K. Neuking

Creep of binary Ni-rich NiTi shape memory alloys and the influence of pre-creep on martensitic transformations

Abstract The present work represents an exploratory study of creep in a Ni-rich NiTi shape memory alloy (50.7 at.% Ni) in the temperature range 470–530°C at stresses betweef 90 and 150 MPa. Creep o...

The effect of stress, temperature and loading direction on the creep behaviour of Ni-base single crystal superalloy miniature tensile specimens

In the present work, we use a miniature test procedure to investigate the tensile creep behaviour of the single crystal superalloy ERBO1. We test precisely oriented [0 0 1], [1 1 0] and [1 1 1] creep specimens and determine the stress and the temperature dependence of characteristic creep rates in limited stress and temperature regimes, where the stress and temperature dependence of characteristic creep rates can be well described by power law and Arrhenius type of relations, with stress exponents n and apparent activation energies Qapp. n-values increase with stress and decrease with temperature. Qapp-values, on the other hand, increase with increasing temperature and decrease with increasing stress. Creep curve shapes gradually evolve from the high temperature low stress to the low temperature high stress (LTHS) regime. This implies that there is a gradual change in elementary deformation and softening mechanisms, which is qualitatively confirmed using transmission electron microscopy. While at high temperatures different loading directions only have a moderate influence on creep, there is a very strong effect of loading direction at low temperatures. The [1 1 0] tests show the fastest deformation rates and the shortest rupture times. In the LTHS creep regime, we confirm the double minimum (DM) type of creep behaviour, which was previously reported but never explained. Further work is required to rationalise DM-creep. The implications of this type of creep behaviour on scatter and on extrapolation of creep data is discussed in the light of previous results published in the literature.

Studies on the cycling, processing and programming of an industrially applicable shape memory polymer Tecoflex® (or TFX EG 72D)

The present investigations were undertaken to find out whether and how often cycling, processing and programming can be repeated, whether repeated programming affects the one way effect and how much irreversible strain the shape memory polymeric material accumulates at a particular temperature. The effect was investigated in dependence of different stress levels, and the effect of both recovery temperature and recovery time was considered. As a model material the commercially and industrially applicable amorphous shape memory polymer Tecoflex® was examined and subjected to 50 programming cycles. Tecoflex® is characterized by a glass transition temperature, Tg, of 74 °C, above which it looses all its strength. During tensile testing at 20 °C (T < Tg), stresses a steady increase to 26 MPa as strains approached the rupture strain of 25%. It is observed that at 60 °C (T < Tg, but near Tg) the material can be strained to more than 2500% before rupture occurs while stresses slowly increase to values less than 0.3 MPa. It turns out that programming, cooling, unloading and heating to trigger the one way effect causes an increase of irreversible strain that is associated with a corresponding decrease of the intensity of the one way effect during the first thermomechanical cycles.

Dislocation reactions and microstructural instability during 1025 °C shear creep testing of superalloy single crystals

Abstract Double shear creep tests of the nickel-base superalloy CMSX6 were performed in the high-temperature/low-stress regime where short dislocation segments move through the narrow γ-channels in between the γ′-cuboids and create long dislocation segments near the surface of the γ′-particles. At the same time there is a change in shape of the γ′-particles (rafting). The double shear technique allows for loading selected glide systems with high Schmid factors. We investigated the microstructure after creep testing in the macroscopic crystallographic shear system (001)[110]. Formation and transformation of the dislocation networks are analysed. This analysis yields an unambiguous determination of the activated glide systems. It is found that several glide systems with relatively high Schmid factors are not activated. Slip on cube glide planes was shown not to be important during primary creep.

In Vitro Comparison of the Sagittal Split Osteotomy With and Without Inferior Border Osteotomy

PURPOSE By adding an osteotomy of the inferior border of the mandibular body to the classic sagittal split osteotomy, the authors expected to prevent unfavorable splits and damage to the inferior alveolar nerve. MATERIALS AND METHODS Thirty-five human mandibles were used to perform 70 sagittal split osteotomies as an in vitro study. Conducted as a split-mouth model, each mandible was split at the midline. One side of the mandible was split using the traditional Obwegeser-Dal Pont technique, and the other side was split in the same manner with an additional osteotomy of the inferior mandible border. The torque used to split the mandible was measured, and the fracture line of the mandible was recorded. RESULTS The average torque associated with the original technique was 1.38 Nm (standard deviation, 0.60 Nm), with a fracture line along the mandibular canal. The average torque required to split the hemimandible with the modified technique was 1.02 Nm (standard deviation, 0.50 Nm), a significant (P < .001) difference, with a fracture line parallel to the posterior ramus of the mandible. The fracture pattern depended significantly on the technique used (P < .001), but not on the applied torque force. CONCLUSION By adding an osteotomy of the inferior mandibular border to the sagittal split osteotomy, less torque was needed to split the mandible. The fracture line was more predictable, even when all the surgical manipulations were performed at a safe distance from the inferior alveolar nerve.

Stress-Strain Behavior of Shape Memory Polymers by 1WE Method: Application to Tecoflex®

Thermomechanical cycles including programming, cooling, unloading and heating to trigger the 1WE were examined for a shape memory polymer (SMP), Tecoflex® (TFX EG-72D). Cycles were performed at 60°C with 50% and 225% strains and the recovery time of 10 min. Strains evolving with time were estimated during the thermomechanical treatments for the total 44 cycles using 50% strains and the total 50 cycles using 225% strains. Recovery ratios for 50% strains and 225% were also estimated. It turns out that programming, cooling, unloading and heating to trigger the 1WE causes an increase of irreversible strain and is associated with a corresponding decrease of the intensity of the 1WE in particular during the first thermomechanical cycles. In parallel scanning electron microscopic study using secondary electron imaging shows a very slight wavy surface structure evolved during cycling.

Comparative studies on the accumulation of strain and recovery ratio of Veriflex®, a shape-memory polymer for a high strain (∊m = 210%): Atomic force microscopic experiments

In the present study, the functional fatigue in the commercial SMP Veriflex®, which is associated with repeating up to maximum 40 programming/one-way effect (1WE) cycles, has been examined. The material is characterized by a glass transition temperature (T g) of 67°C, above which it looses all its strength. An interesting comparative investigation on thermomechanical cycles, including programming, cooling, unloading, and heating to trigger the 1WE, were carried out for Veriflex at 62°C (T < T g, below 5°C but near to T g) and also at 72°C (T > T g, above 5°C but near to T g) for strains of 140% and for a recovery time of 10 min. Accumulation of strain was estimated during the thermomechanical treatments for using strains of 140% at 62°C (T < T g) as well as at 72°C (T > T g). Recovery ratios for strains of 140% at 62°C (T < T g) as well as at 72°C (T > T g) were also estimated. It turns out that programming, cooling, unloading, and heating to trigger the 1WE causes an increase in irreversible strain and is associated with a corresponding decrease in the intensity of the 1WE in particular during the first thermomechanical cycles. Confocal laser scanning microscopic) study shows a very little wavy surface structure evolved during cycling up to strains of 140% at 72°C (T > T g). Infrared study features the chemical nature after cycling, processing, and programming of Veriflex.

Comparative studies on the accumulation of strain and recovery ratio of Veriflex®, a shape-memory polymer infrared and laser experiments

A comparative investigation on thermomechanical cycles including programming, cooling, unloading and heating to trigger the one-way effect (1-WE) was carried out for Veriflex® between temperature (T) and glass transition temperature (T g) at 62°C (T < T g) and 72°C (T > T g) for 140% strains and 10 min recovery time. Accumulation of strain and recovery ratios was estimated during the treatments. It turns out that the process causes an increase in irreversible strain and is associated with a corresponding decrease in intensity of the 1-WE in particular during the first thermomechanical cycle. The confocal laser scanning microscopic study shows a very little wavy surface structure evolved at 72°C. Infrared study features the chemical nature of Veriflex.

Elektronenmikroskopische Untersuchung der Mikrostruktur von pseudoelastischen NiTi-Stents

Kurzfassung Nickel-Titan-Formgedächtnislegierungen (NiTi-FGL) werden seit einigen Jahren kommerziell erfolgreich eingesetzt. Insbesondere im Bereich der Medizintechnik gibt es zahlreiche Anwendungen, bei denen sich NiTi etabliert hat. Ein Beispiel dafür sind Gefäßprothesen (Stents). Auch wenn NiTi-Stents zur Zeit nur einen Anteil von cirka 12% der weltweit mehr als 3,5 Millionen implantierten Stents pro Jahr ausmachen, wird der Markt für endo- und kardiovaskuläre Stents 2006 ein Volumen von 5,5 Milliarden Euro überschreiten [1, 2]. Bei der Stent-Produktion entstehen Oberflächendefekte, welche unter Ermüdungsbeanspruchung zur Rissbildung beitragen können. Für die Hersteller von Stents sind daher Oberflächenqualität und Mikrostruktur mit Hinblick auf die funktionalen Eigenschaften und Ermüdungsbeständigkeit wichtige Faktoren. In der vorliegenden Arbeit wurden NiTi-Stents mit Hilfe von raster- sowie transmissionselektronenmikroskopischen (TEM) Methoden untersucht. Die Probenpräparation für TEM-Untersuchung wurde mit Hilfe einer Fokussierbaren Ionenstrahlanlage (FIB) durchgeführt, weil aufgrund der geringen Dimensionen eine andere Probenpräparation extrem schwierig ist. Dieses Verfahren wird ausführlich beschrieben und die Ergebnisse diskutiert.

Comparative Studies on Thermomechanical Behavior of Veriflex®, a Shape Memory Polymer, for a Low Strain (ϵm = 70%): Laser Experiments

An interesting comparative case study on thermomechanical cycles including programming, cooling, unloading and heating to trigger the 1WE was done using Veriflex® at 62°C (T < Tg close to and below 5°C of Tg) and also at 72°C (T > Tg, close to and above 5°C of Tg) for slightly low strains (ϵm = 70%) and the recovery time of 10 min. Accumulation of strain was estimated during the thermomechanical treatments for using both 70% strains at 62°C (T < Tg), as well as at 72°C (T > Tg). Recovery ratios for 70% strains at 62°C (T < Tg), as well as for 72°C (T > Tg) were also estimated. It turns out that programming, cooling, unloading and heating to trigger the 1WE causes an increase of irreversible strain and is associated with a corresponding decrease of the intensity of the 1WE, in particular, during the first thermomechanical cycles. A LSCM (Laser Scanning Confocal Microscopic) study shows very little change in surface structure which evolved during cycling up to 70% strains at 72°C (T > Tg).