Slow strain rate testing
Slow strain rate testing (SSRT), also called constant extension rate tensile testing (CERT), is a standard[1][2] method of testing of materials, often metals, in which the specimen is subjected to elongation at a constant rate. The load is varied to maintain the constant extension rate. While extended, the material is exposed to an environment (temperature, specific fluid, etc.). The method evaluates the corrosion behaviour of the material in the given environment, often the environmental effects on the material fracture or stress-corrosion cracking susceptibility.[3]
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.[4] 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. The fastest strain rate that will still promote SCC for a given environment-material system is called "critical strain rate", some values are given in the table[5]:
| 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 |
Nevertheless, the method is very suitable for mechanistic studies, as well as for relative ranking of susceptibility to cracking of different alloys, or aggressiveness of environments.[6]
The evaluation of the results
The evaluated parameters are:[6]
- 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 (e.g., by potential drop technique[7]).
The results of the SSRT tests are evaluated using the ratio\[\left[ \frac {\text{result from specimen in test environment}} {\text{result from specimen in inert environment}} \right]\]
The departure of the ratio below unity quantifies the increased susceptibility to cracking.
See also
References
- ↑ Standard ASTM G129-00 (2006), "Standard Practice for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking", ASTM International, 2006.
- ↑ Standard ISO 7539-7:2005, "Corrosion of metals and alloys -- Stress corrosion testing -- Part 7: Method for slow strain rate testing", International Organization for Standardization.
- ↑ G. M. Ugiansky, "Stress corrosion cracking: the slow strain-rate technique", ASTM International, 1979. (Google books)
- ↑ Parkins, R.N., "Slow Strain Rate Testing - 25 Years Experience". In: R. D. Kane (ed.), "Slow strain rate testing for the evaluation of environmentally induced cracking: research and engineering applications", ASTM International, 1993. (Google books)
- ↑ ASM Handbook. Volume 13, Corrosion. ASM International, 1997.
- ↑ 6.0 6.1 V.S. Raja and Tetsuo Shoji, "Stress corrosion cracking. Theory and practice.", Woodhead Publishing Ltd, 2011.
- ↑ "Use of the Potential Drop Technique to Monitor Stress Corrosion Cracking. A short Applications Note." MATELECT application note STE01. Matelect Ltd. 33 Bedford Gardens, London, W8 7EF, UK, undated. (pdf)
