Bending rotation HCF testing of pseudoelastic Ni‐Ti shape memory alloys

In the present work, a computer‐controlled test rig for simultaneous fatigue testing of several pseudoelastic NiTi wires through bending rotation is described. Bending rotation fatigue (BRF) testing represents a displacement‐controlled experiment where a straight wire is bent into a semi‐circle und forced to rotate around its axis. Thus, each point on the wire surface is subjected to alternating tension and compression. A test rig, which allows to control loading amplitudes, rotation frequencies and temperatures is described. We report preliminary results of an experimental program, which aims for a better understanding of fatigue lives, crack initiation, and crack growth in pseudoelastic NiTi wires. It was found that a good surface quality is of utmost importance to avoid early crack initiation. Wöhler curves of pseudoelastic NiTi wires typically show two different regimes depending on the maximum imposed surface strain during bending rotation fatigue testing. Larger strain amplitudes, which are associated with macroscopic formation of stress‐induced martensite, result in relatively low fatigue lives (LCF regime). In contrast, cycle numbers exceeding 10⁷ were obtained for strain amplitudes where no large scale stress‐induced formation of martensite occurred (HCF regime).     

Effect of microstructure on the fatigue of hot-rolled and cold-drawn NiTi shape memory alloys

We present results from a systematic study linking material microstructure to monotonic and fatigue properties of NiTi shape memory alloys. We consider Ni-rich materials that are either (1) hot rolled or (2) hot rolled and cold drawn. In addition to the two material processing routes, heat treatments are used to systematically alter material microstructure giving rise to a broad range of thermal, monotonic and cyclic properties. The strength and hardness of the austenite and martensite phases initially increase with mild heat treatment (300 °C), and subsequently decrease with increased aging temperature above 300 °C. This trend is consistent with transmission electron microscopy observed precipitation hardening in the hot-rolled material and precipitation hardening plus recovery and recrystallization in the cold-drawn materials. The low-cycle pseudoelastic fatigue properties of the NiTi materials generally improve with increasing material strength, although comparison across the two product forms demonstrates that higher measured flow strength does not assure superior resistance to pseudoelastic cyclic degradation. Fatigue crack growth rates in the hot-rolled material are relatively independent of heat treatment and demonstrate similar fatigue crack growth rates to other NiTi product forms; however, the cold-drawn material demonstrates fatigue threshold values some 5 times smaller than the hot-rolled material. The difference in the fatigue performance of hot-rolled and cold-drawn NiTi bars is attributed to significant residual stresses in the cold-drawn material, which amplify fatigue susceptibility despite superior measured monotonic properties.     

镍钛合金在医疗器械领域应用和表面改性研究进展

镍钛合金基于特有的超弹性、形状记忆效应,广泛应用于医疗器械领域。其中超弹性表现为大变形下的弹性应变,应用于血管和腔道介入器械、口腔正畸丝、根管器械等;形状记忆效应可实现低温下易变形、体温下自回复,应用于热激活正畸丝、骨科、矫形外科、缝线等。此外,可通过多样化的表面改性技术,提升镍钛合金的生物相容性、腐蚀抗性、摩擦磨损性能,优化产品性能,拓展其在医疗器械制品中的应用范围。     

Fatigue behaviour of nickel–titanium superelastic wires and endodontic instruments

Endodontic files made of nickel–titanium (NiTi) superelastic wires can be employed in rotary techniques for cleaning and shaping curved root canals, suffering tensile–compressive strain cycles with maximum amplitudes between 3 and 5%. The aim of this work was to study the fatigue behaviour of this material under such high deformation conditions, using NiTi instruments and superelastic wires taken from their production line. One hundred load–unload tensile cycles in the superelastic regime (4% elongation) were applied to NiTi wires. New endodontic instruments were fatigue‐tested simulating the geometrical conditions found in their clinical use. It was found that only small changes took place in the parameters describing the mechanical behaviour of the cycled wires. The measured average number of cycles to failure varies inversely with the maximum tensile strain amplitude in the fatigue tests (r= 0.993).     

Damping Factors and Fatigue Notch Factors of Pseudoelastic NiTi‐Shape Memory Alloys

Pseudoelastic NiTi‐ shape memory alloys (SMAs) provide a high damping capacity and can be used in order to achieve a reduction of peak loads being caused by unexpected shock loading. These “pseudoelastic” properties are related to the formation of martensite M from austenite A, which has been induced by stress; they allow to refer to SMAs as functional materials. Furthermore, these functional materials can operate at high stresses and thus, have to withstand severe mechanical loadings like classical structural materials. In combination, these characteristics provide opportunities for technical applications, e.g., to reduce vibrations or to reduce peak loads caused by shock loading. An extensive knowledge of the functional and structural fatigue behaviour of the material is required to design SMA components. NiTi hollow shaft samples and solid shaft samples have been tested under cyclic torsional loading conditions in a load‐controlled mode. By using these two geometries the influence of the sample geometry on the fatigue behaviour can be investigated. In addition, a test programme has been elaborated in order to investigate the behaviour of the material when subjected to bending. The experimental data have been evaluated describing the transformation behaviour induced by stress concerning transformation stress, apparent shear modulus of the austenite G_A and apparent stiffness τ_Ms (describing the slope of the shear stress‐strain‐curve in the transformation range G_A‐M). These parameters naturally depend on the cycle number, the load amplitude as well as the temperature. Engineering failures are often associated with the presence of notches. In this context, torsion tests on notched samples are planned to be carried out in order to assess the resulting data based on the results obtained from the notch free samples. This will allow to derive simple design rules based on fatigue notch factors, which are needed for engineering design.     

Finite element analysis of hoisting rope and fretting wear evolution and fatigue life estimation of steel wires

This paper is concerned with the finite element analysis of hoisting rope and three-layered strand for the exploration of fretting fatigue parameters and stress distributions on the cross-section. Also, the Archard’s wear law based evolution of fretting wear depth of wires crossed at different angles and implications to fatigue life estimations of fretted wires were presented. The results show that different wires in the rope or strand and distinct material models in the analyses both induce different stress distributions and fretting fatigue parameters. The predicted fretting wear depths of wires show good agreement with experimental results.     

Ermüdungsverhalten einer lasergeschweißten pseudoelastischen NiTi-Legierung unter Biegebeanspruchung

Fatigue behaviour of a laser welded pseudoelastic NiTi alloy under bending stresses The mechanical fatigue of small specimens of a commercial pseudoelastic NiTi alloy produced by laser welding and laser cutting was investigated in strain-controlled bending tests. In comparison with the base material the butt welded specimens showed significantly reduced numbers of cycles to failure. This is mainly caused by higher local strains due to height reductions in the welded joints. Both the welded microstructure and the unwelded one was able to be loaded up to 10⁷ cycles without failure if the strain amplitude remained below 1%.     

Drilling of NiTi Shape Memory Alloys

The machinability of NiTi based shape memory alloys has been examined by conducting drilling experiments. For this reason the cutting parameters cutting speed and feed were varied within a wide range. The machining process was evaluated in terms of tool wear, cutting forces and machining quality. The tool wear was analysed with a scanning electron microscope and the influence of machining on the subsurface zone was evaluated by micro hardness measurements.     

Physical and tribological characteristics of porous NiTi SMA fabricated by powder metallurgy

This paper presents the mechanical and tribological characteristics of the NiTi shape memory alloy (SMA) fabricated by powder metallurgy. This material has prominent applications in micro-electromechanical systems, medical implants, actuator, space and aerospace industries, etc. In every field, wear characteristics plays a dominating role. In present work dry-abrasion wear behavior is determined for NiTi alloy by varying binder percentage. With increasing binder percentage from 2.5 to 15 %, density decreases from 6.5 to 5.3 g/cm3 while porosity increases from 19 % to 51 %. Increasing rotational speed and binder percentage at a constant load the wear rate increases in the NiTi alloy. Due to the presence of hard particles, NiTi exhibits a very small wear rate. The coefficient of friction is also computed for the alloys in present research work. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction were used for the investigation of surface morphology and phases in the NiTi alloy.     

Force‐deflection behaviour of NiTi archwires in a polytetrafluoroethylene (Teflon) bracket system

In orthodontic treatment, archwire forces for tooth movement is influenced by the magnitude of friction encountered during the sliding motion of archwire along the bracket slot. Friction will reduce the archwire force, and subsequently modify the constant force delivery trend into a slope. The aim of this work is to investigate the force‐deflection behaviour of nickel‐titanium (NiTi) shape memory alloy archwire on polytetrafluoroethylene (Teflon) as the bracket. Sliding tests was performed to determine the coefficient of friction of polytetrafluoroethylene‐NiTi material combinations. The force‐deflection behaviour was evaluated from a modified bending tests, at two different configurations; commercial stainless‐steel bracket and hand‐made polytetrafluoroethylene bracket. During test, wires were deflected into different deflections (2 mm, 3 mm and 4 mm) by using four commercial NiTi archwires at different sizes and geometry. From the test; coefficient of friction for polytetrafluoroethylene‐NiTi wire was recorded as small as 0.07. On force‐deflection graphs, bending load on 3 mm and 4 mm deflection were observed higher on stainless steel (SS) bracket compared with polytetrafluoroethylene bracket. Due to small coefficient of friction on polytetrafluoroethylene‐NiTi configuration, the NiTi archwire exhibited the activation and deactivation force within the plateau limit. Greater friction on stainless steel‐nickel‐titanium bracket configuration causes wire to release force in a slope trend. This finding highlights the potential of polytetrafluoroethylene as a material in the bracket slot to minimize friction during orthodontic treatment.     

Selection of drilling conditions for glass fibre reinforced plastics

An experimental investigation has been carried out into the drilling of glass fibre reinforced plastics using HSS twist drills. A test series was conducted using a wide range of cutting conditions and drill geometry, namely cutting speed, feed rate, point angle and helix angle. Decisions relating to the ‘optimum’ drilling conditions were based on the geometrical accuracy and appearance of the produced holes. Drill wear was measured during the test trials and used as a further constraint in ‘optimum’ cutting conditions selection. Speed, feed rate and drill point angle were found statistically to be the most significant parameters influencing hole quality. Drill wear can be successfully correlated to the level of the thrust force. A simple nomogram is included to predict drill wear level from the thrust level or the amount of material removed.     

Failure analysis of drillstring in petroleum industry: A review

Understanding the failure analysis of drillstring and its components i.e., drill collar and drilling bit is one of the essential issues in the oil and gas industry for the high cost of oil well drilling. Different ways such as air drilling, percussion drilling and downhole hydraulic ultra-high pressure (UHP) jet assisted drilling have been often used to improve the rate of penetration (ROP), minimize the cost of drilling per foot and diminish well deviation. Nevertheless, these drilling ways aggravate the working conditions of the downhole drilling tools materials and hence their properties cannot meet the demands of these conditions and consequently causing a risk drillstring failure. The unfavorable geological conditions and the repeated impact for breaking the rock may also cause severe bit bouncing and violent vibration. Tooth loss, tooth fracture, tooth wear and microcracks in addition to drill pipe fatigue from bending stress caused by buckling load are realistic examples of failure modes which occurred in the drilling tools. This study comprehensively describes the reasons causing these failure modes in order to manage risks and achieve high performance of wells and borehole sections. The role of working parameters i.e., bottom hole temperature and solid content on the drilling tools' life time, and the role of predrill predictions of pore pressure or wellbore stability on the drilling process are presented.     

Friction Drilling of IN-713LC Cast Superalloy

Friction drilling utilizes the heat generated from the friction between the rotating conical tool and the workpiece for hole-making. The process produces no chips and incurs less tool wear, thus lengthening the service life of the drill. In this study, IN-713LC, a nickel-based superalloy, was friction-drilled under different rotation speeds and feed rates. Material properties including hardness, roundness, and surface roughness (Ra) were examined after machining. Experimental results reveal that hardness is greater near the hole's wall and decreases with increasing distance from hole's edge. Moreover, higher rotation speed and faster feed rate can yield better roundness and smoother Ra.     

Corrosion fatigue and electrochemical behaviour of steel wires used in bridge cables

In‐service bridge wires often fail prior to the design life subjected to alternating stresses and environmental erosion. In this paper, a novel corrosion fatigue test device, integrating fatigue testing machine and electrochemical accelerated corrosion assemblies, was developed to characterize the corrosion fatigue and electrochemical behaviour of the wires. Using the developed device, corrosion fatigue tests of corroded bridge wires under different corrosion and loading conditions were conducted. Electrochemical characteristics, corrosion fatigue behaviour, failure mechanism, and so forth were investigated according to electrochemical measurements, fracture morphologies and the lifetime of wires. Results evidence the synchronization of corrosion and fatigue and show the accelerated corrosion due to static and fatigue stresses. Additionally, cracking and fracture induced by multiple crack initiation was dominant in corrosion fatigue of corroded wires, and the coexistence of multiple corrosion pits decreased the lifetime significantly.     

φ139.7 mm×10.54 mm G105钻杆刺穿失效分析

采用宏观分析、化学成分分析、力学性能试验、金相检验、扫描电镜断口分析及能谱分析等手段对某规格为φ139.7 mm×10.54 mm的G105钻杆的刺穿原因进行了分析。结果表明:钻杆刺穿的实质是早期疲劳失效;蹩钻、跳钻等钻柱振动引起钻杆上产生的严重交变应力是导致钻杆失效的主要原因;钻井液中的溶解氧对钻杆外表面造成氧腐蚀并形成腐蚀坑,促进了疲劳裂纹的萌生;钻铤直接过渡到钻杆,截面变化突然,使应力集中加剧,也是导致钻杆发生疲劳失效的原因之一。建议在钻井液中添加除氧剂和缓蚀剂,并适当降低钻压,调整钻井参数,避免钻柱剧烈振动。     

The retardation effect of static torsion on fatigue crack growth in strip steel

This study is aimed at analysing the effect of static torsion on Mode I fatigue crack growth behaviour. Experiments have been conducted under constant torque or constant twist angle with different load ratios on SAPH440 and DP780 strip steels. The experimental results showed that the application of the static torsion or twist angle extended the fatigue life of the specimens in all cases. The fatigue life showed a positive relationship with the applied torque or twist angle until some critical value. No additional improvement in fatigue life was found after the critical value. The mechanism of the retardation due to static torsion was discussed and supported by the microscope fractography analysis. An effective stress intensity factor expression was proposed based on the fractography analysis to estimate the fatigue crack growth under constant torque in the low torque condition. The parameters introduced in the expression were found to have relationships with the dimensionless quantity related to torque, tensile load, and specimen geometry. The proposed expression achieved reliable estimates of the SAPH440 and DP780 constant torque experimental results.     

Transformation characteristics and related deformation behaviour in orthodontic NiTi wire

Several orthodontic NiTi wires have been investigated by differential scanning calorimetry measurements and uniaxial tensile tests. An attempt is made to relate pseudoelastic deformation behaviour to characteristics of diffusionless martensitic transformation. Pseudoelasticity is explained by the respective importance of each parameter for use in orthodontics and experimental results are presented in terms of classification.     

Nonlinear models of chatter in drilling processes

We develop a nonlinear model of chatter in drilling that incorporates friction of the material on the cutting tool and its interaction with the axial-torsional mode of vibration seen in twist drills. Stability criteria are determined for both regenerative and non-regenerative chatter, and the effect of tool parameters and the friction law itself on the results is analysed. Our analysis shows that the exact form of the friction law is not critical in the stability calculation, only the size of the friction coefficient for steady cutting, and the slope of the friction law at the steady cutting state. However, its interaction with the geometry of the vibration is crucial. In the laboratory, drilling vibrational instabilities can occur at frequencies less than the natural frequency of the excited mode of the drill, and we find this result depends critically on the shape of the vibration mode projected onto the cutting edge of the drill.     

Statistical model for a deformable solid with a critical volume and some of its applications. Communication 3

It is shown that wear intensity as applied to friction fatigue is governed by relative critical volume with wear. Equations are derived on the basis of energy and fatigue theories using the DBCV model. Similarity criteria are suggested for wear intensity and the life of friction assemblies. Combined indices and damage criteria for friction-mechanical fatigue, and also for the limiting condition of a metal-polymer power system constructed using the DBVC model are presented. Using them it is possible to calculate the reliability of assemblies which operate with repeated alternating loads and simultaneous sliding friction. Equations for calculated and experimental estimation of durability before main fatigue crack formation (life to the first stage of fretting fatigue) are obtained on the basis of a well-known model for fretting wear and the DBCV model. Some ideas for further studies are formulated.Translated from Problemy Prochnosti, No. 11, pp. 21–29, November, 1992.