Double torsion technique

Cutwater, J. O., Murphy, M. C., Kumble, R. G. Berry, J. T. (1974). Double torsion techniques as a universal fracture toughness test method. Fracture Toughness and Slow-Stable Cracking, ASTM Special Technical Publication 559, pp. 121-31, American Society for Testing and Materials. 

The measured V — Ki diagrams of alumina and the composites prepared with the colloidal route and different volume fraction of unstabilized zirconia are plotted in Fig.l4 AI2O3, A7.5Z, AIOZ and A15Z correspond to nanocomposites with 0, 7.5, 10 and 15 vol % ZrOi respectively and A7.5Z-2Y is a composite with 7.5 vol % Zr02, stabilized by 2 mol % Y2O3. Subcritical crack growth laws were measured using the double torsion technique and the relaxation method. 

The techniques and test geometries that have been used to measure subcritical crack growth in ceramics are several, but they share a common principle, namely, the subjection of a well-defined crack to a well-defined stress intensity Ki, and a measurement of its velocity v. The technique considered here, the advantages of which are elaborated upon below, is the double torsion geometry shown in Fig. 12.8. For the double torsion specimen, K is given by... 

K. Schmidt-Rohr, A double-quantum solid-state NMR technique for determining torsion angles in polymers. Macromolecules, 1996, 29, 3975-3981. 

As mentioned above, the determination of atomic level structure, i.e., the backbone torsion angles for an oriented protein fiber, is possible by using both solid-state NMR method described here and specifically isotope labeling. This is basically to obtain the angle information. Another structural parameter is distance between the nuclei for atomic coordinate determination. The observation of Nuclear Overhauser Enhancements (NOEs) between hydrogen atoms is a well known technique to determine the atomic coordinates of proteins in solution [14]. In the field of solid-state NMR, REDOR (rotational echo double resonance) for detection of weak heteronuclear dipole interactions, such as those due to C and N nuclei [15, 16] or R (rotational resonance) for detection of the distance between homonuclei, are typical methods for internuclear distance determination [17,18]. The REDOR technique has been applied to structure determination of a silk fibroin model compound [19]. In general, this does not require orientation of the samples in the analysis, but selective isotope labeling between specified nuclear pairs in the samples is required which frequently becomes a problem. A review of these approaches has appeared elsewhere [16]. 

Most recently, a double-quantum solid-state NMR technique has been reported [47]. This gives detailed structural information such as torsion angles for unoriented amorphous polymers including unoriented polypeptides with NMR tensor correlation in the labeled segments. This new solid-... 

The most common technique used for heteronuclear distance measurement is rotational-echo double resonance - REDOR [239,240,241]. It is now relatively commonplace to label proteins with the spin-1/2 isotopes and N, and C-N distance constraints can be derived using REDOR [242, 243] and used to determine elements of secondary structure. More importantly for molecules of interest to the pharmaceutical industry is the measurement of distances between and the quadmpolar deuterium ( H) nucleus [244]. Introduction of the H isotope is relatively straightforward and the measurement of distances at natural abundance over several bonds can be used to provide torsional constraints [245]. The measurement of distances (up to 6.8 A)... 

In addition, the rotational-echo double resonance (REDOR) is a powerful technique to determine distances between two hetero-nuclei (141). Methods have also been developed that permit the detection of torsional angles (142). 

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