Under-microscope Mechanical Pulse System for Studying Deformation Processes at High Strain Rates

Abstract

We present a miniature experimental system that allows real time optical observation of deformation processes in materials. The system can apply controllable force pulses with strain rates of up to 2000 s−1. The force pulse is generated by electrical activation of two antagonistic NiTi actuating wires that load and unload the sample. The magnitude and duration of the applied force pulse are adjusted by controlling the energy transferred to the loading wire and setting the time delay between the activation of the two wires. The actual force profile acting on the sample is measured directly using a dedicated force sensor. The entire setup is placed under an optical microscope equipped with a high-speed camera that enables monitoring the macroscopic displacement as well as the evolution of the microstructure during the mechanical loading. The capabilities of the method for studying deformation mechanisms are demonstrated by visualization of stress-induced twinning reorientation in Ni-Mn-Ga under different strain rates. High speed imaging allows measuring dynamic stress-strain curves that reveal the strain-rate sensitivity of the twinning reorientation stress. In addition, the proposed method allows obtaining velocity-stress relations for individual twin boundaries, which suggest that twin boundary velocity is determined not only by a kinetic relation but also by local interactions with other defects.