Institute of Fundamental Technological Research

In this work, thermomechanical behavior of the superelastic Ti–26Nb, Ti–25Nb–0.3O and Ti–25Nb–0.3N (at. pct) shape memory alloys (SMAs) under load-unload tension was investigated using coupled techniques of infrared thermography and digital image correlation. Local and average characteristics were analyzed in the context of particular deformation stages. In the case of all the SMAs, during loading, first the temperature decreases due to the thermoelastic effect, which can serve to estimate true elastic strain. During further loading, the temperature significantly increases due to the forward stress-induced phase transformation, whereas during unloading, the temperature significantly decreases due to the reverse phase transformation. The average values of the temperature change generated due to the elastocaloric heating and cooling were 15.95 K, 14.94 K, 16.05 K and 17.79 K, 14.44 K, 19.83 K in the case of the Ti–26Nb, Ti–25Nb–0.3O and Ti–25Nb–0.3N SMAs, respectively. The kinematic fields demonstrated that the deformation during loading and unloading is inhomogeneous. It starts with the appearance of thin parallel bands perpendicular to the loading axis. These bands create larger areas with higher strain upon further loading and gradually reappear upon unloading. The results advance the comprehension of the thermal and kinematic aspects of tensile deformation of superelastic Ti-Nb-based SMAs

This work concerns an experimental investigation of the Ti–25Nb and Ti–25Nb–1O (at%) SMAs in the initial stage of tensile deformation using acoustic emission (AE) and digital image correlation (DIC). The stress-strain responses of the considered SMAs are different. The Ti–25Nb SMA exhibits shape memory effect due to the stress–induced martensitic transformation from the cubic β phase to the orthorombic α″ phase. In the case of the Ti–25Nb–1O SMA, the addition of 1 at% of oxygen results in a nonlinear superelastic behavior with small hysteresis and an increased yield stress. The stress-induced phase transformation in the Ti–25Nb–1O SMA is hindered due to the addition of oxygen interstitials. The difference between the deformation mechanisms and the resulting mechanical behaviors of the SMAs was clearly reflected by the recorded AE signals and deformation fields. It was shown that the AE can serve to track the development of the stress–induced phase transformations in the Ti–25Nb and Ti–25Nb–1O SMAs during tension. The AE signals were correlated to the strain fields of the SMAs, which showed a Lüders–type deformation of the Ti–25Nb SMA and a distinct but still inhomogenous deformation of the Ti–25Nb–1O SMA. The results of this study show that DIC and AE techniques are effective tools for monitoring phase transformations of the Ti–25Nb and Ti–25Nb–1O SMAs during tensile loading

Shape memory alloys, Oxygen interstitials, Martensitic transformation, Acoustic emission, Digital image correlation

shape memory alloys, interstitials, infrared thermography, digital image correlation

The concrete microstructure was successfully reconstructed using the complementarity of X-ray and neutron computed tomography (CT). Neither tomogram alone was found to be suitable to properly describe the microstructure of concrete under this study. However, by merging the information revealed by the two modalities, and using image segmentation, noise reduction, and image registration techniques we reconstruct the concrete microstructure. Void, aggregate, and cement paste phases are successfully captured down to the images' spatial resolution, even though the aggregate consists of multiple minerals. The coarse-aggregate volume fraction of the reconstructed microstructure was similar to that of the mixing proportions. Furthermore, image-based finite element analysis is performed to demonstrate the effects of microstructure on stress concentration and strain localization.

concrete microstructure, X-ray tomography, neutron tomography, image segmentation, complementarity field, image-based analysis

Ni-free shape memory alloys, interstitials atoms, stress-induced phase transformation, acoustic emission