R. Schaller

SPECTRA ESTIMATION FOR COMPUTER-AIDED STRUCTURE DETERMINATION

A recently developed program for estimating 1H-NMR chemical shifts has been interfaced to a structure generator. It provided predicted chemical shifts for 89% of the protons occurring in ca. 110 00...

A computer program for the automatic estimation of 1H NMR chemical shifts

A computer program has been developed for predicting 1H NMR chemical shifts. It automatically finds the various substructures of a given molecule for which additivity rules are available. Several strategies have been used to widen the range of applicability. with 200 test compounds, over 90% of the assigned chemical shifts of protons bonded to a carbon atom could be predicted. The mean deviation between observed and predicted values was 0.08 ppm with a standard deviation of 0.19ppm.

New parameters for predicting 1H NMR chemical shifts of protons attached to carbon atoms

Abstract A parameter set has been compiled for predicting the 1 H NMR chemical shifts of protons attached to carbon atoms. It is based on simple additivity rules and currently comprises 728 base values and about 2000 increments. For methyl groups, increments for α- and β-substituents have been developed and parameter sets previously reported for methylene and methane groups have been amended. New rules have been added for allenes and acetylenes. The parameter set is part of a recently published computer program for estimating proton chemical shifts [1]that automatically searches for appropriate substructures and applies various strategies to find suitable parameters for lacking substituents.

Transient internal friction during thermal cycling of Cu–Al–Ni single crystals in β1′ martensitic phase

Pronounced transient internal friction, accompanied by shear modulus defect and reversible torsional deformation, has been revealed during thermal cycling of Cu–Al–Ni single crystals in the β1′ martensitic phase. These phenomena are associated with microplastic straining of the martensitic phase due to anisotropy of thermal expansion of the martensitic variants.

Fatigue Resistant Silicon Nitride Ceramics Due to Anelastic Deformation and Energy Dissipation.

Abstract To investigate the link between internal friction and fatigue resistance of sintered silicon nitride at elevated temperatures, impulse excitation, torsion pendulum and uniaxial tension–compression tests were performed. At low stress-amplitudes, an internal friction peak is observed and associated with the glass transition of amorphous intergranular phases (IGPs). This peak constitutes a reliable means of energy dissipation for otherwise brittle ceramics. The high amplitude uniaxial tests have revealed a new and much larger internal friction effect than previously could be expected from low amplitude test results. Rheological analysis indicates that the underlying deformation is truly anelastic. Within the investigated stress-amplitude range (30–150 MPa), the damping is shown to increase linearly with stress-amplitude. As a consequence, energy dissipation in a cyclically loaded component will increase locally at stress concentrations such as crack tips. This results in an increased crack propagation resistance, and explains earlier observations of a positive fatigue effect in sintered silicon nitride at high temperature.

Low frequency internal friction associated with precipitation in AlMgSi alloys

The low frequency internal friction spectrum of AlMgSi alloys exhibits a relaxation peak in the intermediate temperature range (between 400 and 500K). To interpret this peak, different models were proposed in the literature. As the interpretation of the peak is still not completely clear, a study of the damping spectrum has been performed in commercial AlMgSi (6082) alloys, in order to develop high damping alloys for industry.

DETECTION OF SHOCK-WAVE-INDUCED INTERNAL STRESSES IN Cu-Al-Ni SHAPE MEMORY ALLOY BY MEANS OF ACOUSTIC TECHNIQUE

University of Antwerpen (RUCA), IMS, Middelheimlaan 1, B-2020, Antwerpen, Belgium(Received April 12, 2000)(Accepted in revised form June 21, 2000)Keywords: Copper alloy; Martensitic transformation; Impact shock-wave loading; Elastic properties;Internal frictionIntroductionA response of materials, exhibiting the thermoelastic martensitic transformation (TMT), on the appliedmechanical stress leads to the superelasticity, plasticity of the martensitic phase, shape memory effect,high damping capacity, depending on a variety of parameters such as alloy composition, temperature,magnitude of the applied stress, etc. [1,2]. A predictability of such response is of a great importancefrom the engineering standpoint. Since the TMT is a diffusionless phase transition, the determinationof a threshold time to induce the TMT is a challenging fundamental problem. Another aspect, relatedto both of the above mentioned issues is the performance of TMT materials under the ultimateconditions of high-energy impact loading by stress pulses with short duration. In the present work theacoustic technique has been used to detect the structural changes induced by shock-wave loading ofCu-Al-Ni crystals. The same acoustic technique provides us with an estimate of the upper time limit toinduce the TMT and plastic deformation of the martensitic phase.ExperimentalPlate-shaped samples with dimensions of about 1 3 3 3 30 mm

Internal friction under low-amplitude torsional and high-amplitude uniaxial load in silicon nitride

High temperatures stimulate thermally activated deformation mechanisms. For single crystal ceramics the consequences are limited up to temperatures of about 1,400 C. However, for polycrystalline ceramic materials it has been observed that at temperatures typically in excess of 1,000 C, the deformation behavior becomes increasingly time-dependent. This phenomenon is known to be affected by grain boundary processes. In ceramics produced with liquid phase forming sintering additives the latter will remain at least partially in the grain boundaries after processing. Ceramics containing such amorphous intergranular phases (IGPs) are even more prone to changes of the mechanical behavior at high temperatures, since at temperatures exceeding the glass transition temperature of the IGPs their viscosity drops. Silicon nitride has been evaluated to determine internal friction.

Design criteria and implementation of hypermedia tools for structure elucidation of organic compounds with spectroscopic methods

Abstract Hypermedia open up new possibilities for organizing and storing information. They are ideally suited to interrelate data according to various principles. Different logical structures encompassing the pieces of information can be implemented simultaneously.

The effect of shock-wave loading on transformation temperatures, elastic and anelastic properties of β′ 1 Cu-Al-Ni martensitic single crystals

Abstract The effect of high-velocity impact loading on the structure, transformation temperatures, elastic and anelastic properties has been studied for a Cu-Al-Ni shape memory alloy in the β′1 martensitic phase. The impact loading of crystals has been performed by means of a light gas gun, producing compressive plane-strain wave pulses with a duration of about 2 × 10−6 s. The normal component of stress in the direction of the strain wave propagation ranged from 0.5 to 5 GPa and was used as a characteristic of the impact magnitude. The influence of the impact loading on the transformation temperatures and the structure of martenistic variants cannot be discerned in the present experimental results. In contrast with the macroscopic properties of crystals, the elastic and anelastic properties, studied at a frequency of about 100 kHz, are found to be strongly influenced by the impact loading. The difference between the effects of the impact on elastic and anelastic properties and on the macroscopic performance of crystals is interpreted on the assumption that these properties are related to different structural entities. Changes in the system of partial dislocations in the faulted martensite and variations in internal stresses are considered as basic contributors to the observed elastic and anelastic effects. The stability of the temperature range and hysteresis of the martensitic transformation are ascribed to the rather stable (under the present impact conditions) martensitic variant structure.