Double torsion test specimen

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. 

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.

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. 

NOTE - there are other testing specimens that are suitable for fracture toughness or fracture energy measurements of brittle materials. These include for instance compact tension (CT) and double torsion (DT) specimens. Readers will find more details in [CHE 75]. 

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. 

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).

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 slow growth of cracks in poly(methyl methacrylate) is an ideal application of linear elastic fracture mechanics to the failure of brittle polymers. Cracks grow in a very well-controlled manner when stable test pieces such as the double-torsion specimen are used. In this case the crack will grow steadily at a constant speed if the ends of the specimen are displaced at a constant rate. The values of Kc or % at which a crack propagates depends upon both the crack velocity and the temperature of testing, another result of the rate- and temperature-dependence of the mechanical properties of polymers. This behaviour is demonstrated clearly... 

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