Stress Corrosion Crack Growth in Copper for Waste Canister Applications
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95 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
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straining of the material. Unfortunately, none of the authors has elaborated on what this information is worth in the context of the long term integrity of the canister. In the absence of convincing arguments for the significance of a critical potential it is reasonable to seek other approaches for the assessment of SCC susceptibility of the canister. One possibility is to accept that cracks may form but that the growth of is extremely slow under storage conditions. One would then use the so called fracture mechanics approach where it is assumed that the crack growth rate is a function of the environment and the applied stress
.
•cracks
K1SCC
K
Stress intensity factor, K
intensity factor. Figure 1 shows a relationship frequently observed for metals susceptible to stress
corrosion cracking. According to Figure 1 the crack growth can be divided into three different stages, I, II crack growth and III and four stress intensity factor ranges: the ranges corresponding to the three stages and the range below KISCC in which no crack growth takes place. Stage I is characterized by a strong dependence on the stress intensity factor KI often adequately described by the following equation:
Figure
1. The three stages of stress corrosion
da dt
where ; is the applied stress, a the crack length and C a constant. In stage 11 the crack growth is weakly dependent on K1 and controlled by diffusion processes in the crack environment. In stage III, finally, mechanical crack growth processes interact with the chemical process and at a high Kj-value a final purely mechanical failure occurs. In the context of the long term canister integrity stage I is the most interesting. The approach to an assessment of canister integrity could then be to determine the stress corrosion crack growth behaviour of copper over a range of stress intensity factor and a range of environmental conditions in the hope that relationships according to eq. 1 were found for the influence of stress intensity factor and some other relationships for the influence of environment. These relationships would then be used for extrapolations to canister conditions and, if the procedure was successful, extrapolated growth rates would be so low that the long term integrity of the canister was secured. The purpose of the present project was to demonstrate the possibility of obtaining crack growth data for copper as a function of applied stress intensity factor as a pre-project to a more long term project for determining crack growth data. The reason for carrying out a preproject was simply that since pure copper is a very soft and ductile metal it is not at all certain that the fracture mechanics approach to stress corrosion cracking will work. At the time of the start of the project some work on crack growth had been done in Finland where useful data had been obtained [4] although a few problems with branching of cracks had been observed [5]. More recently testing on sub-sized CT
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