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Validation of Class B S-N curve of forged steel connectors

By Yanhui Zhang

TWI completed a project to validate the fatigue design curve of forged J-lay connectors for a Member company in the oil and gas sector.

This was done through a fatigue testing programme, using test specimen designs and strip specimens extracted from forged J-lay connectors with different surface finishes.

TWI also derived a procedure for correcting the Class B curve for mean stress effect, to reduce conservatism at low stress ratios. This was based on the results of fatigue tests obtained at stress ratios 0.1 and 0.4, as well as the use of the Goodman diagram.

Historical approach to fatigue design

The fatigue design of forgings in steel catenary riser (SCR) systems is commonly based on the BS 7608 Class B S-N curve, which was derived from longitudinal butt weld data. However, there is very little direct evidence to support this approach. As the surface finish of machined components is better than that of welded components featuring weld ripples, it follows that their fatigue performance should be better than Class B.

In addition, the welded specimens used to generate the Class B data will have contained very high tensile residual stresses, resulting in fatigue performance that reflects the most severe applied tensile mean stress conditions. On the other hand, there is substantial data suggesting that the Class B S-N curve may be too high.

Generating data for forged parts

TWI carried out a testing programme to produce a relevant fatigue database for establishing a suitable fatigue design procedure for plain steel components, based on Class B if appropriate. The database focused on large forged steel pipeline connectors, taking into account the effects of stress ratio and surface finish.

Programme of work

Fatigue tests were performed on strip specimens extracted from two forged J-lay connectors with different surface finishes (Figure 1). TWI used finite element modelling of the stress distribution, with the aim of designing strip test specimens that retained the features expected to influence fatigue strength. This modelling indicated that complete removal of the collar, leaving that region flush with the neighbouring outside surface, considerably reduced the bending stress induced by the thickness change, without significantly changing the stress concentration factors (SCFs) on the outside surface. The final design of the test specimen is shown in Figure 2. It retained the original surface finish and geometry, and hence SCFs at the critical locations, while reducing secondary bending stress.

The specimens were tested under constant amplitude tensile axial loading in a 1000kN servo-hydraulic fatigue testing machine at a frequency of 2–5Hz in laboratory air at ambient temperature. Two stress ratios were used, R=0.1 and 0.4. The fatigue lives of all strip specimens, including those that ran-out, are plotted in Figure 3 in terms of local stress range, which is the product of nominal stress range and SCF. The data showed that the stress ratio significantly affected fatigue endurance.

The fatigue test results were assessed as the basis for the fatigue design of full-scale forged components, with particular reference to SCRs. In this context, it is often necessary to consider higher tensile mean stress conditions than those investigated here. To illustrate a possible approach to fatigue design under higher stress ratios, the present results were analysed by modifying the well-known Goodman correction to calculate the fatigue limit at a higher mean stress.

Figure 3 illustrates possible design curves for various R values, including R=0.1 and 0.4, in comparison with the present experimental data. Each set of data lies well above the corresponding design curve. The Class B curve coincides with the calculated curve for R=0.76. This seems entirely reasonable since the Class B curve was derived from fatigue test data obtained from welded specimens containing very high tensile residual stresses. These would have had the effect of producing a very high effective stress ratio for any applied cyclic stress. Based on the test data and the Goodman mean stress correction, TWI was able to develop an alternative fatigue design approach that included the effect of the applied stress ratio.

Figure 1 Forged steel connectors from which the test specimens were extracted
Figure 1 Forged steel connectors from which the test specimens were extracted
Figure 2 Finite element modelling to design the fatigue test specimen (only half of the model is shown – symmetrical at the mid-length)
Figure 2 Finite element modelling to design the fatigue test specimen (only half of the model is shown – symmetrical at the mid-length)
Figure 3 The test data and the design curves derived for plain steel operating at different stress ratios
Figure 3 The test data and the design curves derived for plain steel operating at different stress ratios

Future developments

The finding from this study were published at the OMAE 2016 conference and will be considered for introduction into BS 7608.

References

BS 7608 (2014) ‘Guide to fatigue design and assessment of steel products’, British Standards Institution, London.

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

Avatar Yanhui Zhang Consultant – Fatigue Integrity Management

Yanhui has a background in metallurgy, and graduated from the University of Science and Technology Beijing in 1982 with a Bachelor’s degree, before obtaining a PhD from the Open University, UK in 1992. Before joining TWI in 2001, he worked on Ni-based super-alloys as a Postdoctoral Researcher at the University of Cambridge. Yanhui’s expertise includes fatigue design, fatigue and creep life evaluation, engineering critical assessment (ECA), fatigue and creep testing, and failure investigation. He is also highly experienced in establishing relationships between mechanical properties and the microstructure of materials. Yanhui has published over 60 academic papers in journals and at international conferences.

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