Double torsion

Double torsion single-edge crocked plote... 

There are a number of methods of determining fracture toughness, but the one used so far for AB cements is the double torsion method introduced... 

Double torsion test specimens take the form of rectangular plates with a sharp groove cut down the centre to eliminate crack shape corrections. An initiating notch is cut into one end of each specimen (Hill Wilson, 1988) and the specimen is then tested on two parallel rollers. A load is applied at a constant rate across the slot by two small balls. In essence the test piece is subjected to a four-point bend test and the crack is propagated along the groove. The crack front is found to be curved. 

The double torsion test specimen has many advantages over other fracture toughness specimen geometries. Since it is a linear compliance test piece, the crack length is not required in the calculation. The crack propagates at constant velocity which is determined by the crosshead displacement rate. Several readings of the critical load required for crack propagation can be made on each specimen. 

To calculate fracture toughness using the double torsion test piece, the following equation is used ... 

Cutwater, J. O., Murphy, M. C., Kumble, R. G. Berry, J. T. (1974). Double torsion techniques as a universal fracture toughness test method. Fracture Toughness and Slow-Stable Cracking, ASTM Special Technical Publication 559, pp. 121-31, American Society for Testing and Materials. 

In the study of crack growth in epoxy polymers the double-torsion and compact-tension specimens have been the most widely used. The values of KIc may be obtained from 1,2)... 

Fig. 2 a and b. Schematic load versus displacement curves for crack propagation in epoxy polymers obtained using a double torsion specimen a Stable continuous propagation, b Unstable stick/slip propagation... 

Double torsion Single-edge cracked plate... 

From the character table, the symmetry eigenvectors for the double torsion without cog-wheel effect in benzaldehyde are easily derived on the basis of the double free rotor solutions. It is found ... 

The authors studied the glassy fracture behavior of the homologous series of DGEBA/DDS networks listed in Table 2. The fracture specimen employed was the double torsion test piece. Fracture data were collected over the temperature range Tg — 120 to Tg — 20 K, and all testing was performed at a single slow crosshead rate of 0.05 cm/min. This test rate was chosen because it minimized hysteretic effects and made all the networks fracture unstably over most of the temperatures investigated. 

Fig. 12. Stoichiometric DGEBA/DDS network fracture toughness versus temperature. Fracture toughness from double torsion specimen at crosshead rate of 0.05 cm/min. Network T, s shown by dashed lines. O Initiation Arrest 9 Stable crack growth. (After LeMay...

The invariance condition can be achieved by other geometries, providing that C° a so that dCJda is a constant, and that of double torsion, as illustrated in Fig. 7, has proved useful for polymers In this case ... 

A quantitative approach to the reinforcement of interfaces with block copolymers is necessary. Such an approach requires a way to evaluate the strength of the interface separately from any change in the morphology or microstructure of the blend. While classic fracture mechanics tests such as compact tension and double torsion could be used, the asymmetric double cantilever beam (ADCB) test used in the first such study [17] has been adopted by all successive workers to date. In this experimental geometry, shown schematically in Fig. 3, a wedge (usually a razor blade) is inserted at the interface. In some versions of the test the... 

Two of the more common test configurations are shown in Fig. 11.6. A third geometry not shown here is the double torsion test, which in addition to measuring Ki. can be used to measure crack velocity versus K curves. This test is described in greater detail in the next chapter. 

The techniques and test geometries that have been used to measure subcritical crack growth in ceramics are several, but they share a common principle, namely, the subjection of a well-defined crack to a well-defined stress intensity Ki, and a measurement of its velocity v. The technique considered here, the advantages of which are elaborated upon below, is the double torsion geometry shown in Fig. 12.8. For the double torsion specimen, K is given by... 

The load-relaxation method of Double-Torsion (DT) test is adopted in this study (Evans, 1972). A schematic illustration of a DT specimen is shown in Figure /, where the notations of the specimen are noted. In this study, the guide groove was set upward. 

Atkinson, B. K. 1979b. Fracture toughness of Tennessee sandstone and Carrara marble using double torsion testing method, Int. J. Rock Mech. Min. Sci. Geomech. Abstr., 16, pp.49-53. 

Glassy fracture energies were measured using single edge notch (sen) and double torsion (DT) specimens (Figure 3). Rubbery fracture measurements above Tg en5)loyed only the SEN specimen. 

Figure 3. Fracture test specimens (a) Single edge notch (sen) (h) Double torsion (DT).

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