Stiffness in torsion

ISO 458, Parts 1 2, Plastics - Determination of stiffness in torsion of flexible materials, 1985. 

In ISO standard,the size of sample, having length of 70 mm, width between 6 and 6.4 mm, and thickness between 1 and 5 mm differs from ASTM standard. The equipment, its general principle of operation, and results calculation are essentially similar to the description above for ASTM standard.A special method is developed to describe behavior of plasticized compounds.Plasticized PVC compositions are characteristic in respect to the appearance of curves of the logarithm of the stiffness in torsion as a function of temperature, which have form of reversed S. Three regions were identified in these curves ... 

ISO 458-2 1985 Plastics - Determination of stiffness in torsion of flexible materials - Part 2 Application to plasticized compounds of homopolymers and copolymers of vinyl chloride 180 472 1999 Plastics - Vocabulary... 

ISO 458, Plastics - Determination of Stiffness in Torsion of Flexible Materials - Part 1 General Method, 1985. 

Since flexible shafting is relatively stiff in torsion and compliant in bending, flexible couplings inherently possess a imique combination of characteristics that permit them to accommodate both lateral and angular misalignment while being subjected to torsional loading. 

This test method covers the determination of the stiffness characteristics of plastics over a wide temperature range by direct measurement of the apparent modulus of rigidity. The samples can be tested by one of two standards ASTM D1043-10 Standard Test Method for Stiffness Properties of Plastics as a Function of Temperature by Means of a Torsion Test or ISO 458-1 1985 Plastics - Determination of stiffness in torsion of flexible materials - Part 1 General method and Part 2 Application to plasticized compounds of homopolymers and... 

The shape of AFM cantilevers (much thinner than the width) results in torsional deflection when forces push the tip laterally as in friction measurements (when the tip is sliding) or lateral stiffness measurements (when the tip is stuck). 

The moduli of elasticity, G for shear and E for tension, are ratios of stress to strain as measured within the proportional limits of the material. Thus the modulus is really a measure of the rigidity for shear of a material or its stiffness in tension and compression. For shear or torsion, the modulus analogous to that for tension is called the shear modulus or the modulus of rigidity, or sometimes the transverse modulus. 

Figure 5.65 provides theoretical evidence that resonance-assisted H-bonding can serve as an effective mechanism for switching a methyl rotor from one preferred conformation to another, or for controlling the stiffness of torsional motions in alkylated amides. In particular, the torsional potentials of proteins (more specifically, the Ramachandran b angle at Ca) should be sensitive to N—H- O and related H-bonding interactions involving the amide backbone. In principle, this electronic... 

In-Plane Shear Properties. The basic lamina in-plane shear stiffness and strength is characterized using a unidirectional hoop-wound (90°) 0.1 -m nominal internal diameter tube that is loaded in torsion. The test method has been standardized under the ASTM D5448 test method for in-plane shear properties of unidirectional fiber-resin composite cylinders. D5448 provides the specimen and hardware geometry necessary to conduct the test. The lamina in-plane shear curve is typically very nonlinear [51]. The test yields the lamina s in-plane shear strength, t12, in-plane shear strain at failure, y12, and in-plane chord shear modulus, G12. 

For many applications low-temperature flexibility of the plasticized composition is also important. Plasticizers of low viscosity and low viscosity-temperature gradient are usually effective at low temperature. There is also a close relationship betv/een rate of oil extraction and low-temperature flexibility plasticizers effective at low temperature are usually rather readily extracted from the resin. Plasticizers containing linear alkyl chains are generally more effective at low temperature than those containing rings. Low-temperature performance is evaluated by measuremen t of stiffness in flexure or torsion or by measurement of second-order transition point, brittle point or peak dielectric loss factor. 

For an ideal solid, Hooke s law holds the stress, cr, applied is proportional to the deformation, e, and the proportionality constant is the modulus of elasticity E, so a = E e. Besides E also other quantities play a role, such as the shear modulus, G, in a shearing deformation or torsion, which is related to E. For the sake of simplicity we shall mainly use as a representative quantity for the elastic stiffness in any geometry of loading. 

Some engineering components operate in torsion, and shear modulus can be measured in this mode, In practice, it is largely restricted to measuring stiffness of rubbers, flexible plastics, and coated fabrics as a means of characterizing low-temperature perfomance. A strip geometry is used when force and deflection are related by... 

Interactions both between non-bonded united atoms (CH2) on different chain segments and between united atom pairs separated by four or more bonds in the same chain segment were calculated using a Lennard-Jones potential with a cut-off distance = 2.5bonded interactions are included, accounting for the stiffness in the bond length, bond angle and torsion. The interaction potentials are, in that order,... 

Markolf, in 1970, demonstrated that the sheer stiffness in the horizontal plane of the disc was 150 N. This was considered to be a high value and was thought to be clinically significant, showing that a large force was required to cause abnormal horizontal displacement of an intact vertebral disc unit. It showed that it should be relatively rare for the annulus to fail under pure shear stress. Clinical evidence suggested that annular disruption was more frequent, as the disc does not fail under shear stress alone it fails under a combination of bending, sheer, torsion and tension stresses. Dr. Farfan, in 1970, demonstrated that it took only a force of 31 N to cause failure of the disc under axial rotation. 

Figure 13.11 illustrates the change in torsional stiffness that can occur as the temperature drops and compares polyurethane with other rubbers. There is only a small increase in the torsional stiffness as the temperature is decreased from -2(fC to about — 25" C, but then the increase in stiffness occurs very rapidly. The second-order transition point is between — 30 and —40°C and it is dependent upon the regularity of the molecular structure. Lower transition temperatures can be obtained by the use of mixed glycol polyadipates in place of the common polyethylene glycol adipate. For example, it is common commercial practice to use a blend of polyethylene... 

The design of the gear at this stage included a continuous web, centrally located through the thickness, and radial ribs outside the central web. This was a fairly stiff design, in torsion or diametral compression. 

In the so-called torsional braid analysis,the liquid is absorbed in a long strip of soft braided thread or filter paper which has very slight torsional stiffness, and torsion pendulum measurements are made by the methods of Chapters 6 and 7. Both storage and loss components of the dynamic torsional response may be attributed primarily to the viscoelastic liquid, and examined on a relative basis, although absolute measurements are not possible. Care must be taken that a contribution from the braid does not lead to erroneous conclusions about the properties of the viscoelastic liquid.25a. a flexural braid method has also been utilized. 

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