Slow strain rate (SSR) testing, also known as constant extension rate testing (CERT), is a modification of the constant load tension test as shown schematically in Fig. 1. In this case, the constant load has been replaced by a slow extension of the specimen until failure. A more detailed description of this test method is given in ASTM G129. The benefit of SSR testing is that it produces a result in a reasonably short time, usually within 1-7 days in most cases, depending on strain rate. It also reduces incubation time to the onset of cracking in susceptible materials through the application of the dynamic plastic straining.

The plastic strain causes an accelerated disruption of surface films thereby overcoming the initial period of incubation that can result in unacceptably long test durations. However, in some cases, the plastic strain can add complications to the interpretation of the test results because most materials are not prone to this degree of straining in actual service.

The main benefit of the SSR test is that it allows the evaluation of the effect of metallurgical variables such as alloy composition, heat treatment and processing and/or environmental parameters (e.g.. aeration, concentration, inhibition, etc.) in a relatively short period of testing. Strain rates utilized for SSR testing are typically in the range of 1- 4 x10^-6 s^-1. At a strain rate of 4 x10^-6 s^-1, the testing speed is about 1% strain per hour and failure of most engineenng materials will occur within a day or two. In some cases, slower strain rates are required.

Additionally. in some cases, longer exposure periods prior to testing may be necessary if long-term formation of corrosion films is a critical step in the cracking process. One example of this effect is the formation of water scales on austenitic stainless steels, which can exacerbate SCC in chloride-containing waters. However, in most cases, short-term testing can provide meaningful data on the roles of many variables on Environmentally Assisted Cracking (EAC).

Evaluation for susceptibility to EAC is normally obtained through the comparison of the results of tests conducted in a corrosive environment vs. corresponding data obtained in an inert environment (i.e., an environment that has been shown not to promote EAC or significant corrosion in the material being tested). In most cases these tests are conducted in air. Direct examination of the specimen gage section for EAC and documentation of fracture mode are also important to a full interpretation of the SSR test results. The SSR test results that are used include time to failure, plastic elongation to failure, reduction in area, ultimate tensile strength, and load at fracture. These data are usually presented in terms of their ratios vs the corresponding value from a test conducted in an inert environment. Ratios in the range 0.8-1.0 normally denote high resistance to EAC whereas low values (i.e.< 0.5) show high susceptibility. In some cases, hydrogen can cause loss in ductility without indication of brittle cracking in the specimen. This is usually a less important situation than when direct evidence of embrirtlement has been observed, particularly if the material still exhibits a high tensile strength ratio.