Bend tests, embrittlement

The specimen may be subjected to simple bending tests to determine whether any embrittlement has occurred. 

Fig. 8.102 Constant-rate bend test for determining hydrogen embrittlement of wires. A, drive unit B, pulley C, semicircular base D, travelling arm E, axial pin F, fixed arm C, wire specimen (after Zapffe and Haslem )...

An alternative procedure is the reverse-bend test in which the specimen in the form of wire or strip is bent repeatedly backwards and forwards over a mandrel until it fractures, the number of bends indicating the degree of embrittlement. However, this method is considered to be less sensitive than the single-bend test. 

The oxygen concentration was monitored principally by hardness tests. Hardness is generally believed to be a better indicator of the extent of interstitial contamination than chemical analysis, which is subject to scatter and inaccuracy because of samphng difficulty. Bend tests (at room and liquid argon temperatures) and room-temperature Olsen cup formabihty tests were conducted to determine the hardness levels at which the materials embrittled. 

In the bending test the RCP also showed a difference among the radiation treatments. Both y and X-radiation caused about the same loss of flexural ductility historically expected for this particular resin, but the EB treatment caused no embrittlement all the way to 100 kGy. 

A field cold bend may be made to a shorter minimum radius than permitted in (a)(2) above, provided the completed bend meets all other requirements of this section and the wall thickness after bending is not less than the minimum permitted by para. PL-3.7.1(a). This may be demonstrated through appropriate testing. Note that cold bending may make line pipe more susceptible to the effects of hydrogen embrittlement. 

When the surface is put into tension (by bending, or by the application of a tensile test) cracks form readily in the embrittled layer and often propagate into the interior, leading to failure (Figure 1). Even though the interior may be relatively undegraded chemically, a crack in the embrittled layer may provide sufficient stress concentration to cause brittle fracture or a ductile tearing failure instead of deformation and draw-ing. 

If the test material is suspected of being subject to dealloying (such as dezincification) or intergranular corrosion, a cross section of the specimen should be microscopicaUy excimined. If embrittlement is suspect, simply bending the specimen may determine this susceptibiUty. 

FIG. 19—Comparison of single-edge-notched bend cantilever beam and wedge-opening load test results for hydrogen embrittlement threshold of ironHiickel-cobalt steels [28],... 

After test exposure, sample evaluation for hydrogen may include tensile, notched tensile, bend, ductility (for example, drawn cup), and/or impact Charpy tests, hydrogen analysis, or cross-sectional microstructural examination, or a combination thereof. Uniaxial, smooth-specimen tension testing is generally of litde value in diagnosing the subde embrittling effects of hydrogen. Titanium alloys tend to... 

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