13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 2. Experimental results 2.1 Mechanical tests The material studied is a 16MND5 steel (similar to A508 Cl3 steel) that originates from a forged nozzle shell of a pressurized water reactor of 220 mm thickness [4]. CT specimens precracked at a ratio a0/W ~ 0.56 (where a0 is the fatigue precrack length and W the width) are tested using ASTM E1921 standard recommendations [5]. Isothermal fracture toughness tests were performed using specimens of two different thicknesses (B=12.5 and 25 mm), leading to a reference temperature, T0, equal to -104.7°C [4]. WPS experiments presented in this study were performed using standard CT specimens with a 25 mm thickness. Four different types of WPS cycles were conducted: LCF (Load-Cool-Fracture), LCDKF (Load-Cool-Decreasing K-Fracture), LCIKF (Load-Cool-Increasing K-Fracture) and LCOIKF (Load-Cool-Oscillating Increasing K-Fracture) (Fig. 1). For all the WPS cycles, the loading sequence was: (i) Loading up to 500 N under displacement control with a crosshead speed of 0.25 mm/min, (ii) loading up to KWPS under extensometer control at 0.1 mm/min, (iii) Cooling from TWPS down to Tfrac under load control with a temperature rate of -1.5°C/min, (iv) Reloading up to fracture under extensometer control at 0.1 mm/min if the specimen survived the transient. All values of K presented here, at WPS loading and fracture levels, are elastic values, thereafter noted Kel, calculated according to ASTM E 1921 [5]. 2.1 Synthesis of the experimental data The whole set of WPS experiments that were performed in this study are presented in Table 1. The first four cycles give us a reference base which will be useful for comparison to the main object of the study, LCIKF cycles. All specimens survived to the transient and failed during reloading phase. From all the WPS tests performed, the following observations can be made: • LCF and LCDKF cycles performed on the 16MND5 RPV steel confirmed the numerous experimental results on ferritic steels published in the literature, i.e. no propagation if the load is held constant or decreased during cooling ; • LCIKF cycles performed in this study confirm and extend the previous results [6, 7] that a critical slope has to be exceeded to induce cleavage during cooling.
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