Torsion, mechanical properties

Much more information can be obtained by examining the mechanical properties of a viscoelastic material over an extensive temperature range. A convenient nondestmctive method is the measurement of torsional modulus. A number of instmments are available (13—18). More details on use and interpretation of these measurements may be found in references 8 and 19—25. An increase in modulus value means an increase in polymer hardness or stiffness. The various regions of elastic behavior are shown in Figure 1. Curve A of Figure 1 is that of a soft polymer, curve B of a hard polymer. To a close approximation both are transpositions of each other on the temperature scale. A copolymer curve would fall between those of the homopolymers, with the displacement depending on the amount of hard monomer in the copolymer (26—28). 

Mechanical Properties. Although wool has a compHcated hierarchical stmcture (see Fig. 1), the mechanical properties of the fiber are largely understood in terms of a two-phase composite model (27—29). In these models, water-impenetrable crystalline regions (generally associated with the intermediate filaments) oriented parallel to the fiber axis are embedded in a water-sensitive matrix to form a semicrystalline biopolymer. The parallel arrangement of these filaments produces a fiber that is highly anisotropic. Whereas the longitudinal modulus of the fiber decreases by a factor of 3 from dry to wet, the torsional modulus, a measure of the matrix stiffness, decreases by a factor of 10 (30). 

Torsion property As noted, the shear modulus is usually obtained by using pendulum and oscillatory rheometer techniques. The torsional pendulum (ASTM D 2236 Dynamic Mechanical Properties of Plastics by Means of a Torsional Pendulum Test Procedure) is a popular test, since it is applicable to virtually all plastics and uses a simple specimen readily fabricated by all commercial processes or easily cut from fabricated products. 

The mechanical properties of a polymer describe how it responds to deforming forces of various types, including tensile, compressive, flexural, and torsional forces. Given the wide range of polymer structures, it should be no surprise that there is a correspondingly wide... 

Pure crystalline silicon is a brittle material with a gray metallic appearance. Its mechanical properties, such as Knoop hardness (950-1150 kg mm-2), Young s modulus (190 GPa for (111), 170 GPa for (110), 130 GPa for (100)), torsion modulus (4050 kg mm-2) and compression breaking strength (5000 kg cm-2) vary slightly with crystal orientation. Silicon has a low thermal expansion coefficient (2.33x 1(T6 K-1) and a high thermal conductivity (148 W K-1m-1). Crystalline silicon melts at 1413 °C (1686 K). 

ISO 6721-2 1995 Plastics - Determination of dynamic mechanical properties - Part 2 Torsion-pendulum method... 

A torsional pendulum (Figure 5.80) is often used to determine dynamic properties. The lower end of the specimen is clamped rigidly and the upper clamp is attached to the inertia arm. By moving the masses of the inertia arm, the rotational momentum of inertia can be adjusted so as to obtain the required frequency of rotational oscillation. The dynamic shear modulus, G, can be measured in this manner. A related device is the dynamic mechanical analyzer (DMA), which is commonly used to evaluate the dynamic mechanical properties of polymers at temperatures down to cryogenic temperatures. 

Cohen, R.E., Tschoegl,N.W, Dynamic mechanical properties of block copolymer blends—a study of the effects of terminal chains in elastomeric materials. I. Torsion pendulum measurements. Intern. J, Polymeric Mater. 2, 49-69 (1972) II. Forced oscillation measurements. Ibid 2, 205-223 (1973) III. A mechanical model for entanglement slippage. Ibid (in press). 

On the other hand, San Miguel and Duran (Ref 125) showed that the mechanical properties of polyurethane solid proplnt were degraded significantly by gamma irradiation dosages greater than 106 R. The tests used to determine the effects were by swelling, torsion, uniaxial tension and multiaxial tension... 

When instead assemblies of helices are taken into account, it is well known that for many aspects DNA duplexes in solution can be treated as a charged anisotropic particle [2]. Accordingly, steric, electrostatic, and Van der Waals interactions, together with the mechanical properties of the helix (bending and torsional rigidity), play a major role in the formation of DNA mesophases. In addition, all these different kinds of interactions combine in a subtle and still poorly understood way to generate other forces relevant for the case of DNA. A notable example is the helix-specific, chiral interaction, whose importance for DNA assemblies will be discussed below. 

The mechanical properties of cortical bone (specifically, the femur, tibia, humerus, and radius) of various species (specifically, horse, cattle, pig, and human) in tension, compression, and torsion are listed in Table I. It should be noted, for example, that human femur tensile strength (namely, 124 MPa) (Yamada, 1970) is in the same order of magnitude to that of cast iron (170 MPa) (Beer and Johnston, 1981) but, surprisingly, low in weight (Kaplan et al., 1994 Fung, 1993). These unique properties of bone are a direct consequence of the synergy of its molecular, cellular, and tissue arrangement. 

Mechanical Properties of Cortical Bone in Tension, Compression, and Torsion"... 

CNTs are valued for their novel properties mechanical strength, chemical inertness, electronic and thermal conductivity. Mechanical properties of CNTs, especially their extreme flexibility and strength equal to steel with one-sixth the mass, is most impressive. An excellent resistance of carbon nanotubes to bending has been observed experimentally and studied theoretically. CNTs are not nearly as strong under compression. Due to their hollow structure, they undergo buckling when placed under compressive, torsional or bending stress.16... 

Tonami et al.179) studied atactic-PMAA (at-PMAA)-PEO complex membranes by means of infrared spectroscopy, stress relaxation measurement and torsional analysis of dynamic-mechanical properties. They pointed out that the polymer complex was formed through hydrogen bonds between the ether... 

Changes in properties of materials during aging form the crux of much research. Changes in thermal and mechanical properties are the most commonly used parameters in studying aging because they are easily detectable. For example, Struik (I) studied the effect of physical aging on torsional and tensile creep compliance of about 40 totally amorphous materials, Chapman (9) examined the effect of physical... 

Various methods (1-1) have used to determine the dynamic mechanical properties of polymers. Many of the instruments described are well known and are widely used (torsional pendulum, rheovibron, vibrating reed, and Oberst beam ASTM D4065-82). Newer instruments like the torqued cylinder apparatus (4), resonant bar apparatus (5) and Polymer Laboratories Dynamic Mechanical Thermal Analyzer (6) are becoming more popular in recent times. 

This allowed the determination of the reduced parameter (T-T ) during cure of the resin under a variety of time-temperature profiles. The dynamic mechanical properties were then estimated as a function of (T-T ). This estimate is compared with the results of torsion impregnated cloth analysis of the curing of the ATS resin. 

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