Slow strain rate testing

Slow strain rate testing (SSRT), also called constant extension rate tensile testing (CERT), is a popular test used by research scientists to study stress corrosion cracking. It involves a slow (compared to conventional tensile tests) 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.[1] The test has also been standardized[2][3] and two ASTM symposia devoted to it.[4][5]

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:[6]

Critical strain rates
Metal-environment systemCritical strain rate, s−1
Aluminium alloys - aqueous chloride solutions10−4 to 10−7
Copper alloys - ammonia/nitrite solutions10−6
Titanium alloys - chloride solutions10−5
Steels - solutions of carbonates, hydroxides, or nitrates, or liquid ammonia10−6
Magnesium alloys - chromate/chloride solutions10−5
Stainless steel - chloride solutions10−6
Stainless steel - high temperature water solutions10−7

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 (e.g., breakage, or from other "failure" criteria)
  • ductility (by elongation to fracture or the reduction of the area)
  • ultimate tensile strength (from the maximum load)
  • area under the elongation - load curve (which represents the fracture energy)
  • percent of ductile/brittle fracture on the fracture surface
  • threshold stress for cracking

The results of the SSRT tests are evaluated using the ratio:

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.

References
  1. M. Henthorne, "The Slow Strain Rate Stress Corrosion Cracking Test - A 50 Year Retrospective", Corrosion, Vol 72, December 2016, NACE International.
  2. Standard ASTM G129-00 (reproved 2013), "Standard Practice for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking", ASTM International, 2013.
  3. Standard ISO 7539-7:2005 (last reviewed 2014), "Corrosion of metals and alloys -- Stress corrosion testing -- Part 7: Method for slow strain rate testing", International Organization for Standardization.
  4. G. M. Ugiansky (ed.), "Stress corrosion cracking: the slow strain-rate technique", STP 665, ASTM International, 1979.
  5. R. D. Kane (ed.), "Slow strain rate testing for the evaluation of environmentally induced cracking: research and engineering applications", STP 1210, ASTM International, 1993.
  6. ASM Handbook. Volume 13, Corrosion. ASM International, 1997.
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