loading cycles in the load range of 0.5 to 3 kN, at a frequency of 8 Hz, were applied to each of the three specimens (SBS_1, SBS_2, and SBS_3), of thickness B = 12 mm and length W = 17.4 mm. A crack mouth opening displacement (CMOD) resistive extensometer was employed to control their cracking progress during fatigue, which was interrupted after reaching distinct crack extensions, as follows: 9.45 mm for SBS_1, 10.22 mm for SBS_2, and 11.14 mm for SBS_3. In the subsequent fracture tests, illustrated in Figure 2b, the same CMOD extensometer was used to monitor the failure process. Figure 2c shows the macroscopic image of one of the tested specimens, revealing the fatigue crack and its final fracture.  
The fatigue behavior of the bar material was assessed by means of cyclic tensile tests made with 5 mm diameter cylindrical specimens, in accordance with the sketch given in Figure 3a. Each specimen was surface notched (Figure 3b) to a depth of 0.5 mm before being subjected to several cyclic loading steps at room temperature at a frequency of 8 Hz.  
Figure 1: Images showing the: (**a**) tensile testing of smooth cylindrical specimens; (**b**) the moderately ductile fracture in tension of the steel; (**c**) representative microstructure of steel (the white arrows are indicating some inclusions); (**d**) higher magnification detail illustrating two grouped, globular inclusions.  
Figure 2: (**a**) Geometry and test configuration of SBS specimens (B = 12 mm, W = 17.4 mm); (**b**) image showing the fracture toughness testing of SBS specimens; (**c**) macroscopic image of one of the fracture surfaces of specimens with the marked crack length, a, measured according to ASTM E1304 [24].  
During each step, the load amplitude was maintained constant and decreased between each two successive steps by 20% to assure that the applied load is constantly well below the yield strength of the specimen in the corresponding cracked condition. In order to maximize the number of experimental data, after each cyclic loading step, the specimens were unloaded and the cracked areas were progressively marked by heat tinting, as it is shown in Figure 3c. In addition, the crack size was determined from the specimen stiffness obtained after each fatigue step by measuring the load-CMOD curve slope during the static unloading process. The CMOD measurements were made by using a resistive extensometer, mounted on the surface of the specimen, just ahead of the notch, on a 12.5 mm length. At the end of each fatigue test, the specimens were broken in tension. The failure loads, together with the final crack dimensions, were further used to find out the critical stress intensity factor K\({}_{\text{c}}\) for its further comparison with the determined fracture toughness measured from the SBS tests.  
Lastly, the fracture morphologies in the fatigue crack propagation regime and in the final stage of fracture, further examined by SEM (equipped with an energy-dispersive X-ray analyzer--EDS, Oxford Instruments, Abingdon, Oxfordshire, UK), complement the macroscopic evaluation of the fracture behavior of the bar steel.