Resonance fatigue testing in TWI’s pressure pit facility

Resonance fatigue testing has become the industry standard test method for qualifying the fatigue strength of girth welds made using new welding procedures.

TWI recently carried out a series of resonance fatigue tests on thick walled pipes for TWI member Petrofac. The internal pressure required to ensure that the applied stress was tensile at the root resulted in a high level of stored energy in the test specimens. In order to complete the tests safely, the tests were performed on a resonance fatigue machine located in TWI’s pressure pit test facility.

Background

Petrofac was contracted by their client to undertake the detail design work for a gas injection pipeline. The pipeline has a design pressure of 600bar and therefore the pipe requires a wall thickness of close to 60mm. The pipeline will experience fatigue loads in service so it is important that the welds can be shown to have sufficient fatigue strength.

Petrofac contracted TWI to carry out a programme of resonance fatigue tests to determine the in-air fatigue performance of sample welds.

Objectives

Carry out resonance fatigue tests on six specimens in order to determine whether the girth welds qualify to the target BS7608 Class F2 fatigue design curve
Ensure that the weld root is in tension throughout the tests.

Solution

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 the weld root was in tension during testing, a mean stress was applied via internal water pressure. Since the pipes were thick walled, a high internal pressure (16,800psi, 1158bar) was required to produce a 100MPa axial mean stress. This ensured that the weld root was in tension throughout testing.

The pipes also had a large diameter so the stored energy associated with the pressurised water was higher than could be accommodated by TWI’s

standard safety containment frames. In order to carry out the tests safely, a resonance test machine was therefore relocated to TWI’s pressure pit test facility. This is a fully reinforced pit with a burst proof lid so that the test specimen is fully enclosed during the test. In the unlikely event that there were to be a catastrophic failure of a test specimen, all fragments would be safely contained without any risk to the operators.

In this project, six resonance fatigue tests were successfully carried out. Five of the tests ran until through-wall cracking occurred, as detected by a decrease in the measured internal water pressure (see the Figure).

Analysis of the test results showed that the welds did have the required Class F2 fatigue performance.

Project Outcome

TWI’s extensive test facilities allowed tests to be carried which met Petrofac’s requirements.

The welds tested achieved the required target of BS7608 Class F2 fatigue performance.

A thick walled specimen on test in a TWI designed resonance fatigue test machine, located in TWI’s pressure pit test facility. Water can be seen escaping from a through-wall fatigue crack

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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