Yanjun Zheng

Temperature memory effect of a nickel–titanium shape memory alloy

An incomplete transformation cycle induces a kinetic stop in the following complete transformation cycle in shape memory alloys. Therefore, the kinetic stop can be regarded as a memory of the previous arrest temperature. Herein, we show that the temperature memory effect of a nickel–titanium shape memory alloy can be expanded to be operational in a very wide temperature range by prestraining and constraining, which may be exploited for various practical applications.

Constrained thermoelastic martensitic transformation studied by modulated DSC

The constrained thermoelastic martensitic transformation of TiNiCu wires embedded in an epoxy matrix was studied by modulated differential scanning calorimetry (MDSC). Results showed that the non-reversing heat flow (NHF) signal of TiNiCu wires in a free condition is quite small whether the wires are pre-strained or not, while the NHF signal of the TiNiCu wires embedded in the epoxy matrix increases significantly with increasing level of pre-strain. Analysis showed that this significant increase of the NHF value is related to a term which is a function of the stress rate (dσ/dT). If the interface is perfectly bonded, this term contributes to the reversing heat flow (RHF) signal, while if the interface is debonded, this term contributes to the NHF signal.

Thermal response of glass fibre/epoxy composites with embedded TiNiCu alloy wires

Abstract Adaptive materials based on glass fibre/epoxy composites have been produced by embedding TiNiCu alloy wires. Prestrained TiNiCu wires can generate large recovery stresses in a constrained condition and, therefore, modify the physical and mechanical properties of the composites if they are heated above the reverse transformation temperature of the wires. In this paper it has been shown that the rate at which the recovery stress increases with temperature, d σ /d T , decreases with increasing prestrain level. For wires with a high prestrain level (4–6%), the recovery stress increases immediately upon heating, even when the temperature is still below the reverse starting temperature, A s . As a result, the absolute value of the strain rate at which the composites contract decreases with increasing prestrain level, and the composites with a high prestrain level (4–6%) contract immediately upon heating. No direct relationship was found between the interfacial failure temperature and the prestrain level, but the contracting strain that the composites can provide before the interfacial failure decreases with increasing level of prestrain.

Qualitative and Quantitative Evaluation of the Interface in Activated Shape Memory Alloy Composites

The interfacial quality of shape memory alloy (SMA) composites was experimentally evaluated by differential scanning calorimetry, and quantitatively calculated using a combination of Šittners model and a composite cylinder model. Results have shown that the damage level of the interface in a heating process can be quantitatively evaluated by measuring the transformation enthalpy of the subsequent heating cycle and comparing it with the S-shaped enthalpy kinetic curve. A combination of Šittners model and a composite cylinder model is capable of calculating both the thermal behavior and the triaxial stress state of SMA composites.

Partial transformation behavior of prestrained TiNi fibers in composites

The effect of partial cycling on the reverse martensitic transformation of TiNi fibers embedded in a cement composite has been studied. Results show that after a partial transformation cycling, a part of the martensite is transformed and then recovered. The remaining martensite is stabilized by plastic deformation due to the recovery stress.

Shape Memory Alloy Wires Turn Composites into Smart Structures Part I: Material requirements

Composites containing thin Shape Memory Alloy (SMA) wires show great potential as materials able to adapt their shape, thermal behavior or vibrational properties to external stimuli. The functional properties of SMA-composites are directly related to the constraining effect of the matrix on the reversible martensitic transformation of the embedded pre-strained SMA wires. The present work reports results of a concerted European effort towards a fundamental understanding of the manufacturing and design of SMA composites. This first part investigates the transformational behavior of constrained SMA wires and its translation into functional properties of SMA composites. Thermodynamic and thermomechanical experiments were performed on SMA wires. A model was developed to simulate the thermomechanical behavior of the wires. From the screening of potential wires it was concluded that NiTiCu, as well as R-phase NiTi appeared as best candidates. Requirements for the host composite materials were surveyed. A Kevlar-epoxy system was chosen. Finally, the quality of the SMA wire-resin interface was assessed by two different techniques. These indicated that a thin oxide layer seems to provide the best interfacial strength. A temperature window in which SMA composites can be safely used was also defined. The manufacturing and properties of the SMA composites will be discussed in Part II.