PSI - Issue 68

Jan Kec et al. / Procedia Structural Integrity 68 (2025) 1091–1097 Jan Kec / Structural Integrity Procedia 00 (2025) 000–000

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section with 10 mm sides were used. Three specimens were taken from the pipe in circumferential (T-L) and longitudinal (L-T) orientation. The specimens for the FT evaluation were compact tension (CT) shape and were manufactured in accordance with ASTM E 1820 and ASTM E 1681. The thickness of the specimens was 20 mm, the height was 48 mm, the width was 40 mm, and the mechanical notch length was 18 mm. All specimens had a circumferential (T-L) orientation. Fatigue pre-cracking was performed on an Instron 8500 servohydraulic testing machine at room temperature and ambient humidity. The loading of the CT specimens was performed through clevis with flat-bottomed holes and pins. The fatigue loading control parameter was a constant force ΔF = 20 kN and load asymmetry R = 0,1. The crack opening displacement (COD) was measured using an Epsilon 3541-010M-040M-ST Clip-On extensometer. Fatigue crack growth was measured using the compliance technique. The equation for determining the crack length using the compliance technique and determining the SIF is given in ASTM E 1820. The CT specimens for FT were initially fatigue-cracked and then bolt-loaded to a SIF of K Iapp = 84,5 MPa.m 0,5 , see Fig. 1. (b). SIF corresponds to an internal semi-elliptical crack with a depth of a = 10 mm (0,46.t) and a surface length of 2c = 60 mm (2,79.t) loaded at 100 bar. The Alias Hida software, using the RCC-MR approach, was used to calculate the SIF of the internal semi-elliptical crack. The equation provided by ASTM E 1681 was used to calculate the SIF of bold-loaded CT specimens. The FT test used the methodology of determining fracture mechanical parameters on a single specimen using the compliance technique. A constant COD rate of 1 mm/min was chosen for loading and unloading. The starting point of COD was 0,5 mm and subsequent increments were always in 0,20 mm. The equations for the determination of SIF, J-integral and elastic compliance are given in ASTM E 1820.

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Fig. 1. (a) pre-loading of tensile specimen; (b) bolt-loading CT specimen.

2.2. Results For TT, during the transition from elastic to elasto-plastic deformations, the longitudinal specimens (black points and curves in Fig. 2.) exhibited continuous yielding and for this reason, the yield strength at 0,2% plastic deformation (R p0,2 ) was evaluated. In the case of circumferential specimens (red points and curves in Fig. 2.), discontinuous yielding was observed between elastic and elasto-plastic deformations, and therefore the lower (R eL ) and upper (R eH ) yield strengths were evaluated. The specimens from the longitudinal direction exhibit yield strength, ultimate strength (R m ) and elongation (A) values within the specified interval defined by ISO 3183 for X70ME steel. For circumferential specimens, it is possible to see the upper yield strength exceeding the maximum permissible interval and also exceeding the ultimate strength of the material, a noteworthy phenomenon. It can be seen in Fig. 2. (a), there is an observable reduction in strength properties after 3000 hours of pre-charging. Subsequently, the strength properties remain approximately the same values at higher pre-charge time (6000 hours). The slight fluctuations in strength