Slow strain rate testing
Slow strain rate testing, also called constant extension rate tensile testing, is a popular test used by research scientists to study stress corrosion cracking. It involves a slow dynamic strain applied at a constant extension rate in the environment of interest. These test results are compared to those for similar tests in a, known to be inert, environment. A 50-year history of the SSRT has recently been published by its creator. The test has also been standardized and two ASTM symposia devoted to it.
Effect of strain rate
The important characteristic of these tests is that the strain rate is low, for example extension rates selected in the range from 10−8 to 10−3 s−1. The selection of the strain rate is very important because the susceptibility to cracking may not be evident from result of tests at too low or too high strain rate. For numerous material-environment systems, strain rates in range 10−5 - 10−6 s−1 are used; however, the observed absence of cracking at a given strain rate should not be taken as a proof of immunity to cracking. There are known cases wherein the susceptibility to stress-corrosion cracking only became evident at strain rates as low as 10−8 or 10−9 s−1. Nevertheless, the method is very suitable for mechanistic studies, as well as for relative ranking of susceptibility to cracking of different alloys, or the aggressiveness of environments and the effect of temperature, pH, metallurgical condition etc.The fastest strain rate that will still promote SCC for a given environment-material system is sometimes called the "critical strain rate", some values are given in the table:
| Metal-environment system | Critical strain rate, s−1 |
| Aluminium alloys - aqueous chloride solutions | 10−4 to 10−7 |
| Copper alloys - ammonia/nitrite solutions | 10−6 |
| Titanium alloys - chloride solutions | 10−5 |
| Steels - solutions of carbonates, hydroxides, or nitrates, or liquid ammonia | 10−6 |
| Magnesium alloys - chromate/chloride solutions | 10−5 |
| Stainless steel - chloride solutions | 10−6 |
| Stainless steel - high temperature water solutions | 10−7 |
The importance of other test parameters
Electrode potential and other environmental factors such as temperature, pH and degree of aeration can greatly impact the results off this accelerated stress corrosion cracking test, as can the specimen surface finish and metallurgical condition.The evaluation of the results
The evaluated parameters are:- time to specimen failure
- ductility
- ultimate tensile strength
- area under the elongation - load curve
- percent of ductile/brittle fracture on the fracture surface
- threshold stress for cracking
The departure of the ratio below unity quantifies the increased susceptibility to cracking.
The test is best used in combination with electrochemical measurements and other stress corrosion cracking tests.