Slow strain rate testing

Slow Strain-Rate Test In its present state of development, the results from slow strain-rate tests (SSRT) with electrochemical monitoring are not always completely definitive but, for a short-term test, they do provide considerable useful SCC information. Work in our laboratory shows that the SSRT with electrochemical monitoring and the U-bend tests are essentially equivalent in sensitivity in finding SCC. The SSRT is more versatile and faster, providing both mechanical and electrochemical feedback during testing. 

Edwards e/a/. carried out controlled potential, slow strain-rate tests on Zimaloy (a cobalt-chromium-molybdenum implant alloy) in Ringer s solution at 37°C and showed that hydrogen absorption may degrade the mechanical properties of the alloy. Potentials were controlled so that the tensile sample was either cathodic or anodic with respect to the metal s free corrosion potential. Hydrogen was generated on the sample surface when the specimen was cathodic, and dissolution of the sample was encouraged when the sample was anodic. The results of these controlled potential tests showed no susceptibility of this alloy to SCC at anodic potentials. 

The sometimes contradictory results from different workers in relation to the elements mentioned above extends to other elements . Some of these differences probably arise from variations in test methods, differences in the amounts of alloying additions made, variations in the amounts of other elements in the steel and the differing structural conditions of the latter. Moreover, the tests were mostly conducted at the free corrosion potential, and that can introduce further variability between apparently similar experiments. In an attempt to overcome some of these difficulties, slow strain-rate tests were conducted on some 45 annealed steels at various controlled potentials in three very different cracking environments since, if macroscopic... 

Fig. 8.15 Effects of potential upon the stress-corrosion cracking of various steels in CO3-HCO3 solution in slow strain rate tests (after Parkins et al )...

Fig. 8.17 Effects of applied potential upon the time to failure ratio in slow strain rate tests of C-Mn steel, with and without a 6% nickel addition, in boiling 8 m NaOH, 1 m NaFICOj + 0.5 m Na2COj at 75°C, and boiling 4.4m MgCl2 (after Parkins elat and...

The strains needed to initiate cracks in both the annealed and the sensitised materials were obtained using tapered slow-strain-rate specimens and the data are given in Fig. 8.36. As can be seen, there is little temperature dependence of the strain needed to initiate cracks in sensitised material whereas the annealed material was most susceptible to cracking at about 250°C. These results indicate the complicated response of Type 316 stainless steel to applied potential and demonstrate that, even though environmentally-assisted cracking may be generated by severe test methods, in this case the slow-strain-rate test, the results obtained must be used with care. For instance, the cracking of the annealed material at low potentials... 

Fig. 8.36 Minimum strains for initiating stress-corrosion cracks in annealed and in sensitised 316 during slow strain rate tests in S ppm chloride content water...

While the conventional slow strain-rate test offers many benefits, it does suffer from a tendency to overstate the susceptibility of materials to hydrogen embrittlement. Thus structural steels of modest strength will fail even under conditions giving relatively low rates of hydrogen entry. This is... 

Another modification to the slow strain-rate test involves the superimposition of a low amplitude sine wave ripple on the slow uniform extension (Fig. 8.47). In effect this produces higher strain rates (which appear to be more damaging for hydrogen embrittlement), while still giving a long test duration, with adequate time for the accumulation of hydrogen in the steeps. 

Fracture Mechanics Tests One problem of both sustained load and slow strain-rate tests is that they do not provide a means of predicting the behaviour of components containing defects (other than the inherent defect associated with the notch in a sustained load test). Fracture mechanics provides a basis for such tests (Section 8.9), and measurements of crack velocity as a function of stress intensity factor, K, are widely used. A typical graph of crack velocity as a function of K is shown in Fig. 8.48. Several regions may be seen on this curve. At low stress intensity factors no crack growth is... 

In addition to examining pre-exposure effects, the slow strain-rate testing technique has been used increasingly to examine and compare the stress-corrosion susceptibility of aluminium alloys of various compositions, heat treatments and forms. A recent extensive review draws attention to differences in response to the various groups of commonly employed alloys which are summarised in Fig. 8.57. The most effective test environment was found to be 3 Vo NaCl -F 0.3 Vo HjOj. The most useful strain rate depends upon the alloy classification. 

The equipment required for slow strain-rate testing is simply a device that permits a selection of deflection rates whilst being powerful enough to cope with the loads generated. Plain or precracked specimens in tension may be used but if the cross-section of these needs to be large or the loads high for any reason, cantilever bend specimens with the beam deflected at appropriate rates may be used. It is important to appreciate that the same deflection rate does not produce the same response in all systems and that the rate has to be chosen in relation to the particular system studied (see Section 8.1). 

The representation of the results from slow strain-rate tests may be through the usual ductility parameters such as reduction in area, the maximum load achieved, the crack velocity or even the time to failure, although as with all tests, metallographic or fractographic examination, whilst not readily quantifiable, should also be involved. Since stress-corrosion failures are usually associated with relatively little plastic deformation, the ductility... 

Fig. 8.96 Average slress-corrosioD crack velocity from monotonic slow strain rate tests at 1.5 X 10 s conducted over various restricted ranges of stress on a cast Ni-Al bronze in seawater at 0.15 V(SCE). The stress range traversed in each test is shown by the length of the bar. (after Parkins and Suzuki )...

Method for the preparation and use of precracked specimens Method for slow strain rate testing... 

For comparison, the following relationship has been found for an X70 pipeline steel (slow strain rate test using compact tension specimens with the precrack located in the heat-affected zone of a weld) ... 

G129, Standard Practice for Slow-Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking, ASTM, West Conshohoken, PA, 1995. 

TMO 198-98, Slow Strain Rate Test Method for Screening Corrosion Resistant Alloys for Stress-Corrosion Cracking in Sour Oil Field Service, NACE International, Houston, TX, 1998. 

Fig. 18. Polarization curves measured for Alloy 600 in lithiated water at 350 °C and percentages of IGSCC fracture in slow strain rate test specimens with humps [40], Reproduced with permission.

Next, whenever diffusion is considered to proceed simultaneously along with a non-steady state loading history, such as if slow strain rate tests were simulated, the stress-field is obviously time dependent, and so, the stress dependent element matrices do, too. Besides, when large geometry changes occur, the deformed distances become the diffusion paths of interest, so that coordinates x must be continuously updated with deformation displacements, and thus, they also become time dependent. As a result, all the element matrices in equations (13) must be updated throughout the simulation histories, i.e., they... 

---