School of Civil and Environmental Engineering

Ph.D. Thesis Defense Announcement

Experimental Testing on Strain Rate Dependency of Shape Memory Alloy Materials under Quasistatic and Dynamic Loading

By

Nan Gao

Advisors:

Dr. Lauren Stewart and Dr. Reginald DesRoches

Committee Members:

Dr. Lawrence F. Kahn (CEE), Dr. Laurence J. Jacobs(CEE), and Dr. Russell Gentry(ARCH)

Date & Time: Monday, November 6th, 3017

Location: Mason Conference Room 2119

Shape memory alloys (SMAs) are smart materials that have great potential in structural engineering because of their remarkable mechanical properties such as superelasticity and shape memory behaviors. Specially designed SMA-based devices can provide significant energy dissipation capacity and introduce considerable re-centering ability to structures. The stress-strain relation and the corresponded energy dissipation and re-centering capacity are dependent on loading rates while the responses of SMAs under intermediate strain rates are hard to obtain using conventional experimental techniques. This research develops an innovative high-loading-rate tensile testing system that can test SMA specimens under intermediate strain rates, bridging the gap between quasistatic strain rates from conventional servo-hydraulic testing machines and high strain rates from typical Kolsky bar tests. The key component of this testing system is an inverter mechanism that can transfer the impact at the input end from a high-speed actuator into tensile forces applied onto any specimen attached to the output end. This system can test not only material tensile specimens to obtain stress-strain results of a certain material but also structural components composed of different materials or subcomponents to obtain force-deformation results of the components. The testing system is verified and calibrated through a series of validation tests on aluminum tensile specimens. Experimental results are compared with theoretical estimation and finite element simulation to corroborate that the system can obtain reliable force-deformation results in a repeatable and controllable manner. Utilizing this system, the performance of SMAs under intermediate strain rates can be assessed. This research conducts two types of experimental tests on SMAs: a quasistatic cyclic loading test on a bracing system based on an SMA ring and a series of high-loading-rate tensile tests on varies SMA specimens. The performance of SMAs under the scenarios of both earthquakes and blast is accessed and thus the potential applications of SMAs in both seismic engineering and blast protection are evaluated.