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B. Žužek et alii, Frattura ed Integrità Strutturale, 34 (2015) 160-168; DOI: 10.3221/IGF-ESIS.34.17 162 Charge C Si Mn P S Cr Al Cu V ESR 0.53 0.29 0.94 0.010 0.003 1.07 0.025 0.18 0.16 CCC 0.54 0.33 0.97 0.012 0.007 1.10 0.006 0.17 0.17 Table 1 : Chemical composition of CCC and ESR spring steel charges (mas. %). Mechanical testing Mechanical properties involved tensile test, hardness measurement, as well as fracture and impact toughness analysis. Tensile test according to EN ISO 6892-1 A224 was performed on standard B10x50 mm specimens using universal testing machine Instron 8802, and all standard tensile test parameters were determined. Fracture toughness measurement were performed on non-standard circumferentially notched and fatigue-precracked tensile-bar specimens (Fig. 1), designated CNPTB specimens [26]. Fatigue pre-crack of about 0.5 mm was created under rotating-bending loading before the final heat treatment [27]. Using Instron 8802 tensile-test machine and cross-head speed of 1.0 mm/min load at fracture was measured and fracture toughness KIc calculated using Eq. 1 [28]: 3 2 1.27 1.72 Ic P D K D d            (1) where P is the load at failure, D the outside non-notched diameter (10 mm), and d the diameter of the instantly fractured area. Eqn. (1) is valid for 0.5 < d / D < 0.8 and linearly elastic behavior, as displayed by all investigated specimens. Figure 1 : A) CNPTB specimen, circumferentially notched and fatigue-precracked (units in mm), B) detail on notch marked on Fig.1A. Impact toughness was determined on charpy V-notch specimens (KV 2 ) according to standard EN ISO 148-1 using 300 J pendulum. Tests were performed at room and sub-zero temperatures of 0°C, -20°C and -40°C in order to reveal ductile to brittle transition temperature (DBTT), carried out for specimens tempered at 475°C. Finally, the Rockwell hardness measurements (HRC) were performed on each cylindrical specimen using an Instron B2000 hardness testing machine. Dynamic testing Fatigue properties were determined under bending as well as tensile-compression loading. Bending fatigue testing was performed on a Rumul Cracktronic resonant machine using standard 10x10x55 mm Charpy V-notch test specimens. The testing resonant stress frequency was 175 Hz using a sinusoidal waveform at a stress ratio R of 0.1. A constant amplitude bending stress, ranging between 275 MPa and 430 MPa was used. A criterion of specimen failure was a drop of inherent oscillation for more than 3%, where fatigue cracks occurred in a depth of up to 3 mm. Fatigue testing under tension-compression loading were conducted in servo hydraulic Instron 8802 dynamic test machine under the conditions of R = -1 and sine wave of 30 Hz. Tests were performed on polished hourglass-shaped fatigue specimens with a gauge length of 38 mm and neck diameter of 7.5 mm. Maximum tensile stresses were in the range between 500 and 780 MPa. P P L 0 D d 60° detail  18  10  7,1  12 15 L =120 t 10 L =60 v L =50 0 A B