Torsion-tube test

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... 

Two-in. multiaxial (axial/torsional/intemal pressure) composite tube test method [43]... 

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. 

The losipescu test (Figures 17.48 and 17.49) has two opposing V-notches to impose pure shear in the middle of the specimen (ASTM D 5379) and gives results that compare well with the torsion tube method [64,66]. 

Sawada Y, Shindo A, Torsional properties of carbon-fibers, Carbon, 30(4), 619-629, 1992. Swanson SR, Merrick M, Toombes GR, Comparison of torsion tube and losipescu in-plane shear test results for a carbon fibre reinforced epoxy composite, Composites, 16, 8220, 1985. 

Apart from the short beam shear test, which measures the interlaminar shear properties, many different specimen geometry and loading configurations are available in the literature for the translaminar or in-plane strength measurements. These include the losipescu shear test, the 45°]5 tensile test, the [10°] off-axis tensile test, the rail-shear tests, the cross-beam sandwich test and the thin-walled tube torsion test. Since the state of shear stress in the test areas of the specimens is seldom pure or uniform in most of these techniques, the results obtained are likely to be inconsistent. In addition to the above shear tests, the transverse tension test is another simple popular method to assess the bond quality of bulk composites. Some of these methods are more widely used than others due to their simplicity in specimen preparation and data reduction methodology. 

The same test method, although not standardized, can be used to characterize the laminate in-plane shear behavior. This is accomplished by winding a multiorientation (hoop/helical and/or helical only) tube. Other test methods that can be used to measure in-plane shear stiffhess/strength of filament wound composites are discussed by Tamopol skii and Kinds [45]. These methods include schemes for torsion of intact rings and split rings. Both of these ring test methods are used to evaluate the in-plane shear modulii G0r and G0z for a filament-wound laminate. 

Further work will be focusing on lifetime prediction methods for gas turbine materials (superalloys) and experiments on fracture mechanics in superalloys as well as finite element calculations of multiaxial loaded tubes and validation of multiaxial tests (tension, torsion, internal pressure). 

ISO 2535 [68] describes a method using a prescribed cure system with measurement of time from point of accelerator addition to onset of gelation at a temperature of 25 C. The resin is contained in a test tube with temperature control using a water bath. A glass rod is rotated at 1-2 rev/min by a torsion wire connected to a geared electric motor. The gel time is taken as the time taken for the wire to twist an amount corresponding to a liquid viscosity of 50 Poise for measuring this point and is shown in the standard. 

A range of shear tests exists to satisfy all the requirements for in-plane and some throughthickness shear properties. Although torsion of tubes or rods is often recommended, it is rarely used in practice and has not been standardized. Equally, the rail shear test only appears as an ASTM guide [ASTM D 4255], It is not favored, since a large specimen is required and the test suffers from premature failure initiated at the bolt holes. Several other methods that have been standardized are discus.sed below. 

Great attention should therefore be paid when planning experimental activities. Testing hoop-wound tubes under combined tension—torsion loading is a good... 

The torsion test is generally considered to be the most accurate shear test and Figure 17.45 shows a thin walled tube subject to axial tension, which gives a state of pure shear and produces reliable measurements for both strength and modulus, but it is costly procedure. 

Figure 17.45 Tube subjected to axial torsion. Source Reprinted from Course on Mechanical Testing of Advanced Fibre Composites, University of London, Imperial College, Sep 1995.

Fuel pin and fuel element stability tests have been conducted both In alr/water and freon loops, in the latter case under nucleate boiling conditions. These tests have all shown the fuel pins to be inherently very stable in the reference design, but have revealed some cluster vibration at about 7 c/s which is initiated by flow around the neutron scatter plug. An early type of twisted strip scatter plug was rejected because it set up torsional oscillations in the fuel the current torpedo shaped plug can be stabilized by a device already developed, but this will not be applied to reactor fuel unless the instability proves to be damaging to fuel or pressure tubes. 

We seek then to test the hypothesis that the instantaneous vector rate of displacement of any small arc of filament, referred to the local Frenet frame, is determined only by the local curvature and torsion of the filament, and by the twist of the surrounding concentration field. These three scalar quantities comprise a complete description of the local stacking of plane rotors into a continuous three-dimensional vortex tube, so they should be uniquely associated with its local movement. If so, then these three values at a given point s along the filament at time t indirectly (through the displacement vector) specify the same quantities att + dt. They thus specify the dynamics of the filament, and so determine the behavior and stability of the organizing center. Are there dynamically... 

Schematic of the modified Hopkinson bar used by Sato and Ikegami (1999) to test butt joints of tubes under tension and torsion...

Torsion is a variation of pme shear in which a structmal member is twisted in the manner of Figme 6.1d torsional forces produce a rotational motion about the longitudinal axis of one end of the member relative to the other end. Examples of torsion are found for machine axles and drive shafts as well as for twist drOls. Torsional tests are normally performed on cylindrical sohd shafts or tubes. A shear stress t is a function of the applied torque T, whereas shear strain y is related to the angle of twist, in Figure 6.1d. 

Analysis of craze initiation stresses in tensile samples containing circular holes, in thin-walled biaxial tube samples, and for rods in combined tension-torsion tests... 

With sufficient interest and participation within the CMC community, new mechanical test standards for CMC tubes are planned for torsional shear strength and flexural strength. 

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