How It Works
Rotating bending tests with the resonance method allows for parts to be tested at full scale with a high test frequency of around 30Hz. The technique involves exciting a test specimen close to its first mode of vibration by applying a rotating radial force to one end. A bending moment is generated in the specimen, which rotates about the pipe axis, resulting in all of the longitudinal fibres in the specimen experiencing the same bending moment. The industrially-accepted approach is to test three specimens at each of three stress ranges, equalling nine specimens in total. High and medium stress range tests would run until through-wall cracking occurred while low stress range tests can be stopped as ‘runouts’ (above the target life but before cracking has occurred).
Resonance Fatigue Testing at TWI
Our facilities have the capability to test pipes and mechanical connectors with external diameters from 6 to 36 inches in rotating bending, at either zero mean stress, a tensile axial mean stress from internal pressurisation (10,000psi), or mechanically applied tensile axial mean stress (up to 2000kN).
Although there is no set standard to define a resonance fatigue test, our years of experience in this area has allowed us to create an industry standard method for testing full-scale girth welds, which is now a widely adopted method for testing mechanical and other subsea components.
Our resonance testing laboratory boasts seven TWI-designed bespoke resonance fatigue test machines, allowing us to conduct tests with:
- zero mean stress
- a tensile axial mean stress introduced by internal pressurisation with water (up to 10,000psi)
- a tensile axial mean stress introduced mechanically (up to 2000kN)
The Need for Reliable Testing
Small changes in welding process or parameters can greatly influence the geometry and properties of girth welds, and thereby affect their fatigue strength. Where high reliability is required, validation fatigue testing of new welding procedures is specified to ensure safety of these systems.
The resonance technique is a quick and efficient means of fatigue testing the full circumference of girth welds and mechanical connectors in full-scale pipes. Resonance fatigue testing is mainly used by the oil and gas industry for testing girth welds in risers, pipelines and flowlines to generate S-N curves. Connector designers use resonance fatigue testing to verify finite element predictions of fatigue life and of the mode and location of fatigue failure.
Benefits
Resonance tests typically run at 30Hz, so samples can be tested to failure quickly. Our tests are run continuously, achieving 2.5 million cycles per day.
Resonance fatigue testing has become the industry standard method of testing full-scale girth welds for high integrity pipes and risers, and is also widely adopted for fatigue testing mechanical connectors and other subsea components.
Operators choose TWI for our expertise (especially in interpretation of results), our experience and our capability.
For more information on our resonance fatigue testing services, please email contactus@twi.co.uk.