BS ISO 22848:2021 pdf download – Corrosion of metals and alloys — Test method for measuring the stress corrosion crack growth rate of steels and alloys under static- load conditions in high-temperature water
4 Principle of test Stress corrosion cracking is a phenomenon in which a crack grows in an environment when stress is applied to a susceptible material. Thus, stress corrosion cracking is affected by three general factors: the material, stress and environment. The SCC growth rate is affected by the stress intensity factor, K I . The SCC growth rate, da/dt, is defined as the time derivative of the crack length. While there is often no clear distinction between static loading and some very slowly increasing monotonically or cyclic loading, the primary interest in most SCC growth testing is the behaviour under static loading.
By applying a static load to a specimen with a crack at a known K I and continuously measuring the crack length (a) using the PDM, the crack growth rate (da/dt) can be continuously obtained. Often the best insight into the effects of environment and temperature are obtained by making periodic changes while continuously measuring the SCC growth response.
This document specifies the preparation of specimens, the control of the testing environment, the method of transitioning from fatigue crack to SCC, and the determination of the growth rate of a crack using fracture mechanics specimens in high-temperature water environments, with an emphasis on light water reactors. Although the minimum requirements and basic procedures of SCC growth rate testing in high- temperature water are summarized in this document, it should be noted that there are complex inter- dependencies of many influential parameters on stress corrosion cracking phenomena, and subtle variations in test conditions can have a major impact on the reproducibility and credibility of the data. Extensive efforts to obtain high-quality SCC growth rate data have been undertaken over the last four decades and many key issues must be understood and implemented (see References  to ). For example, reliable SCC transitioning prior to static loading is an essential element, and specific procedures have been developed to help achieve well-behaved response.
5.1 Specimen orientation The specimen orientation in the test material is designed in accordance with ISO 7539-6. The relative orientation of the crack plane and growth direction in the test material shall be specified in relation to the product form (such as plate rolling direction or pipe longitudinal direction) and, if applicable, also specified in relation to the weld direction and additional cold work (e.g. for rolling or forging). When the specimen is taken in or near a weld, the location of the crack plane of the specimen in relation to the weld fusion line shall be provided because a very pronounced effect on SCC behaviour is expected when the crack in the specimen propagates in the heat affected zone or weld metal of the test material, and the properties can vary from the weld root to the weld crown.
5.2 Specimen geometry Many specimen geometries have been used for crack growth testing (see ISO 7539-6). The most common specimen is a compact tension (CT) specimen with a side-groove design, shown in Figure 1. The specimen thickness, B, is usually between 12,5 × 10 −3 m and 25,4 × 10 −3 m. Smaller or larger specimens are sometimes used but shall be justified from K-size criteria (see 5.4). The specimen width, W, is typically two times the specimen thickness (B). Side grooves on both sides of the specimen are recommended to help maintain in-plane crack growth, but are not obligatory. The depth of each side groove is typically 5 % of B, and they are typically hemispherical.
The more complex contoured double cantilever beam (CDCB) specimen is also used because the stress intensity factor is practically considered constant over a certain range of crack lengths under constant- load conditions. However, note that the criteria for a CT specimen given in 5.4 is not applicable to a CDCB specimen. Details for a CDCB specimen are given in Annex A.