A test rig, Figure 1 above, was designed and manufactured for testing mooring chain sections of 76mm link in diameter. Six full-scale fatigue tests were conducted: three on high strength steel, grade R4 and three on high strength steel, grade R5. In each test, a chain line of eleven links was loaded under tension into free corrosion conditions in seawater, Figure 1.
When a link failed, it was replaced with a Kenter (temporary) link to enable continuation of the test, until three link failures had been achieved in each test. Magnetic particle inspection (MPI) was carried out to characterise the arising cracking location, crack size and crack growth. Fracture surfaces were examined to reveal any features associated with fatigue crack initiation.
The testing programme generated sixteen link failures: eight from grade R4 and eight from grade R5. Statistical analyses were performed to determine whether the fatigue data associated with R5 was significantly different from that of R4. The fatigue data obtained in this testing programme was also compared with the mean curve for grades R3 and R4, provided by the earlier Noble Denton (ND) Joint Industry Project on fatigue of mooring chains, and the design curves in the API RP 2SK and DNVGL-OS-E301 standards, Figure 2.