|M.Sc Student||Uria Heller|
|Subject||A novel Experimental System Based on Shape Memory|
Alloys for Studying the Dynamic Mechanical
Behavior of Materials at the Millisecond
|Department||Department of Design and Manufacturing Management||Supervisors||Full Professor Shilo Doron|
|Dr. Faran Eilon|
|Full Thesis text - in Hebrew|
One of the major challenges in experimental techniques commonly used for studying the dynamic response of materials is the ability to accurately control the stress state within the tested sample. In particular, existing methods for high strain rate experiments, such as conventional Kolsky bar and plate impact experiments lack the ability to apply load-controlled conditions. In this work we present the design, development and testing of a novel experimental system that generates stress-controlled pulses with an approximately rectangular shape and durations in the ms time scale.
This experimental system poses two additional important advantages. First, it is suitable for moderate strain rates in the range of 1 - 100 s-1. This range appears in many situations, such as in ballistic impacts, vehicle crashes, and a variety of actuation mechanisms that are based on shape memory materials. Nevertheless, there is a lack of experimental systems that are suitable for performing tests in this range of strain rates. Second, the overall size of the experimental apparatus is small and allows mounting the system on a microscope stage. This advantage enables the visualization of the sample’s microstructure evolution during the mechanical test, which enables quantitative investigation of the deformation processes that take place under these strain rates.
The system's operating principles are based on experience gained from recent projects related to ultra-fast actuation of shape memory alloy wires subjected to electric pulses at the -scale. Such a fast pulse leads to instantaneous heating of the shape memory alloy wire to a temperature above its phase transformation value. Heating takes place at much faster rates than the rates of the phase transformation. Under these conditions, it was shown that the stress in the shape memory alloy wires reaches a plateau after few hundreds of μs and the stress amplitude can be maintained over time scales of several ms while its magnitude can be controlled by adjusting the electric energy that actuates the wire. The presented setup incorporates two antagonistic shape memory alloy NiTi wires; the first is used for loading the tested sample and the second for releasing the load. The time delay between the actuation of the two wires determines the stress pulse duration. The experimental system is mounted under an optical microscope equipped with a fast camera, allowing fast in-situ photography of the moving interfaces during the stress pulse.
The first demonstration of this novel method involves studying the dynamics of individual twin boundaries in a twinned ferromagnetic shape memory alloy Ni-Mn-Ga crystal. In this alloy, large deformations of up to 10% take place via the motion of internal material interfaces called twin boundaries. Using the new experimental
System, the temporary velocities of individual twin boundaries are measured under different applied stress values, demonstrating the feasibility of a direct mechanical measurement of the kinetic relation for twin boundary motion. Advantages, limitations and future applications of the new method are discussed, indicating that this technique has the potential to contribute significantly for studies of various mechanical and physical properties that involve high strain rates.