13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- were observed to initiate and grow by the decohesion of carbides from the matrix. Larger voids, in some cases, were shown to nucleate at large particles. These larger voids were observed to be present well below the fracture surface and ahead of the crack tip. Figure 3: Large macroscopic voids with an inclusion Figure 4: Microvoids nucleating at proximity of carbides 3.1 X-ray Tomography The X-ray tomography images of Figure 5 shows the typical distribution of voids below the fracture surface for a specimen extracted from the fracture surface. Voids as small as 10µm in diameter could be resolved with a high degree of confidence. Figure 6 shows a render of the range of shapes and sizes of voids observed. Some voids have dumbbells morphologies possibly indicating coalescence. The ability to use the X-ray tomography technique enabled the imaging of voids in their entirety. Furthermore, this technique demonstrated the ability to visualise and quantify voids and in some cases, clusters of voids up to 3.6mm below the fracture surface. Figure 5: 3D tomographic image of the ferritic steel samples and the void distribution below the fracture surface. 50µm 4µm
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