Test and analysis
Aker Subsea provided four connectors within two 7m long test specimens for testing. The specimen ends were blanked off so that they could be filled with water and pressurised to check that the integrity functioned during the tests.
Usually in a resonance fatigue test, a high internal water pressure is used so that when a through-wall crack develops by fatigue, the internal water pressure is released and the test is stopped automatically. In this case, the design of the connector was such that the internal seals would prevent the release of the internal pressure, so could not be relied upon to stop the test. As an alternative, TWI identified suitable locations for the placement of strain gauges, which would measure a rapid increase in strain when the connector cracked. Data recording software was used to stop the test automatically when the measured strain exceeded set values.
The team tested the connectors in a TWI-designed resonance fatigue test machine. Cracking in both connectors was indicated by the strain gauge readings.
TWI then opened the connectors and carried out dye penetrant and magnetic particle inspection on the fatigue critical locations. This identified a number of cracks in locations that had not been highlighted by the FEA.