Creep in torsion

The requirements that the apparatus can handle small, weak specimens with negligible friction in the moving parts (as discussed in the previous 

A versatile and accurate apparatus has been described by Morrison et in which the torque is applied to the specimen by passing a constant current through a coil suspended in a radial magnetic field. Modifications of this system have been used successfully to study the creep of oriented polymers by McCrum and Morris and by Qayton et al  

Simpler torsional creep apparatus used in studies on oriented polymers has been described by Raumann and by Ladizesky and Ward.  

A major problem in data presentation for anisotropy of creep behaviour arises from the dependence of behaviour on many parameters. In Section 10.2 the modulus for a given degree of molecular orientation under given environmental conditions for a single material was given as a function of angle, time and either stress or strain whilst the material compliance functions for these conditions were functions of time and stress or strain. Changes of temperature, composition, orientation, structure, etc., will of course affect the whole pattern of behaviour. 

It is necessary therefore to establish systematic methods for obtaining and presenting the data so as to give a clear overall description of behaviour involving the minimum of experimental work. The importance of such methods for isotropic material has been reviewed by Turner. In the anisotropic case the problems are more extensive. 

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Measurements of creep in torsion can be made very accurately. The reason is that deformation can be measured by measuring the large deflections of a light beam. A convenient way to simultaneously obtain shear dynamic and transient data is to combine both types of measurements in the same equipment (4). Usually this requires only small modifications of the experimental device. For example, the cross bar in a torsion pendulum can be removed and replaced by weights and pulleys to apply a constant torque to the upper clamp. In this way, a torsion creep apparatus is obtained (Fig. 7.8). The... 

Figure 1 Typical creep cui e of an ice single crystal deformed in torsion up to 7 Vo along the c-axis. The deformation is the measured shear strain on the surface.

Loads on a fabricated product can produce different t3q>es of stresses within the material. There are basically static loads (tensile, modulus, flexural, compression, shear, etc.) and dynamic loads (creep, fatigue torsion, rapid loading, etc.). The magnitude of these stresses depends on many factors such as applied forces/loads, angle of loads, rate and point of application of each load, geometry of the structure, manner in which the structure is supported, and time at temperature. The behavior of the material in response to these induced stresses determines the performance of the structure. 

The most common technique employed to date has been that of creep in uniaxial tension. It was shown above that with the inclusion of lateral strain measurements this is a powerful technique giving access to up to 6 independent creep compliance functions. This is more than for any other known method. It further has the overwhelming advantage over many methods, such as say torsional or flexural creep, that the stress is sensibly uniform over the working volume of the specimen. This advantage is paramount in studies of materials displaying non-linear behaviour in creep since analysis of the non-uniform stress situation in non-linear systems is not well developed. Attempts to overcome the non-uniform stress situation in torsion, by recourse to, say, torsion of thin walled tubes, lead to severe difSculties in specimen preparation in oriented materials, when anisotropy of behaviour is to be studied. 

A detailed examination of the problems of direct measurement of 544(f) and 566(f) in highly cold-drawn LDPE with fibre symmetry was carried out by Ladizesky and Ward using torsional creep apparatus. They found only a small dependence of 544(t) upon time at high draw ratio, in agreement with the above mentioned studies on tensile and torsional creep. The direct determination of 566(fX by experiments in torsion was, however, shown to require a complicated double extrapolation procedure reasonable agreement then being obtained between the measured values and those derived from the measurements of 5t 1 and 5i2 during tensile creep. 

Graph of stress as a function of strain constructed from data obtained in any mechanical test in which a load is apphed to a material and continuous measurements of stress and strain are made simultaneously. It is constructed for tensile, creep, or torsional loadings. 

All creep measurements were carried out in torsion, and the sample was loaded with a maximum shear stress of about 15 kPa for PS and 10 kPa for PC, respectively. 

To conclude the subject of creep in MgO polycrystalline ceramics, notice that the vast quantity of experimental data on this ceramic in the hterature show marked differences between the various sources. Some of the data discrepancies observed in various investigations on MgO may be attributed to the different methods used for specimen preparation, different densities and grain sizes and, particularly, different loading levels and geometries (torsion, tension, compression and bending). Therefore, the message of this section is that the best and most effective way to study creep is under similar conditions. 

Crystalline polymers exhibit more mechanical relaxations than amorphous polymers. It is not an overstatement to remark that the greater number of mechanical relaxations in crystalline polymers is the cause of the substantial difference in properties between crystalline and amorphous polymers (4.N.4). For example in linear (LPE) and branched (BPE) polyethylene at temperatures above — 200 "C, there is a sequence of relaxations (see Fig. 4.12). In branched PE the processes are y-relaxation at — 120 C, -relaxation at — lO C, and a-relaxation at 70 C. The presence of a relaxation is detected most easily by the peak in A this is one reason why this parameter is of value. The relaxation observed in creep in linear PE at room temperature and above (shown in Fig. 4.4) is the a-process. The torsion pendulum is a useful tool for yielding quickly a description of temperature regions where creep or stress-relaxation processes are to be expected. In addition, the relaxation temperatures often mark transitions in ductility the polymer becomes increasingly brittle as it is eooled. 

Collected results for the nine compliance constants are shown in Table 8.9. The 3 axis is the initial draw direction and the 1 axis lies in the plane of the sheet, following the convention indicated in Figure 8.2. in and 533 were obtained from measurements of extensional creep in a dead loading creep machine and refer to the 10s response at 0.1% strain, was obtained from the deformation of an electron microscope grid printed on the surface of the sample [97], and 12 and S23 by the Michelson interferometer method [12]. S22 was determined by increasing the compressive strain of strips under load in a compressional creep apparatus [7]. S55 was determined by the torsion of rectangular samples cut with their long axes parallel to 3 and 1 respectively [18], 44 and see were also determined in this way... 

Plastics - Determination of stiffness in torsion of flexible materials Plastics - Determination of creep behavior... 

The extension of an amorphous material under a tensile force can be resolved into three parts first, an immediate elastic extension. Which is immediately recoverable on removing the tensile force Mcondly, a delayed elastic extension which is recoverable slowly and thirdly, a plastic extension, viscous flow, or creep, which cannot be glteovered. With glass at ordinary temperatures, this plastic exten- ion is practically absent. A very slow delayed elastic extension OOCUrs. This effect can be troublesome in work with torsion fibres. The delayed elastic effect in vitreous silica fibres is 100 times less than in other glass fibres, and viscous flow of silica is negligible below OO C (N. J. Tighe, 1956). For exact work vitreous sihea torsion flbres are therefore used. 

Another effect o(f orientation shows up as changes in Poisson s ratio, which can be determined as a function of time by combining the results of tension and torsion creep tests. Poisson s ratio of rigid unoriented polymers remains nearly constant or slowly increases with time. Orientation can drastically change Poisson s ratio (254). Such anisotropic materials actually have more than one Poisson s ratio. The Poisson s ratio as determined when a load is applied parallel to the orientation direction is expected to... 

Two types of measurements were made on these samples. In the region where moduli are higher than 109 dynes/sq. cm., a Clash-Berg torsional creep apparatus (7) was used. For moduli below 109 dynes/sq. cm., a modified Gehman apparatus (14) was employed. In both cases shear creep compliance, Je(t), was obtained. To convert this to relaxation modulus, Gr(t), the following equation was used ... 

Transducers that use torsion bars or springs of known compliance may also oscillate at the beginning of the test as the step is imposed. This ringing is symptomatic of the detector and not the sample response, and thus needs to be removed by signal filtering. In a creep test this kind of behavior only occurs if the sample is... 

There are a large number of designs for all types of torsional apparatus, mostly home made. In apparatus for torsional creep the measurement of displacement may be made by a variety of methods e.g. optical lever and spot-following recorder, or electrical sensing. The application of the torque may be made by means of weights or electrically. 

In various experiments different elastic constants are being determined with a torsion pendulum, for instance, the shear modulus, G, is measured, with creep or vibrations in elongation or in bending the Young s modulus, (tensile modulus), E. For an isotropic material the relation between E and G is as follows ... 

Recent tests have revealed surprisingly good fatigue and creep resistance for carbon/carbon composites. Figure 29 presents some results of torsion and flexure tests in which the fatigue properties of carbon-fiber-reinforced carbon (CFRC) 3D composites are compared with those of carbon-fiber-reinforced polymer (CFRP) 3D composites (53). 

Again, reliable creep modulus data have to be available in order to apply the deflection equations. Tables 25.2 and 25.3 (see also Fig. 25.3) give the expressions for the deflections and torsional deformations of bars. By means of these equations the modulus of engineering materials may be determined from deflection and torsion experiments. The reader is also referred to, e.g. Ferry (1980), McCrum et al. (1997), Whorlow (1992) and Te Nijenhuis (1980, 2007). 

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... 

Usually, creep deformation of ice single crystals is associated to a steady-state creep regime, with a stress exponent equal to 2 when basal glide is activated . In the torsion experiments performed, the steady-state creep was not reached, but one would expect it to be achieved for larger strain when the immobilisation of the basal dislocations in the pile-ups is balanced by the dislocation multiplication induced by the double cross-slip mechanism. 

In summary, torsion creep tests on well-oriented ice single crystals appear to be a pertinent experiment to try to understand and represent the fundamental mechanisms of deformation in ice single crystals. The presented evidence for the occurence of cross-slip as a rate-limiting process questions the role of dislocation climb as suggested by Louchet (2004) I... 

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