Torsional oscillation test

Figure 3.54 Mechanical loss factor d of Celanese Hostaform C 9021 as a function of temperature torsional oscillation test DIN 53 445 [5].

Fig. 4. Torsional oscillation test according to DIN 53520 on blends of NBR (34% acrylonitrile) with EPDM and their dispersion photographs. Blend ratio 65 NBR/35 EPDM. Logarithmic decrement and shear modulus as functions of...

Grades whose acrylonitrile contents differ widely do not give homogeneous mixtures. As Fig. 6 shows, the mixture is still homogeneous when the acrylonitrile contents of the NBR are 18% and 28 %. But mixtures of grades containing 18 % and 34% acrylonitrile have two maxima in the torsional oscillation test, which indicates that they are heterogeneous. 

Fig. 6. Torsional oscillation test with blends of NBR having different acrylonitrile contents logarithmic decrement and shear modulus versus temperature.

Fig. 8. Free torsional oscillation test. Shear modulus G versus temperature at different blend ratios of NBR (28% acrylonitrile) and PVC (crosslinked...

The glass transition temperature and the temperature-dependent storage modulus (G ) can be measured particularly sensitive with the DMA (Dynamic Mechanical Analysis) in the torsion oscillation test. The evaluation of the glass transition temperature took place in accordance to the method of the half steps level . The measurements were performed at a Rheometric Scientific DMA MK III (-50°C-350 °C). 

There are several other comparable rheological experimental methods involving linear viscoelastic behavior. Among them are creep tests (constant stress), dynamic mechanical fatigue tests (forced periodic oscillation), and torsion pendulum tests (free oscillation). Viscoelastic data obtained from any of these techniques must be consistent data from the others. 

DMA has not traditionally been used by the board fabrication industry however, some evidence suggests that this technique might be a better differentiator for and indicator of a materials inherent thermal stability in the LEA reflow environment since it combines temperature with torsional oscillation. Additionally, this test has the advantage of being very sensitive and accurate with thin film materials. 

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. 

Adams and Coppendale (1976, 1977) used a dynamic technique, for the axial butt joint, using the adhesive layer as a spring in an axially vibrating system. By comparing the frequency reduction when having an adhesive layer instead of a solid bar, they were able to calculate E, the effective Young s modulus. They then tested the same bars in torsional oscillation and obtained the shear modulus G by the same frequency change technique. From E and G, it is possible to calculate Poisson s ratio v, and hence to obtain E. 

Dynamic Mechanical Analysis determines the elastic modulus (storage modulus), viscous modulus (loss modulus) and damping coefficient (Tan 5) as a function of temperature. The test specimens dimension was 3 mm X 13 mm x 20 mm and was the same for those used in the Izod impact test but without a notch. The test specimens were clamped between the movable and stationary fixtures, and then enclosed in the thermal chamber. The frequency, amplitude, and a temperature range of25-220°C were set-up for the material. The analyzer applied torsional oscillation to the test sample while slowly moving through the specified temperature range of 25-220°C. 

Before considering particular test methods, it is useful to survey the principles and terms used in dynamic testing. There are basically two classes of dynamic motion, free vibration in which the test piece is set into oscillation and the amplitude allowed to decay due to damping in the system, and forced vibration in which the oscillation is maintained by external means. These are illustrated in Figure 9.1 together with a subdivision of forced vibration in which the test piece is subjected to a series of half-cycles. The two classes could be sub-divided in a number of ways, for example forced vibration machines may operate at resonance or away from resonance. Wave propagation (e.g. ultrasonics) is a form of forced vibration method and rebound resilience is a simple unforced method consisting of one half-cycle. The most common type of free vibration apparatus is the torsion pendulum. 

Figure 9-8. Types of torsion pendulum, (a) Free oscillation apparatus with inertia member supported by test piece (b) free oscillation apparatus with inertial member supported by a fine wire. In both types of apparatus, a lamp and scale is used in conjunction with the mirror to observe the oscillations. The broken lines indicate compensation devices to produce a constant amplitude apparatus...

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... 

A convenient method for determining transition times and transition temperatures of polymeric materials is dynamic mechanical analysis. One type of instrument which is particularly suitable for polymeric solids is the freely oscillating torsion pendulum (TP). Advantages of the TP include its simplicity, sensitivity, relatively low frequency ( 1 Hz) which permits direct correlation of transition temperatures with static nonmechanical methods (e.g., dilatometry and calorimetry), and its high resolution of transitions A major disadvantage of the conventional TP is that test temperatures are limited by the inability of materials to support their own weight near load-limiting transition temperatures. 

Mechanical Properties. Dynamic mechanical properties were determined both in torsion and tension. For torsional modulus measurements, a rectangular sample with dimensions of 45 by 12.5 mm was cut from the extruded sheet. Then the sample was mounted on the Rheometrics Mechanical Spectrometer (RMS 800) using the solid fixtures. The frequency of oscillation was 10 rad/sec and the strain was 0.1% for most samples. The auto tension mode was used to keep a small amount of tension on the sample during heating. In the temperature sweep experiments the temperature was raised at a rate of 5°C to 8°C per minute until the modulus of a given sample dropped remarkably. The elastic component of the torsional modulus, G, of the samples was measured as a function of temperature. For the dynamic tensile modulus measurements a Rheometrics Solid Analyzer (RSA II) was used. The frequency used was 10 Hz and the strain was 0.5 % for all tests. 

The viscoelastic parameters are generally measured by dynamic oscillatory measurements. Apparatus of three different configurations can be used cone and plate, parallel plates, or concentric cylinders. In the case of cone and plate geometry, the test material is contained between a cone and a plate with the angle between cone and plate being small (<4°). The bottom member undergoes forced harmonic oscillations about its axis and this motion is transmitted through the test material to the top member, the motion of which is constrained by a torsion bar. The relevant measurements are the amplitude ratio of the motions of the two members and the associated phase lag. From this information it is relatively simple to determine G and G". 

In dynamic mechanical tests, the response of a material to periodie stress is measured. There are many types of dynamic mechanical test instruments. Each has a limited Irequency range, but it is generally possible to cover frequencies from 1(E to 10 cycles per second. A popular instrument for dynamic mechanical measurements is the torsion pendulum (Figure 13.6A). A polymer sample is elamped at one end, and the other end is attaehed to a disk that is free to oscillate. As a result of the damping characteristics of the test sample, the amplitude of oscillation decays with time (Figure 13.6B). 

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