Low-energy tensile-impact behavior of superelastic NiTi shape memory alloy wires

Superelastic property of shape memory alloys (SMAs) is becoming increasingly important for impact applications due to their large recoverable strains and high capacity to dissipate energy. In this work, tensile behavior of superelastic NiTi SMA wires at impact strain rates was studied by instrumented tensile-impact technique, which allows to obtain material properties on the order of 1–10² s⁻¹. The results show that even at impact strain rates, martensite can be induced by tension in NiTi. At impact, a plateau stress appears during transformations similar to that at quasi-static strain rates, but 100–150 MPa higher in stress. This is due to the higher temperatures achieved during the deformation due to the close to adiabatic nature of the impact event. The influence of the strain rate over the mechanical behavior of NiTi was spread to the quasi-static strain rates so that the evolution of several parameters was also studied on the range 10⁻⁵–10² s⁻¹. Therefore, forward stress-induced martensitic (SIM) transformation stresses (σ^Ms and σ^Mf) and deformation energy (E_d) increase with strain rate, but they are strain rate independent from 10⁻¹ s⁻¹ at least until 10² s⁻¹. Reverse SIM transformation stresses (σ^As and σ^Af), recoverable strain energy (E_r), and dissipated energy (W_d) depend mainly on maximum strain achieved during the deformation, but for strains corresponding to a load–unload cycle with complete SIM transformation, σ^As, σ^Af and E_r are higher at impact than at quasi-static strain rates, and W_d shows similar values at very low strain rates and at impact.     

Effect of strain rate on properties of superelastic NiTi thin wires

This study deals with the effect of strain rate on tensile and energy absorbing properties of superelastic NiTi thin wires. It also attempts to gain an understanding of the interplay of the ductile behavior, temperature and strain rate effects, energy storage and cycling. The wires are in austenite condition at room temperature and above. The strain rates imposed during testing range from 0.2 to 180%/min (i.e., 0.06–54 mm/min) corresponding to a frequency of 2.77 × 10⁻⁴ to 0.25 Hz for strain amplitudes of 6%. The corresponding frequency for 8% strain amplitude is 2.08 × 10⁻⁴ to 0.18 Hz. It is shown that NiTi SMAs exhibit ductility at both low and high strain rates. This is also true for the cold worked and heat treated conditions both below M_f and above A_f. During tensile testing the stress-induced martensite (SIM) plateau increases in length and translates upwards with increase in strain rate up to a certain value. Similarly, the onset of elastic yield stress also increases with strain rate. At high strain rates the SIM segment and elastically deformed SIM segment overlap. The SIM formation is not able to cope with the externally imposed higher strain rates. This is also the reason for the reduction of hysteresis loop at the high strain rates as observed in the cyclic tests.

The dissipated strain energy density (E_d) increases with increasing strain rate up to a certain value beyond which the E_d decreases. It is clear that the mean point of the superelastic loop shifts to the right and upwards (higher stress and higher strain region) for cyclic testing with increase in strain rates. However, it shifts to the right and downwards (lower stress/higher strain regime) for both the 6 and 8% strain amplitude cycling at constant strain rate. The stabilization of residual strain and E_d is based on the same underlying mechanism relating to SIM formation and occurs at the same numbers of cycles.     

Low-cycle fatigue crack advance and life prediction

A new concept for a fatigue process zone within which the actual degradation of the material takes place is proposed. This zone is described as the region in which the stress distribution of the HRR field approaches the maximum flow stress of the material, with the strain localization caused by a sliding-off process. In high-strain low-cycle fatigue conditions, this new concept is shown to be more realistic for the prediction of the fatigue life than that of previous work which has been based on a rough approximation. The proposed feature of the zone is experimentally supported by microhardness measurements. In particular, although this zone is formulated from continuum mechanics, it reflects microstructural factors such as precipitates and stacking-fault energy. Using the developed fatigue process zone and strain intensification in the zone near the crack tip, a modified analytical model for prediction of the continuous low-cycle fatigue life is proposed. The exponent of the Coffin-Manson law obtained from the present prediction is suggested to be 1/(3 + 1), which is different from the previously reported value of 1/(2/'n + 1), and is shown to be in good agreement with experimental results for five alloy systems.     

Behaviour in torsion of superelastic NiTi

Abstract

Torque–strain curves have been established at 20 and 37°C and for up to 100 strain cycles for NiTi tube with an A _f temperature of—8.5°C. The performance mirrors the behaviour in tension and compression. In the first few strain cycles, the curves exhibit the familiar flag shape, but this gradually changes into a sigmoidal curve on repeated cycling. Changes are rapid at first, but after ~ 40 cycles a steady state condition is approached closely. Attempts to analyse the performance in shear in relation to that in tension and compression draw attention to the problems involved in trying to establish flow or yield criteria for the material.     

NiTi superelastic orthodontic archwires with polyamide coating

Twenty orthodontic archwires with 55.2 % Ni and 44.8 % Ti (% weight) were subjected to a dipping treatment to coat the NiTi surface by a polyamide polymer. It has been selected a Polyamide 11 due to its remarkable long lasting performance. The transformation temperatures as well as the transformation stresses of the NiTi alloy were determined in order to know whether the coating process can alter its properties. The adhesive wear tests have been demonstrated that the wear rates as well as the dynamic friction coefficients μ of polymer coated wires are much lower than metallic wires. The corrosion studies have shown that the use of this polymer, as coating, seals the NiTi surface to prevent corrosion and the release of nickel ions. The average decrease of Ni ions release due to this coating is around 85 %.     

Mechanism of fracture of NiTi superelastic endodontic rotary instruments

The aim is to investigate the premature catastrophic fracture produced for different periods during clinical endodontic treatment of two brands of NiTi endodontic rotary instruments. 3 samples as-received, 6 samples used with patients for 2 and 7?h and 5 samples fractured were studied for each brand of endodontic NiTi rotary instruments. Transformation temperatures (M_s, M_f, A_s and A_f) and enthalpies of transformation were determined by calorimetry. Critical stresses until fracture (σ^β→SIM, σ^SIM→β) were obtained using an electromechanical testing machine. The samples were also visualized by Scanning Electron Microscopy. Calorimetric studies have shown an increase of the M_s and A_s transformation temperatures with time of use as well as a decrease of their stress transformations. Moreover, reverse transformation enthalpies decreased along the time. The enthalpies of transformation decreased because martensitic plates were anchored, which prevented their transformation to austenite; thus losing its superelastic effect. The stabilisation of the martensitic plates induced the collapse of the structure and so the main cause for the fracture. The heat treatment proposed has been increased the life in service of NiTi superelastic endodontic instruments recovering theirs superelastic effect.     

Low-cycle fatigue of austempered ductile irons

The effects of austempering temperature and isothermal transformation time on the low-cycle fatigue (LCF) behaviour in ductile irons have been studied. The fracture surfaces were observed by a scanning electron microscope in order to understand the fracture mechanism of LCF. From the results, it can be concluded that the best LCF behaviour is for the irons austenitized at 950 °C and there is very good cyclic stability at the lower strain amplitude irrespective of the austempering condition. However, there is a little cyclic softening at higher strain amplitudes for all the austempering conditions. Under a larger strain amplitude, the best LCF behaviour is for the specimen that has undergone austempering at a higher temperature, but under a smaller strain amplitude, the best LCF behaviour is for the specimen austempered at 350 °C.     

Low-cycle fatigue life assessment for gas turbine engine disks under flight cycle conditions

We address the effects of the actual flight cycle on durability of gas turbine engine disks under low-cycle fatigue. An approach is proposed which improves reliability of life cycle prediction owing to schematization of flight cycle with a criterion for reaching the maximum intensity of total strain range. Contribution of subcycles to the cumulative damage is demonstrated. __________ Translated from Problemy Prochnosti, No. 1, pp. 129–133, January–February, 2009.     

Vibrations of plates with superelastic shape memory alloy wires

The article illustrates an approach to the passive vibration control of thin plates utilizing prestressed superelastic shape memory alloy (SMA) wires. The SMA wires can freely slide within protective sleeves that are either embedded within the structure or bonded to its surface. The vibration control mechanism combines an effective continuous elastic foundation representing the support provided by SMA wires to the structure with the energy dissipation as a result of the hysteresis occurring in the wires. The other approach to the vibration control employs superelastic wires attached to the structure at discrete points. The mathematical formulation of the problem presented in the article can be adopted for a rigorous computational analysis. In particular, a closed form expression is obtained for the loss factor in large aspect ratio plates supported at the midspan by a system of parallel SMA wires. As follows from numerical examples presented for such plates, the proposed method offers a significant damping, far exceeding that observed in conventional engineering structures.     

Low-cycle fatigue life of SiC-particulate-reinforced Al-Si cast alloy composites with tensile mean strain effects

The low-cycle fatigue behaviour of a SiC-particulate-reinforced Al-Si cast alloy with two different volume fractions has been investigated under strain-controlled conditions with and without tensile mean strains. The composites and the unreinforced matrix alloy showed cyclic hardening behaviour. The composite having a higher volume fraction of the SiC particles exhibited a more pronounced strain-hardening rate. For the tensile mean strain tests, the initial high tensile mean stress relaxed to zero for the ductile Al-Si alloy, resulting in no influence of the tensile mean strain on the fatigue life of the matrix alloy. However, tensile mean strain for the composite caused tensile mean stresses and reduced the fatigue life. The pronounced effects of mean strain on the low-cycle fatigue life of the composite compared to the unreinforced matrix alloy were attributed to the initial large prestrain causing non-relaxing high tensile mean stress in the composite with limited ductility and cyclic plasticity. Fatigue damage parameter using strain energy density accounted for the mean stress effects quite satisfactorily. Predicted fatigue life using this damage parameter correlated fairly well with the experimental life within a factor of 3. Moreover, the fatigue damage parameter indicated the inferior life in the low-cycle regime and superior life in the high-cycle regime for the composite, compared to the unreinforced matrix alloy.     

Low-cycle fatigue life prediction of a polycrystalline nickel-base superalloy using crystal plasticity modelling approach

A crystal plasticity model is developed to predict the cyclic plasticity during the low-cycle fatigue of GH4169 superalloy. Accumulated plastic slip and energy dissipation as fatigue indicator parameters(FIPs) are used to predict fatigue crack initiation and the fatigue life until failure. Results show that fatigue damage is most likely to initiate at triple points and grain boundaries where severe plastic slip and energy dissipation are present. The predicted fatigue life until failure is within the scatter band of factor 2 when compared with experimental data for the total strain amplitudes ranging from 0.8% to 2.4%.Microscopically, the adjacent grain arrangements and their interactions account for the stress concentration. In addition, different sets of grain orientations with the same total grain numbers of 150 were generated using the present model. Results show that different sets have significant influence on the distribution of stresses between each individual grain at the meso-scale, although little effect is found on the macroscopic length-scale.     

High ultimate tensile stress in nano-grained superelastic NiTi thin films

NiTi-films were fabricated by dc magnetron sputtering from melt-cast disc targets. The freestanding films revealed superelastic properties in tensile tests. At 37 °C superelastic properties were achieved showing a closed-loop hysteresis and a plateau of more than 5% strain. The ultimate tensile strength exceeded 1180 MPa for the sputtered films at a maximum strain of 11.5%. This remarkable improvement in mechanical properties over those reported in previous studies correlates with a textured, fine grained (50–200 nm), single phase microstructure, confirmed by transmission electron microstructure. Moreover, these grains revealed a texture which was not found in earlier studies concerning sputtered films. Finally, the prepared specimens did not reveal any evidence of disc or lens shaped Ti₃Ni₄ precipitates but a relatively homogeneous chemistry.     

Deformation energy of NiTi shape memory wires

Deformation energy of NiTi wires with B2 and R phases was studied by the multiple tensile testing (MTT) method. In traditional materials, the total energy required to tear specimens is assumed to be the sum of elastic, uniform plastic, and post-uniform or tearing energy components. For the shape memory alloys, however, this classification is not valid due to their unusual superelastic/shape memory characteristics. Using a modified MTT method, different energy components were calculated by plotting different combination of deformation energies divided by the specimen cross-sectional area against the gage length of the specimens. The slope of the obtained straight line demonstrates the summation of the elastic, superelastic/shape memory, second elastic, and plastic energy per unit volume and its intercept gives the value of tearing energy. It was found that the uniform plastic energy per unit volume for the R-phase wires was considerably higher than that for the B2-phase wires. This caused a marked enhancement in the total deformation energy of the R-phase wire, as compared to the B2-phase wire. The effect of strain rate on the tensile behaviour and deformation energies of these materials was also investigated. Except the plateau stress of the tensile curve which was raised for both wires, the B2-phase wires were almost strain-rate-independent, whereas the R-phase wires were significantly influenced by the variation in strain rate.     

Low-cycle fatigue life prediction of spot welds based on hardness distribution and finite element analysis

Elasto-plastic finite element analysis was carried out for single spot tensile shear spot welds with software ANSYS. According to the relationship between micro-hardness and strength, cyclic material constants of different zones in the periphery of spot welds were determined in accordance with hardness distribution, as well as base material fatigue parameters. Using the local stress and strain obtained from finite element analysis, fatigue lives of spot welds were predicted with Morrow’s modified Manson–Coffin equation and Smith–Watson–Topper damage equation. Life prediction results showed that both equations gave good agreement with experimental data within low-cycle fatigue life regime.