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Resonance fatigue testing to qualify welds in thick walled pipe

Resonance fatigue testing has become the industry standard test method for qualifying girth welds made using new welding procedures. Recently TWI member Pipeline Technique Ltd (PTL) contracted TWI to carry out a series of resonance fatigue tests as part of a large weld qualification programme on thick walled pipe.

The intention was to deploy the welds in deep water. The large water depth necessitated the use of thick walled pipe. PTL was carrying out a substantial weld qualification programme which involved 56 welding procedures in five pipe diameters. The majority of the pipes had a large wall thickness, necessary to withstand the deep-water service conditions.

Some of the girth welds would be subject to cyclic loading in service and so were fatigue critical. Therefore, fatigue testing of the welds made using 13 of these welding procedures was needed to ensure that they met the required target life, based on the fatigue class API X’.

PTL contracted TWI to carry out this large resonance fatigue testing project.

  • Carry out resonance fatigue tests on 13 sets of six specimens in order to determine whether the girth welds qualify to a target based on API X’.
  • Ensure that the weld root and weld cap are in tension throughout the tests.
Figure 1 Without internal pressure, the stress cycle in a resonance fatigue test oscillates about a zero mean stress. Using a positive mean stress via internal pressure, the stress range becomes fully tensile
Figure 1 Without internal pressure, the stress cycle in a resonance fatigue test oscillates about a zero mean stress. Using a positive mean stress via internal pressure, the stress range becomes fully tensile

The resonance fatigue testing technique applies a fully alternating stress range. The applied stress range varies from fully tensile to fully compressive in each cycle about a mean stress equal to zero, i.e., the ‘stress ratio’, R, is equal to -1.

To ensure that weld cap and root were in tension during testing, a mean stress was applied via internal pressure (see Figure 1). The large wall thickness of the pipes in this project meant that very large internal pressures were required to produce reasonably high mean stresses.

TWI first carried out calculations to determine the maximum mean stress that could be safely applied using internal pressure. This was limited by the hoop stress produced (TWI ensure that this would not burst the pipes), and the stored energy limit of TWI’s test machine safety frames. The calculations showed that the maximum pressure that could be safely applied was 22,000psi (1517bar).

TWI has successfully carried out over 60 tests to date on welds made using PTL’s new welding procedures. All of the welding procedures have produced welds which do qualify to the required target based on API X’. Many of the welds have exceeded the requirement and achieved fatigue lives that were above 2.9 x BS7608 Class B, which is a very high fatigue performance, considering that the weld caps and roots were not ground flush to the parent material in these welds.

For more information, please email contactus@twi.co.uk

Figure 2 A thick walled specimen on test in a TWI designed resonance fatigue test machine
Figure 2 A thick walled specimen on test in a TWI designed resonance fatigue test machine
Avatar Dr Carol Johnston Consultant

Carol Johnston is a Consultant in the Fatigue Integrity Management section at TWI. Since 2009, she has been carrying out research and running a range of projects related to the fatigue performance and structural integrity of welded joints. Her main area of work is on validation fatigue testing of welds using the resonance fatigue testing technique, and providing consultancy on fatigue design of welded joints. 

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