Session: 01-07: Multifunctional Composites

Paper Number: 111016

111016 - Evaluation of Interface Strength and Failure Between Nickel-Titanium Shape-Memory-Alloy Wire and Bismuth-Tin Matrix for the Design of Self-Healing Composites 

This paper presents recent research into the interface strength between nickel titanium (NiTi) shape memory wires and off eutectic bismuth tin (BiSn) matrix comprising a self-healing metal-metal composite.

Self-healing materials can lead to a paradigm shift in engineering design of structures by enabling lighter weight & more efficient structures, reducing maintenance requirements, and changing the definition of failure.  Among self-healing materials, metal-metal composites have the potential for some of the most advanced capabilities, being entirely structural without consumable healing agents. 

Despite potential advantages, metal-metal self-healing composites are challenging to synthesize, as a result most research has focused on polymeric or ceramic self-healing materials.  NiTi fiber reinforced off-eutectic BiSn matrix composite structures have demonstrated some of the most advanced self-healing abilities.  After one of these non-autonomic self-healing structures incurs damage, the self-healing capability can be activated through the application of heat.  Increase in temperature activates the shape memory effect in the NiTi, closing fractures.  Continued heating results in melting of eutectic portions of the BiSn matrix which serves to solder the matrix back together. Combined these abilities result in restoration of macro-scale geometry and near 100% restoration of strength without expending any consumable reagents.  Therefore, the process can be repeated indefinitely.

One of the primary challenges in designing this composite structure is understanding the interface between the NiTi and BiSn. The strength and mechanisms of failure at this interface are critical to designing the composites (ie. sizing the fibers) and understanding the resulting strength. This is a challenge because NiTi forms an inert titanium oxide (TiO_2­) surface layer almost instantly upon exposure to air. The TiO₂ layer severely inhibits bonding between NiTi and BiSn (or most any other materials).

This paper presents an experimental and theoretical investigation into the properties of the interface between NiTi and BiSn in both a control state with the TiO₂ present and an experimental state where chemical etching processes were performed in an inert environment to remove the TiO₂ and prevent is re-formation.  Experimental specimens were synthesized, mechanically tested, and microscopically inspected.  Modeling was performed to understand internal states of the structure and theory was developed consistent with and explaining observed results.

The results of this work will quantify the improvement in interface strength between NiTi and BiSn achieved through the performed etching process.  Quantification of this strength will provide vital information for composite design optimization and sizing of wires.  However, these efforts will differ from in-active composites to account for internal loads generated by the healing process and potential development of TiO₂ upon incurring damage.

Presenting Author: Nathan Salowitz University of Wisconsin - Milwaukee

Presenting Author Biography: Nathan Picchietti Salowitz earned the B.S. degree in engineering mechanics from The University of Wisconsin – Madison in 2001 and the M.S. and Ph.D. degrees in Aeronautics and Astronautics from Stanford University in 2006 and 2013 respectively.
From 2003 to 2005 he was a Structural Analyst with Boeing. After earning his Ph.D. he was an Engineering Research Associate in the Structures and Composites Laboratory at Stanford University. From 2014 until 2020 he was an Assistant Professor of Mechanical Engineering at The University of Wisconsin – Milwaukee (UWM) with Affiliate Professor Appointments in Electrical Engineering and Civil Engineering. In 2020 he was promoted to Associate Professor with tenure in the Mechanical Engineering Department at UWM where he continues to present day. He is the founder of the Advanced Structures Laboratory at UWM. His research encompasses physical sensors and active materials with a focus on ultrasonic sensing systems and properties of shape memory alloys for structural health monitoring (SHM) and self-healing materials.
Dr. Salowitz is a member of ASME and IEEE who regularly participates in local student events and national conferences. Dr. Salowitz is involved in the ASME Smart Materials, Adaptive Structures and Intelligent Systems (SMASIS) Division and Structural Health Monitoring (SHM) technical committee. He is active in the international SHM community as an Associate Editor of the journal Structural Health Monitoring and member of the international program committees of the International Workshop on Structural Health Monitoring and Asia/Pacific Workshop on Structural Health Monitoring.