X-ray microtomography investigation of damage fields ahead of cracks in CT and SENT C-Mn steel samples

To reduce over conservative safety margins, toughness may be assessed by Single Edge Notched Tensile (SENT) testing in addition to Compact Tension (CT) testing. Higher crack growth resistance is found during SENT testing compared to CT testing for the studied 15NiCuMoNb5 (WB36) ferritic steel. To identify differences in damage mechanisms for the two samples that might explain these differences, synchrotron microtomography is carried out on stopped cracks cut from CT and SENT samples. They are complemented by post mortem fractography and elasto-plastic 3D finite element simulations. For the CT sample, substantial growth of voids is found, that primarily nucleated on MnS particles, leading to a rough crack with a diffuse crack tip. In contrast, the SENT sample shows limited void growth and a very defined and smooth crack with a damage free fan shaped zone ahead of the crack. Complementary high-resolution fractography also shows very small dimples for the SENT sample that is linked to nucleation on carbides. Damage quantification in regions of interest (ROIs) of 50 mu m length showed void volume fractions up to 7% for the CT sample over hundreds of micrometres while the narrow damage zone of the SENT sample showed void volume fraction below 1%. An internal length of about 150 mu m below which the result of damage analysis does not change has been found for the CT sample. The damage free zone in the SENT sample is attributed to the fact that the strain in this region is low according to finite element simulations. The results indicate that the damage mechanisms change between the two samples and that secondary nucleation on carbides needs to be accounted for to model failure in SENT samples.

Automated summary

Higher crack growth resistance is found in 15NiCuMoNb5 (WB36) ferritic steel during Single Edge Notched Tensile (SENT) testing compared to Compact Tension (CT) testing. Synchrotron microtomography, post mortem fractography, and elasto-plastic 3D finite element simulations are used to identify differences in damage mechanisms and reveal that secondary nucleation on carbides needs to be taken into account in modeling failure in SENT samples.