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Stress Corrosion Cracking Test Selection

In essence, tests for stress corrosion cracking simply require the exposure of the stressed sample of the material or component in question to the environment of interest. However, there are various classes of test with differing objectives: (reference)

These tests are generally designed to test a material for its susceptibility to SCC in an environment that is known to give problems, or to test components to determine whether they have the necessary combination of material properties and residual stress to suffer from cracking. For example, boiling 42% MgCl2 solution is widely used as a test for the susceptibility of austenitic stainless steels to chloride stress corrosion cracking, and this test may be used to rank alloys or to check components for the presence of residual stresses.

These tests essentially describe a specimen and a loading method that stresses the specimen while exposed to the solution. The susceptibility to SCC is then assessed by the time taken for failure of the specimen, or the development of cracks in the surface of the specimen. A common constant displacement test use a U-shaped specimen, produced by bending a flat plate, and then stressed by drawing the arms of the U together with a loading bolt (known as a U-bend test).

Such tests use a specimen with a pre existing crack (often produced by fatigue cycling). The tests may be evaluated simply by recording the time to failure, but it is more common to measure the change in length of the crack with time, and thereby derive a graph of crack growth rate as a function of stress intensity factor. With a suitable loading arrangement and specimen geometry it is possible to arrange for the stress intensity factor to fall as the crack grows, and this provides a useful method of estimating KISCC. The stress corrosion crack is initiated at a relatively high stress intensity factor, but as the crack grows the stress intensity factor falls, until the crack arrests at KISCC.

An efficient strategy to carry out a test would be to start with the most severe and least expensive SCC test, i.e. the slow strain-rate test in which a bar made from the relevant material is exposed to the environment of interest and slowly monotonically strained to fracture. When cracks are found, the susceptibility of the material should then be further evaluated by performing a battery of other tests designed to differentiate between the various mechanisms leading to SCC and hydrogen embrittlement.

Decision tree for the selection of corrosion tests to verify the susceptibility of steels to stress corrosion cracking (SCC)