Researchers at PSI have established a link between the martensitic transformation, microstructural evolution and the mechanical behavior under multiaxial deformation in a NiTi alloy by using a unique combination of in situ high-resolution Digital Image Correlation (DIC), in situ X-ray diffraction and electron microscopy characterization.
Molecular dynamics simulations of transient stress drops have been carried out in different regimes on a nanocrystalline Aluminum sample with average grain size of 12 nm. Besides confirming the interpretation of experimental results obtained during in situ X-ray diffraction, the creep simulations performed at 2 or 3 orders of magnitude lower strain rates than usual reveal deformation mechanisms that have not been observed previously.
Multiaxial mechanical testing of sheet metals is far from trivial, which is mainly related to issues with sample design and fabrication. PSI scientists have developed a new methodology to produce cruciform shaped samples from thin sheet metals based on a novel bottom-up approach. A proof-of-principle experiment based on polymer lamination of an aluminum thin sheet demonstrates the effectiveness of this new approach.
Cruciform experiments are very useful to study non-proportional strain path change behavior of engineering metals and alloys. This work studies the stress response of 6 prominently used cruciform geometries deformed under tension. Results show that for most of the cruciform samples, the gauge stresses are non-linearly coupled to the applied forces in both arms. Cruciform geometries based on the ISO standard are able to decouple these stresses but negligible gauge plastic strains are reached prior to failure.
Researchers at PSI have developed a new unique miniaturized biaxial deformation rig, which allows to apply in-plane biaxial stress states with arbitrary stress ratios and to perform strain path changes on thin-sheet metals. The device is optimized for in situ usage inside a scanning electron microscope and at synchrotron beam lines.