Subject torsional strain

The preference for endo attack in 7,7-dimethylnorbomene is certainly steric in origin, with the 7-methyl substituent shielding the exo direction of approach. The origin of the preferred exo-attack in norbomene is more subject to discussion. A purely steric explanation views the endo hydrogens at C—5 and C—6 as sterically shieldihg the endo approach. There probably is also a major torsional effect Comparison of the exo and endo modes of reproach shows that greater torsional strain develops in the endo mode of... 

Flexible tubes Metallic tubes complying with BS 669, Part 2 or BS 6501 shall be used other than with small portable appliances or with domestic-type appliances, in which case they shall comply with BS 669 Part 1. A manual valve shall be fitted on the inlet side in close proximity to the tube. The pipework shall be adequately supported such that the tube does not support the weight of the attached pipework. Tubes shall be installed so that they are neither twisted nor subjected to torsional strain, have flexing in one plane only, and are not subject to sharp bends near end fittings. 

Torsional braid analysis (TEA) is a particular case where the sample supported by a fiberglass braid is subjected to a torsional strain. 

Rings containing seven to twelve carbon atoms are also subject to torsional strain, and hence these compounds, too, arc less stable than cyclohexane scale models also reveal serious crowding of hydrogens inside these rings. Only quite large ring systems seem to be as stable as cyclohexane. 

The 12 kj/niol of extra energy present in the eclipsed conformation of ethane is called torsional strain. Its cause was the subject of controversy for some years, but most chemists now believe that torsional strain is due to the slight repulsion between electron clouds in the C-H bonds as they pass close by each other in the eclipsed conformer. Calculations indicate that the hydrogen-hydrogen distance is 255 pm in the staggered conformer but only about 229 pm in the eclipsed conformer. 

If cyclopentane were planar, as Baeyer had predicted, it would have essentially no angle strain, but its 10 pairs of eclipsed hydrogens would be subject to considerable torsional strain. So cyclopentane puckers, allowing the hydrogens to become nearly staggered. In the process, however, it acquires some angle strain. The puckered form of cyclopentane is called the envelope conformation because the shape resembles a squarish envelope with the flap up. 

Gel texture properties for both heat-treated and pressure-treated gels were determined by torsion. The gels were cut into 2.8 cm lengths and formed into an hourglass shape with a 1 cm die on a lathe-type apparatus (Gel Consultants Inc., Raleigh, NC). The samples were subjected to torsional strain in a modified Brookfield viscometer (Gel Consultants Inc., Raleigh NC). Shear stress and shear strain, at failure, were calculated using equations developed by Hamann (1983). 

The torsion-tube test described by Whitney, Pagano, and Pipes [2-14] involves a thin circular tube subjected to a torque, T, at the ends as in Figure 2-29. The tube is made of multiple laminae with their fiber directions aligned either all parallel to the tube axis or all circumferentially. Reasonable assurance of a constant stress state through the tube thickness exists if the tube is only a few laminae thick. However, then serious end-grip difficulties can arise because of the flimsy nature of the tube. Usually, the thickness of the tube ends must be built up by bonding on additional layers to introduce the load so that failure occurs in the central uniformly stressed portion of the tube (recall the test specimen criteria). Torsion tubes are expensive to fabricate and require relatively sophisticated instrumentation. If the shearing strain y 2 is measured under shear stress t.,2, then... 

For thin-walled cylinders subject to in-plane (axial and circumferential) loading and axial torsion, Whitney and Halpin [8] have developed an analytic solution for strains. Their analysis is valid in the central region of the cylinder, end support effects are neglected. 

The analysis of the stresses and strains in beams and thin rods is a subject of great interest with many practical applications in the study of the strength of materials. The geometry associated with problems of this type determines the specific type of solution. There are cases where small strains are accompanied by large displacements, flexion and torsion in relatively simple structures being the most relevant examples. Problems of this type were solved for the elastic case by Saint Venant in the nineteenth century. The flexion of viscoelastic beams and the torsion of viscoelastic rods are studied in this chapter. 

Shear deformation occurs in structural elements such as those subjected to torsional loads and in short beams subjected to transverse loads. Shear S-S data can be generated by twisting (applying torque) to a specimen at a specified rate while measuring the angle of twist between the ends of the specimen and the torque load exerted by the specimen on the testing machine (ASTM D 732). Maximum shear stress at the surface of the specimen can be computed from the measured torque that is the maximum shear strain from the measured angle of twist. 

Figure 13 shows a torque cell consisting of a circular shaft mounted with four strain gages on two 45° helices that are diametrically opposite to each other gages 1 and 3 mounted on the right-hand helix sense a positive strain, while gages 2 and 4 mounted on the left-hand helix sense a negative strain. The two helices define the principal stress and strain directions when the circular shaft is subjected to pure torsion. 

Dynamic mechanical analyzers can be divided into resonant and defined frequency instruments. The torsion pendulum just described is, for example, a resonant instrument. The schematic of a defined-frequency instrument is shown in Fig. 4.155. The basic elements are the force generator and the strain meter. Signals of both are collected by the module CPU, the central processing unit, and transmitted to the computer for data evaluation. The diagram is drawn after a commercial DMA which was produced by Seiko. At the bottom of Fig. 4.155, a typical sample behavior for a DMA experiment is sketched. An applied sinusoidal stress, o, is followed with a phase lag, 6, by the strain, e. The analysis of such data in terms of the dynamic moduli (stress-strain ratios, see Fig. 4.143) at different frequencies and temperature is the subject of DMA. 

Modulus of Rigidity Rate of change of strain as function of stress in specimen subjected to shear or torsion loading. 

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