During the high-temperature tensile experiments, backprojection images were captured to allow for subsequent 3D XCT reconstruction so that the damage accumulation behavior that affects strength could be better understood. To distinguish voids or cracks from artifacts, a number of sequential sliced images were compared. If black patterns could be observed at the same location within more than four sequential images, they were defined as defects, and vice versa.
imagix 4d crack
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In case of Nimonic 80A, intergranular cracks were observed on the fractured surface rather than internally. According to the fractography analysis and ex situ XCT images of fractured sample obtained at room temperature, surface cracks originated in an intergranular manner from the rim of the sample gauge. At similar temperature conditions, Sharma et al.[31] mentioned similarly that cracking tended to initiate around \(\mathrm\gamma '\)-phases and created intergranular fractured surface, which is consistent with our results. Since Nimonic 80A exhibited less ductility than Inconel 718, one can say that cavity nucleation occurred more favorably than for Inconel 718. Such cavitation can occur preferentially at grain boundaries lying transverse to the tensile direction.[32] Since our results showed that the initiated cracks were largely perpendicular to the tensile direction, tensile cracks were produced by such void coalescence at the grain boundaries. For damage creation with cavity nucleation dominating, other works have also shown that cracks were initially preferentially at the sample surfaces.[29]
Thus Nimonic 80A also exhibited damage accumulation which depended upon the strain rate conditions. At slower strain rate, a smaller number of surface cracks were formed and they grew to much larger size, whilst higher strain rates generated a larger number of smaller cracks on the surface. Necking behavior was not as apparent as for Inconel 718, but the slower strain rate in Nimonic 80A also caused a deeper necking profile than in the higher strain rate case. Due to the comparatively more necked profile, at slower strain rate, the tensile stress was concentrated in the necked part, which resulted in the cavity growth and eventually significant cracking in this region.
The apparatus allows collection of backprojection images with resolution sufficient to detect plastic deformation, void growth, and crack initiation. Void growth and crack initiation were detected if they exceeded 30 \(\mu \)m, and this gives an indication of the spatial resolution which can be achieved.
Studies on the grades Nimonic 80A and Inconel 718 have been carried out, with the rationale that these are widely used alloys of lesser and greater ductility, respectively. For testing at 800 C, Nimonic 80A exhibited surface cracks which are the sites of fracture initiation; for Inconel 718, internal void growth after necking was preferred. In both cases, the XCT system proved to be capable of detecting these failure events.
For Inconel 718 at 800 C and higher strain rates, void growth did not occur uniformly; nevertheless, its temporal and spatial distribution could be detected and quantified. For slower strain rates, void coalescence was more concentrated within the necked region. The surface cracks in Nimonic 80A were generated widely over a wide range of the gauge volume at higher strain rates, whilst the slower strain rate generated a smaller number of larger cracks. 2ff7e9595c
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