Thermomechanical training and structural tests for adaptive SMA bumps with two-way shape memory effect

In this paper, shape memory alloy (SMA) bumps with two-way shape memory effect (TWSME) are trained by simple but effective training approaches, which provides a new idea for the actuations of the shock control bump (SCB) on airfoil. Two different configurations of bump structures (2D and 3D SMA bumps) are designed and fabricated. The bumps are required to exhibit TWSME so that it can change its shape by heating and cooling between two stable states at austenitic phase and martensitic phase, respectively. To obtain the TWSME, the material is trained in the range of martensitic finish temperature and austenitic finish temperature whilst a displacement boundary condition is imposed. A set of fixtures, which can be assembled to the universal testing machine (UTM), are designed to achieve the clamped boundary condition during thermal cycles of the training process. After training, SMA bumps with the TWSME, that bulge at low-temperature and become flat at high, are obtained. Structural tests and deformation control are then carried out afterwards to show the deformation performance of the trained SMA bumps.

Investigation of the Residual Stress in a Multi-Pass T-Welded Joint Using Low Transformation Temperature Welding Wire

We investigated whether low transformation temperature (LTT) welding materials are beneficial to the generation of compressive residual stress around a weld zone, thus enhancing the fatigue performance of the welded joint. An experimental and numerical study were conducted in order to analyze the residual stress in multi-pass T-welded joints using LTT welding wire. It was found that, compared to the conventional welded joint, greater tensile residual stress was induced in the flange plate of the LTT welded joints. This was attributed to the reheat temperature of the LTT weld pass during the multi-pass welding. The formerly-formed LTT weld pass with a reheat temperature lower than the austenite finish temperature converted the compressive residual stress into tensile stress. The compressive residual stress was generated in the regions with a reheat temperature higher than the austenite finish temperature, indicating that LTT welding materials are more suitable for single-pass welding.

Influence of Ausforming Treatment on Super Elasticity of Cu-Zn-Al Shape Memory Alloy for Seismic Energy Dissipaters

In order to develop the application of the more cost-effective copper-based shape memory alloys (SMAs), rather than nickel–titanium as earthquake energy dissipaters, the influence of ausforming-induced plastic deformation on phase transformations, microstructure, super elasticity and mechanical properties of the shape memory alloy Cu-26Zn-4Al was examined. These specific SMA properties were targeted by applying appropriate parameters of the thermomechanical (the so-called ausforming) process: beta-phase homogenization at 800 °C for 20 min, one-step hot rolling at 800 °C and water quenching. The results showed significant microstructural changes, increased mechanical resistance and change in the phase transformation behavior. The SMA treated by ausforming retained the reversible austenitic–martensitic transformation ability, with the appearance of the super-elastic effect up to 6% of strain recovery. Although some strengthening occurred after hot rolling (an increase in true yield strength of 125 MPa was detected), all phase transformation temperatures were decreased. The smallest decrease was detected for the austenite finish temperature (32.8 °C) and the largest for the martensite finish temperature (42.0 °C), allowing both the expansion and the lowering of the temperature range of super elasticity, which is favorable for construction applications. It is concluded that it is possible to achieve an optimal combination of adequate strength and improved transformation behavior of Cu-Zn-Al alloy by applying the ausforming treatment.

Vibration Absorption in a Nonlinear Metamaterial Beam Incorporating Shape Memory Alloys

Locally resonant metamaterials are capable of demonstrating low-frequency vibration absorption due to the formation of stop-bands. In this work, the multi-mode vibration absorption capability of an adaptive nonlinear metamaterial beam is investigated. The metamaterial beam is idealized as a hinged-hinged finite Euler-Bernoulli beam with a von-Kármán geometric type nonlinearity that is attached to a distributed cellular array of shape memory alloy (SMA) spring–mass resonators. Numerical studies are performed to evaluate the effects of dissipation and change in elastic modulus due to material phase change of SMA pseudoelasticity on the dynamic response of the beam. Using a modal analysis approach, stop-bands are generated at the first three nonlinear frequencies of the beam. The frequency response demonstrates a hardening behavior at a temperature significantly higher than the austenite finish temperature while conversely demonstrating a softening behavior at a temperature slightly above the austenite finish temperature.

Determination of the critical points in solid-state phase transformation of some hypoeutectoid steels

This study allowed, by dilatometric analyses, both to highlight the solid state transformations that occurred during the continuous heating of two hypoeutectoid steels, as well as to investigate the effect of the heating rate on the critical points at which these transformations occurred. The eutectoid transformation (the pearlite dissolution into austenite) was carried out in a temperatures interval, ranging between pearlite dissolution start temperature (Ac1) and pearlite dissolution finish temperature (denoted Acfp in this article). Increasing the heating rate determined a displacement of the critical points in solid-state phase transformation to higher temperatures; these displacements were more significant for the Acfp point, than for the critical points Ac1 and Ac3.

Perspective ways to improve the strength properties and resistance to hydrogen induced cracking of low-alloy pipe steels

The perspective ways to improve the strength properties and resistance to hydrogen induced cracking (HIC) of low-alloy pipe steels were established. The possibility to improve the strength properties and resistance to HIC of plates due the additive of 0.15% molybdenum while decreasing finish temperature of accelerated cooling from 560 to 420°C was found. The influence of heating in the α-, (α+γ)and γ-regions followed by air cooling on the mechanical properties and resistance to HIC of various alloying systems pipe steels plates was studied. The possibility of increasing the strength while maintaining the resistance to HIC of plates by tempering at heating in the α-region was shown.

Behaviour of Superelastic Nickel Titanium Shape Memory Alloy Material under Uniaxial Testing and its Potential in Civil Engineering

Smart structures are defined as structures that able to adapt and maintain structural characteristics in dealing with changes of external disturbance, environment and unexpected severe loadings. This ability will lead to improve structural safety, serviceability and structural life extension. Shape memory alloys is one of the smart materials which has potential to be integrated in structural system to provide functions such as sensing, actuation, self-adapting and healing of the structures. The unique characteristic of shape memory alloys material is the ability to ‘remember’ its original shape after deformation. Nickel Titanium superelastic shape memory alloy wire is popular and widely used in many engineering fields and owned fully recovery of maximum strain of 6%-13.5% which is among the best shape recovery limit in alloy materials. The austenite finish temperature plays important role in stress-strain behaviour of superelastic shape memory alloys where higher stress required to complete martensite transformation with the increase of austenite finish temperature. The similar behaviour also is observed in the case of higher strain rate. The behaviour of superelastic shape memory alloys need to be studied before implementing in the structural system, so the targeted improvement for the structural system can be achieved.