Current projects primarily focus on linking mechanical behavior of materials with the underlying microstructure.  Specific investigations involve in metallic & polymer biomaterials, shape memory materials, and nanometer-scale materials.

 

Shape Memory Polymers

Shape Memory Alloys

Small-Scale Metal Deformation

 

Shape Memory Polymers

Shape memory polymers (SMPs) are a part of a broad class of materials termed "smart materials", due to their ability to respond an outside stimulus. In this case, shape memory refers to the polymer's ability to recover a previously defined shape most often triggered by an increase in thermal energy. This relatively new type of shape memory material has been proposed for a variety of applications (e.g. biomedical devices, switchable adhesives, sensors/actuators). Because the shape memory behavior in polymers is a function of polymer chemistry and processing history, rather than an intrinsic property, a wide range of possibilities exist to tailor thermomechanical properties for optimized design. While previous work has given a broad perspective on strain recovery, stress actuation, and biocompatibility, utilization of SMPs is in it's infancy. Our research involves tailoring thermomechanical properties for various polymer chemistries. We have also focused on developing biodegradable SMPs.

Shape Memory Alloys

NiTi shape memory alloys are capable of undergoing a reversible thermo-elastic martensitic solid-state phase transformation.  In essence, NiTi experiences relatively large amounts of inelastic deformation and subsequently recovers deformation after load removal or upon the application of heat due to rearranging its atomic lattice structure.  It is therefore possible to accomplish a relative shape change that is entirely reversible via this stress-induced martensitic phase transformation.  NiTi already has applications in micro-systems primarily due to the extremely high work output per unit volume provided by the martensitic phase transformation.  In fact NiTi has the potential to be an ideal nano-scale actuator, because the deformation recovery is inherent to the material.  Recent small-scale research specifically focused on phase transformation behavior of NiTi has demonstrated inconsistent results, indicating there may be a fundamental shift in phase transformation behavior as size scale is reduced.  Relying on a strong background in both bulk and nano testing of NiTi, we have continued our ongoing investigation into the martensitic transformations at small scales using cutting edge experimental techniques.  Additionally, because the phase transformation is both stress and temperature induced, understanding the role of temperature at small scales is also of interest.  Isolating the particular mechanisms of deformation in NiTi will provide insight into the nature of phase transformations as well as present an opportunity to tailor shape recovery properties at nanometer scales

Small-Scale Metal Deformation

Knowledge of material properties at the nano-scale will fundamentally influence the way materials will be manufactured and utilized.  Future materials may be designed at multiple length scales and built from quantum and nanometer sized components in order to exploit their unique functionality.  However, knowledge of the fundamental mechanical properties at this scale is lacking, and is necessary in order to incorporate them into any proposed devices or systems.  For example, metals with a critical dimension in the nanometer regime have demonstrated a dramatic increase in strength.  While some estimates have demonstrated stresses approaching the theoretical strength of the material, variations in strength measurements are in the giga-Pascal range.  Understanding and quantifying

mechanical behavior is an essential part of creating small-scale systems, and is possible using advanced experimental methods.  In an effort to build on our previous sub-micron compression testing, our continuing research investigates deformation mechanisms in miniaturized metals with emphasis on dislocation plasticity, diffusional creep, fatigue, and fracture.  Furthermore, the emergence of nanostructures (e.g. nano-particles, nano-tubes, nano-wires, and nano-belts) offers the opportunity to directly test mechanical properties of nanoscale building blocks rather than thin films or bulk material.  Formation of nano-structures typically lends itself to the creation of unique material structures, further encouraging the need to establish a link between this structure and the mechanical behavior at small-scales. Based on the fundamental insight gained, optimized systems can be developed to be transferred into useful technical devices.