The new accelerated cooled (TMCP) steels are potentially a suitable replacement for the traditional normalized steel used in offshore structures. This research addressed the fatigue behaviour of offshore welded structures in TMCP steel, as compared with those made from traditional normalized steel. In particular, the effect on fatigue life of different parameters, including plate thickness, cathodic protection and the size effect, were studied. The following materials were tested: TMCP steel equivalent to BS355 and normalized Fe 510 EU25 steel. The experiments included fatigue tests on full scale welded tubular X-joints and on smaller scale T-butt specimens, in air and in seawater (10 deg C) with cathodic protection (-850 mV). The tubular X-joints were tested under axial loading while the T-butt specimens were tested under 4-points bending. Constant amplitude sinusoidal loading (R = 0.1) was used, with frequencies of 1 and 2 Hz respectively for tests in air on tubular and T-butt specimens, and 0.35 Hz for all tests in seawater. Both joint types were tested at four different stress levels, ranging from 115 to 285 MPa. The experiments showed that the fatigue behaviour of welded joints in the TMCP steel was similar to that in the normalized steel, both in air and in seawater with cathodic protection. Therefore, the same design criteria can be used for both steels. As regards the influence of plate thickness, different results were obtained from the tests on reduced scale and full scale joints. For the reduced scale T-butt welds (in air and seawater), Gurney's formula was found to be valid with a value of n increased up to 0.3. Moreover, the influence of plate thickness on the fatigue performance in seawater seemed to depend on the applied stress. For high applied stress, the effect of thickness was small, and vice versa. The influence of thickness on the fatigue performance of tubular X-joints was less than for the T-butt specimens. In this case, a value of the exponent of n = 0.2 was found to be most suitable. Regarding the influence of cathodic protection, the results showed that, in general, it halved the reduction in fatigue life due to free corrosion in seawater, with an even greater benefit in the low stress/high cycle regime.
Analisi del comportamento a fatica di acciaio TMCP da raffreddamento accelerato, per impieghi off-shore, in prove di laboratorio e in piena scala
Ermüdungsverhalten von thermomechanisch behandelten (kontrolliert abgeschreckten) Stählen für die Meerestechnik in Laborversuchen und unter praxisnahen Bedingungen
Fatigue behaviour of TMCP accelerated cooled steel for offshore structures, in laboratory and full scale tests
1998
78 Seiten, 38 Bilder, 19 Tabellen, 16 Quellen
Report
Italienisch
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