Marco Urbano

SMA Numerical Modeling Versus Experimental Results: Parameter Identification and Model Prediction Capabilities

In this work, we briefly review the one-dimensional version of a well-known phenomenological shape memory alloy (SMA) constitutive model able to represent the main macroscopic SMA macroscopic behaviors (i.e., superelasticity and shape-memory effect). We then show how to identify the needed parameters from experimental results and, in particular, from strain-temperature tests. We finally use the obtained material parameters to test the prediction properties of the model, comparing numerical results with some experiments (different from those used for the identification), and we discuss model capabilities and further required enhancements.

A Numerical/Experimental Study of Nitinol Actuator Springs

This study deals with the numerical modeling, simulation and experimental analysis of shape-memory alloy (SMA) helicoidal springs. An experimental campaign is conducted on both SMA straight wires and helicoidal springs that experienced the same annealing process. Then, we use such experimental results to investigate three phenomenological constitutive models able to represent SMA macroscopic behavior. In particular, after the identification of all the material parameters from experimental results on SMA wires, we inspect the thermo-mechanical behavior of SMA helicoidal springs by comparing numerical predictions to experimental data. Finally, we discuss models capabilities and some aspects characterizing SMA material behavior.

Theoretical and Experimental Investigation on SMA Superelastic Springs

The mechanical behavior of superelastic springs is investigated in this study. The goal is to evaluate the device response and to exploit the material superelastic behavior, main concerns being material and geometrical response nonlinearity. The investigation is made of two parts, i.e., an experimental campaign and a numerical model proposal. Experimental tests have been performed on superelastic SMA coil springs considering load history in tension and compression for three different spring geometrical configurations. Tested specimens experience a maximum elongation larger than the original spring axis length. The response is not symmetric and under compression it is affected by buckling instability. Nevertheless, experimental results show a very good superelastic behavior with no damage and with negligible residual displacements. Numerical analyses have been performed to reproduce the experimental campaign results. A simple finite element model is proposed. Experimental and numerical result agreement is very good. The numerical model turns out to be a powerful design tool even for the very complex geometrical and material nonlinear conditions under investigation. Hence, it is proposed as a useful tool for spring design validation and response prediction.

Theoretical and Experimental Study of the Shape Memory Effect of Beams in Bending Conditions

In this study, the shape memory effect of SMA beams under complex stress conditions is studied by means of a finite element model. The 1D version of a well-established SMA constitutive model is utilized in the numerical computations and the required parameters are obtained experimentally starting from thermal cycling tests in tension under different constant loads. After being calibrated, the model is used to compute the deformation of beams loaded in bending and undergoing thermal cycling; three-point bending and cantilever configurations are considered in this stage. Finally, the response predicted by the model is compared to experimental results and model capabilities are discussed. In particular, insight of the stress and strain evolution in bending is provided.

Inclusions Size-based Fatigue Life Prediction Model of NiTi Alloy for Biomedical Applications

Current standards consider the size and distribution of inclusions in semi-finished material, but do not place requirements on final biomedical devices made of NiTi shape memory alloys. In this paper, we analyze this by comparing the fatigue performances of NiTi superelastic wires obtained by different processes through a simple bilinear model of fatigue response in terms of strain life. The fracture surfaces of failed wires are analyzed through SEM microscopy and data regarding the presence of particles, and their morphology is recorded and analyzed using Type-I extreme value distribution. The results show a strong correlation between the fatigue limit of wires (in terms of strain) and the predicted extreme values of inclusions at fracture origin. Then, following the concept of treating the inclusions as ‘small cracks,’ a simple relationship between fatigue limit strain range and inclusion size is proposed based on ΔKth data from the literature. The model is compared with the fatigue data obtained from the tested wires.

Investigation on the Hysteretic Behavior of NiTi Shape Memory Wires Actuated Under Quasi-Equilibrium and Dynamic Conditions

The functional characterization of SMAs for actuation is typically performed by measuring the specimen deformation under constant load during a controlled thermal cycling across transformation temperatures. Under dynamic actuation, transformation temperatures different from those measured in quasi-equilibrium conditions have been observed. The aim of this work is to better investigate and understand these phenomena. Direct and indirect heating of shape memory wires under several loading conditions are examined in detail. According to the experimental results, the hypothesis is to consider the observed differences as an effect of the thermal cycling rate on the internal friction. However, the presented data seem do not fully confirm this idea. Further experiments will be carried out in order to directly measure the internal friction of the material under the same working conditions.

Pumping characteristics of metal films in a vacuum glass vessel: Experimental and theoretical issues

The evaluation of the pumping characteristics of metallic films deposited onto glass surfaces in a vacuum environment is a very important issue for several industrial and research applications. Many years ago, an optimized experimental setup and method were defined to measure the pumping characteristics of barium films inside cathode ray tubes (CRTs). [P. della Porta and F. Ricca, Advances in Vacuum Science and Technology (Pergamon, New York, 1960), Vol. II, p. 661 and P. della Porta and L. Michon, Vacuum 15, 536 (1965)]. However, some technological limitations, related both to the particular experimental configuration and to the method used, prevented the possibility of extending this approach to a more general case, including adsorbing materials different from barium, deposited onto surfaces having a geometry different from that of a CRT. The progress in vacuum technology makes it possible today to use a large variety of components to assemble an experimental vacuum apparatus. Moreover, the availability...

Modeling Permanent Deformations of Superelastic and Shape Memory Materials

In this paper we propose a modification of the polycrystalline shape memory alloy constitutive model originally proposed by Souza. By introducing a transformation strain energy with two different hardening coefficients, we are able to take into account the effect of the martensitic transformation of unfavorably oriented grains occurring after the main plateau. By choosing a proper second hardening coefficient, it is possible to reproduce the correct stress strain behavior of the material after the plateau without the need of introducing a much smaller Young modulus for martensite. The proposed modification is introduced in the model comprising permanent deformation effects. Model results for uniaxial stress tests are compared to experimental results showing good agreement.

Study of the Influence of Inclusions on the Behavior of NiTi Shape-Memory Alloys in Thermal Cycling by Means of Finite Element Method

Despite the number of articles on the subject, the influence of inclusions on the behavior of nickel titanium shape memory alloys is still controversial. Numerical simulation can play a fundamental role in providing insight into this subject. As far as superelastic materials are concerned, it has been shown by means of finite element simulations that, in wires loaded in rotary bending conditions, the presence of inclusions greatly increases the stress distribution in the cross section, and that the maximum stress increases as the distance between the inclusion and the neutral axis increases. In this work, a similar approach is used to analyze the effect of inclusions on thermal cycles of wires loaded in tension. By means of a thermomechanical constitutive model implemented in ANSYS, the alternate stress/strain field in the presence of a particle is computed. The particle is either located close to the wire surface or in the center of the wire section. An empirical damage evolution law is applied to the stress strain field showing a region adjacent to the particle where premature mesocrack nucleation is likely to take place.

Fivefold enhancement in stability of organic light-emitting diodes with addition of non-evaporable getter pumps

We demonstrate a significant enhancement of the operational stability of organic light-emitting diodes (OLEDs) by fabricating the devices under ultrahigh-vacuum (UHV) conditions in the region of 10−10–10−11 Torr. The UHV condition is achieved by utilizing non-evaporable getter pumps together with regular turbo molecular pumps in an OLED deposition chamber. The short-term stability of the OLEDs is prolonged fivefold because of the removal of detrimental gases, especially water. We suggest that ultra-clean fabrication conditions are indispensable for revealing the true intrinsic degradation mode of OLEDs. Additionally, we analyze the effect of operating residual gas analyzers during the device fabrication.