PSI - Issue 42

Mimoun Elboujdaini et al. / Procedia Structural Integrity 42 (2022) 1033–1039 Mimoun Elboujdaini / Structural Integrity Procedia 00 (2019) 000 – 000

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inclusion. Aluminate inclusions have very poor deformability [1], and therefore, they remain spherically shaped throughout the pipe manufacturing processes.

3.2. Initiation of SCC There are generally three stages for the cracking process: (i) generation of an environment that causes cracks to initiate; (ii) Initiation of cracks; and (iii) propagation of cracks until failure occurs.

The present study is focused on crack initiation, types of cracks, crack behavior , role of crack initiation sites, inclusions, factors governing crack growth, and the crack coalescence process in the context of low-pH SCC in linepipe steel. 3.3. The Aim The aim of this work, the validation of the results under accelerated test conditions based on the assumption that the micro-processes, microstructural features, and specific sites associated with initiation of cracks in laboratory tests are those associated with the initiation of SCC in operating pipelines. In this context, the following aspects were studied for SCC initiation: 1. The sites where cracks are initiated in relation to metallurgical factors, such as nonmetallic inclusions, Figs. 1 to 2, grain boundaries, specific phases, or other forms of surface discontinuity or surface defect. 2. The role of applied mechanical loading conditions , including: the effects of stress level (for static and cyclic loading), strain rate (for dynamic loading), stress ratio, R, and loading frequency, f; 3. The time dependence of crack development, including: (i) Existence of an incubation time, (ii) Changes in crack number density, crack size with time, dormancy, and (iii) The crack growth rate. The cracks are found to become dormant and hence are innocuous. Significantly, they are always found near the site of ruptures, Fig. 3, so it is felt that these cracks are sometimes precursors to rupture. 4. The spatial distribution of cracks is also relevant. Spacing affects crack interaction and coalescence and significantly influences the overall cracking behavior, and hence, the lifetime of a pipeline. The linking or coalescence process for neighboring growing cracks (Fig. 4) was proposed [2, 3] as a means to encourage growth (Fig. 5) and was shown by measurements to be more likely associated with crack tip dormancy [4].

Fig. 1. (a) Development of pits from inclusions and (b) nucleation of cracks at pits in a sample of pipe steel. The specimen was tested in NS4 solution saturated with CO 2 at σ max = 70% YS. In (c) EDS spectrum reveals the elements Na, Mg, Al, Si, P, S, Cl, K and Ca.