Shape Memory

The fatigue thresholds of nickel-titanium alloy with martensite start temperature of 80°C were measured in air from room temperature to 150°C and load ratios of 0.1 to 0.9. The ΔK versus Kmax fundamental threshold curves are characteristic of ductile metals at all temperatures tested and are consistent with the existence of two threshold parameters, ΔKth* and Kmax,th*, for cyclic and

After being severely and quasi-plastically distorted, shape memory materials (SMMs) are able to recover their original shape at the presence of the right stimulus. In recent years we have seen significant progress from shape memory alloys (SMAs) to shape memory polymers (SMPs). In this paper, we summarize the most recent advances in SMMs. The focus is on the new features found in traditional SMMs, namely SMAs and SMPs, and a newly emerging type of SMM, namely shape memory hybrid (SMH), which enables enthusiasts to design SMMs with tailored properties/features for a particular application without the aid of experts (“do-it-yourself” manner).

Thermomechanical and functional properties of shape memory polymer (SMP) have been presented. Taking advantages from high quality testing machine and infrared camera, mechanical characteristics and temperature changes of the shape memory polyurethane specimens subjected to tension and simple shear processes were recorded, worked out, completed and discussed. The material is said to demonstrate a shape memory properties since it can

Many applications involve actuated devices made of shape memory alloys, but the lack of efficient numerical tools hinders the development of such technologies. Software using a finite element method like ANSYS allows the user to predict complex responses of a system without extensive programming. In this paper, a homemade phenomenological 1D bilinear model is programmed through the USERMAT procedure in

The different crystal structures of ferromagnetic Ni$_2$MnGa have been calculated using density functional theory (DFT) with special emphasis on the modulated structures 10M and 14M. These are important for understanding the stability of Ni$_2$MnGa martensites and their functionality as shape-memory materials. The modulated structures have been optimized in the calculations and their properties are discussed in relation to the structures without modulation. The occurrence of the modulated structures is related to the soft TA$_2$ phonon mode observed in Ni$_2$MnGa. The latter is related to the specific nesting behavior of the Fermi surface in Ni$_2$MnGa. Particular shapes of the modulated structures are stabilized by the covalent interaction mediated by the \textit{p}-orbitals of Ga and \textit{d}-orbitals of Ni. The role of this interaction becomes clear seen when considering the phonon dispersion spectrum of Ni$_2$MnGa, where some characteristic anomalies occur in the coupling of acoustical vibrational modes and the optical modes of Ni.

Over the past several years, NiTi shape memory alloys have seen a treme ndous increase in applications, which exploit the materials' ability to repeatedly recover inelastic strains up to 8 percent. Despite our solid understanding of the relationship between the deformation modes and the transformations in NiTi, we do not have a clear understanding of the fracture initiation processes. In this paper, we present a research methodology to characterize the effects of crystallographic orientation and grain size on the deformations and transformations in Nitinol. The approach involves a combination of full -field strain measurements using Moiré interferometry and finite element modeling. We explore the effects of localized stress raisers such as notches and grain boundaries, and discuss implications for medical device design.