Constant extension rate test

The two major drawbacks to the Kj c measurement through the use of iT-decreasing specimens are, as said in Sect. 15.5, the long waiting times due to incubation and the subcritical crack growth that may occur immediately during the 

15 Fracture Mechanics Approach to Stress Corrosion Stress intensity factor (ksiVin) 

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Controlled Strain-rate Tests Controlled strain-rate tests were first developed by Parkins (see Ugiansky and Payer ) for the study of stress-corrosion cracking. These took the form of constant strain-rate tests (also known, perhaps more accurately, as constant extension-rate tests). Since then alternative forms of test have been developed to modify the conditions under which the specimen is exposed. 

The conditions are severe, the tests are rapid and the imposed conditions of a slow strain-rate are similar to those occurring at a crack tip. For reasons discussed below, experiments should be done potentiostatically. A recent conference (1 0) was devoted to the constant extension rate test, organized by A.S.T.M. 

As an example, provisions for electrochemical measurements during CERT (Constant Extension Rate Testing) are shown in Fig. 3.4 [21,22]. 

N. Winter, A. Atrens, W. Dietzel, V. Song, K.U. Kainer, Comparison of the hnearly increasing stress test and the constant extension rate test in the evalnation of transgranular stress corrosion cracking of magnesium. Mater. Sci. Eng. A 472 (2008) 97—106. 

In cases where susceptibility to HEC, SCC, liquid metal embrittlement (LME), or other EAC mechcinism may be suspected, it is common to evaluate the actual mechcinical behavior of the alloys under consideration in the corrosive environment of interest. For decades, these techniques were developed and conducted in low-pressure ciqueous environments and are referred to as slow strciin rate (SSR) testing or constant extension rate testing (CERT) [/2]. The important aspect of such techniques is the requirement for these tests to be conducted in a range of strain rate or crosshead extension rate in which the material under evaluation is going to experience cracking if it is susceptible. 

An HE mechanism was postnlated. Kannan and Raman [112] fonnd small decreases of strength and dnctility for AZ91 in a simnlated body finid nsing the constant extension rate test (CERT) these decreases nsnally indicate SCC althongh the decreases were claimed to be insignificant. 

Tsuchiya et al. [109] carried out constant extension rate test (CERT) experiments on sensitized 304 and 316 stainless steels at 4 x 10 s at temperatures firom 290 to 550°C at 25 MPa in high purity water with a dissolved oxygen content of 8 ppm. No signs of IGSCC were observed at temperatures above 400°C (Fig. 4.24), but both alloys had small cracks on the side surface when exposed to SCW. 

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