13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- observations showed a higher concentration of voids for pre-cracked specimens than for notched tensile specimens. Additionally, a greater analysis of the area below the fracture surface has shown that the void volume fraction can extend to a few millimetres below the fracture surface with large clusters of voids extending up to 3.5mm below the fracture surface. The aim of this paper is to extend the observations made in this previous work by using 3D X-ray tomography to further quantify the void volume fraction below the fracture surface in pre-cracked CT specimens of A508 Class 3 RPV ferritic steel. The methodology and observations will be discussed as well as its implications for the calibration and application of the GTN model. 2. Experimental 2.1 Material The material used throughout this experiment was an A508 Class 3 ferritic steel. The specimens were extracted from the outer ring of an upright wedge-shaped block originating from a larger ring forging. All the specimens were extracted from the same location and in the same orientation. The chemical composition (wt%) of the ferritic steel was evaluated using spectographic analysis and the results are indicated in Table 1. Table 1: Chemical composition in wt% of A508 Class 3 steel. 2.2 Mechanical testing The tensile properties of the material were determined using standard round-bar test specimens oriented in the hoop direction. Three tensile specimens were tested on a Zwick 1464 at room temperature using a strain rate of 0.025% s-1 according to BS EN ISO 6892 procedure [9]. Ten fracture toughness tests were performed according to the ESIS P2-92 [10] standard using CT specimens with standard dimensions of thickness, B = 25mm, width, W = 50mm and a crack length to specimen width ratio, a/W = 0.53. Specimens were 20% side-grooved following fatigue pre-cracking. Tests were performed using both the unloading compliance and the multi specimen methods. Out of the ten tested CT specimens, two were left intact in order to preserve the crack tip for analysis. 2.3 Metallographic analysis The cracked and parent material was imaged using optical and scanning electron microscopes. The parent material was imaged to characterise the general microstructure of the ferritic steel with a specific interest on grain size and inclusion/particle type and distribution. The cracked specimens were analysed to characterise the ductile fracture mechanism and distribution of voids below the C Si Mn P S Cr Mo Ni Al Co Cu Sn Ti V 0.18 0.23 1.3 <0.005 <0.005 0.25 0.55 0.81 0.02 0.01 0.04 0.005 <0.01 0.01
RkJQdWJsaXNoZXIy MjM0NDE=