Various testing methods are used to assess the ability of new and in-service materi2t~ to resist hydrogen-sulfide cracking and to determine the effectiveness of anticorrosion measures. Stress is created in test specimens by constant deformation, constant loading, or a constam strain rate.The testing of specimens under constant deformation simulates corrosion cracking of structures subject to residual stresses. The indeterminacy and instability of the stress level in specimens, and significant duration and ambiguity are classed as disadvantages of these tests, since stress relaxation as a result of crack growth may slow, or even terminate crack propagation.Constant-load tests simulate corrosion cracking under operating stresses. Their advantage consists in the fact that the stress level is accurately defined, and the cross-sectional area of the specimen decreases with increasing corrosion cracking, and specimen failure will occur earlier than during constant-deformation tests with increasing mess.When tests are conducted by these methods, the tendency of the material to hydrogen sulfide cracking is, as a rule, determined by the lime to failure at a different previously adopted test base. These tests yield useful information; the results may, however, be deceptive, since the time to failure is summed from the incubation period, suberitical crack growth, and the time of complete fracture. A material possessing high crack stability (resistance to initiation of a corrosion crack) does not fail during the base test time as a result of which it may be classified as not inclined to hydrogen sulfide eraeidng, but may, in that ease, have low crack resistance, i.e., low resistance to the propagation of corrosion cracks. To ascertain this, it is necessary to subject specimens to prolonged testing; in the majority of eases, this is unacceptable from the practical standpoint, since it is difficult to hold steady the conditions under which the medium acts.To accelerate the tests, the composition or temperature of the corrosive medium is varied, and a notch is also applied to, or a preliminary crack created in the specimen. A disadvantage of these methods is their siL-~nifieant noncorrespondenee to the operating conditions of the structure; in that ease, only the crack resistance is determined, and the incubation period, which is the controlling period in hydrogen-sulfide-containln~ media, is excluded. Moreover, these methods cannot be used to investigate the effectiveness of various means of surface treatment to improve the stability of the test specimens.Many drawbacks of traditional methods of testing may be eliminated, using corrosion tests under a constant strain rate. This method of testing represents a variety of tension tests during which the specimen is tensioned in a corrosive medium at a constant strain rate to complete failure. The correlation between this method of testing and the practical situation is less obvious; under operating conditions and in all types of tests for hydrogen sulfide cracking, the suberitieal ...
