Double torsion test

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 ... 

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. 

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. 

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. 

Figure 4. Typical schematic load-displacement traces for double torsion test specimen (a) continuous (stable) crack growth (b) discontinuoxis (unstable) crack growth showing initiation and arrest loads.

Alternative geometries and test methods to determine the fracture toughness of ceramics and hard materials have been developed over the last decades Herzian indentation [88], the double torsion test [89], and compact tension tests [90 92]. 

Because strain measurements are difficult if not impossible to measure, few values of yield strength can be determined by testing. It is interesting to note that tests of bolts and rivets have shown that their strength in double shear can at times be as much as 20% below that for single shear. The values for the shear yield point (kPa or psi) are generally not available however, the values that are listed are usually obtained by the torsional testing of round test specimens. 

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. 

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... 

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. 

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

Figure 8.21 Double torsion fracture mechanics test geometry.

Since it is usually convenient to measure the crack speed in a specimen in which K does not vary with a for a given load or loading rate, the double-torsion and tapered-cantilever beam tests are used (Fig. 16). In the first we have... 

The calculation of G and for a number of geometries, such as peeling, double cantilever, double torsion, or blister test, can be found in textbooks. We will concentrate on the case of adherence of punches (and especially of a sphere) which is conceptually an important topic via which to understand the connection between adherence, mechanics of contact, and fracture mechanics, or, more simply, what is an area of contact. 

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