PSI - Issue 7

Ivo Černý et al. / Procedia Structural Integrity 7 (2017) 431 – 437 Ivo Černý / St ructural Integrity Procedia 00 ( 2017) 000–000

433

3

pipeline steels, containing semi elliptical external surface axial crack, was carried out using ALIAS HIDA software developed within the European Framework Programme project – Deschanels et. al. (2006), Jovanovič et. al. (2001). 2. Experimental programme 2.1. Evaluation of defects During recent internal inspection of an operated high pressure pipeline of the diameter DN700, distinct, quite large indications of cracks were found in one of the pipe sections. A part of the section was taken from the ground for a comprehensive experimental programme containing analyses of the defect type, dimensions and character and also fatigue test at variable internal pressure. Direct current potential drop (DCPD) method was used to assess actual cracks. The NDT indications were of surface length between 40 mm and 100 mm (Fig. 2). Potential electrodes were spot welded to the surface near four crack indications. Measured potentials were recalculated to crack depths using the analytical curve according to the modified Johnson´s formula, Černý (2001, 2004). The calculated values corresponded sequentially to depths 1.30 mm, 0 mm, 2.17 mm and 1.60 mm. Eventually the pipe was sectioned and metallographical cuts perpendicular to the cracks in their centres were prepared.

Fig. 2. NDT indication of 41 mm length

Fig. 3. Example of one of the crack-like defects

Fig. 4. Example of inclined defect occurred during pipe manufacture

Fig. 5. Fatigue crack growth from near-surface defect during pressure fatigue test

The metallographical examination provided results, which differed from the DCPD measurement. The most important difference was the fact that practically all cracks were considerably inclined – Figs. 3, 4. Some of the defects evidently arose already during the pipe manufacture (Fig. 3), but some defects indicated a presence of an additional growth e.g. by stress corrosion mechanism near the crack tip (Fig. 4). The inclination could be considered