The torsion test

During the torsion test, tension weights as shown in Table 4-13 shall be applied to the wire tested. 

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 torsion test is generally considered to be the most accurate shear test and Figure 17.45 shows a thin walled tube subject to axial tension, which gives a state of pure shear and produces reliable measurements for both strength and modulus, but it is costly procedure. 

Rnnm Tftmpftrature Tests. The torsion test results from the 650X Si3N4-lncoloy 909 system, the 950X Sl3N4-lnconel 718 system, and the 650X SiC-Incoloy 909 system are shown in Table 1. The fracture behavior of the all of the joints was similar. 

Using Equation 2 and 602 MPa for xmaxi fh calculated value of xpredicted at room temperature was 448 MPa while the measured values from the torsion tests were in the range of 75.6 - 246.4 MPa. The measui >d values change to 128.8 410.6 MPa when the stress concentration factor (Kt = 1.7) associated with the change in diameter at the Joint (12.7 mm ceramic to 19.5 mm metal) Is accounted for. The measured values are 30 - 90 % of values predicted by FEA. The relatively large scatter In the strength values of the joints should be attributed to the handling of ceramic parts and to the difference in the ideal joints modelled and the real joints tested. 

For polymers, the torsion test is often the test of choice because, as discussed in Chapter 2, the time dependent (viscoelastic) behavior of polymers is principally due to the deviatoric (shear or shape change) stress components rather than the dilatoric (volume change) stress components. Typically, constant strain rate tests are often used for either tension, compression or torsion as discussed in Chapter 3. If the material is linear elastic, the stress rate is proportional to the strain rate as the modulus is time independent. That is. 

Fig. 29. Geometry of the torsion test specimen. Redrawn from ref. [47].

A description of the torsion test can be seen in Fig. 29. The derivation of the strain energy release rate has been recently reviewed [47]. 

In Figure 8, the torsion test showed extremely large test frame crosshead displacement of 11 mm because of slack in the complex fixturing system and also because of deformation of the soft aluminum coupler relative to the hard CVD SiC. The latter effect is discussed in the next section. The stress-displacement curve indicated britde failure of the joined torsion specimen in... 

Figure 8. Representative shear stress versus machine cross head displacement curve for the torsion test.

Failure of the substrate material (CVD SiC) was observed for the torsion specimens, as shown in Figure 11. Deformation of the aluminum coupler and hence the relative rotation between the coupler and the joint specimen can be seen, which contributes partly to the large crosshead displacement observed in the torsion test. 

Of aU the test methods studied in this paper, the torsion test showed the highest apparent shear strength of 142.4 MPa. Torsion test has seen some development work recently and is deemed promising because of its true shear loading condition and its small specimen size. Less stress concentration near the joint is expected for torsion test compared to the other three tests... 

To obtain better understanding of diffusion bonding processes for joining SiC to SiC, this paper presents the relationships among the process condition, microstmcture, and mechanical properties. For shear strength evaluation in this work, the DNS test was used for quick estimation of the shear strength, and the torsion test was used for better-defined shear strength evaluation. 

Figure 1. Optical microscope images of Mo foil-bonded SiC specimens for the torsional test (a) Top-view image, and (b, c, and d) side-view images.

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