Andrew England

There are limited data on the very high‐cycle fatigue for structural steel welds for over 10 million cycles. The purpose of this research is a fatigue performance comparison of the welds made of steels S355JR+AR and S275JR+AR. The goal of reaching the gigacycle fatigue domain is achieved using the ultrasonic fatigue testing at 20 kHz. Fatigue samples are prepared to investigate the influence of two surface conditions—polished and precorroded. Fatigue failures are driven primarily by welding porosity with the fatigue life duration dependent on the size and location of pores. Visual comparison of fatigue data for steels poses a challenge, because of the vast scatter of experimental data points. Therefore, a statistical approach is used with a fatigue performance parameter applied to the fatigue data for welded samples to quantify the quality of the structural materials.

The very high cycle fatigue behavior of S275J2+N flux‐core arc welded joints was investigated using the ultrasonic fatigue testing method at a loading frequency of 20 kHz. A bespoke specimen design featuring the weld toe was employed to more suitably represent in‐service welded joints than typical ultrasonic fatigue testing specimens. This revealed that fracture occurs above 10 million cycles, beyond the classically accepted fatigue limit. Additionally, comparative fatigue tests were performed at 10 Hz to investigate the frequency effect on the fatigue behavior. A 35% increase in fatigue strength was measured at 20 kHz when compared to 10 Hz, indicating that a correction must be considered in the use of ultrasonic fatigue testing data for design purposes. Fracture surfaces for both test frequencies showed similar morphologies and typical characteristics for fatigue failures of ductile metals.

The design life of welded structures and components extends into the very high cycle fatigue (VHCF) regime across various applications. However, the availability of data on the fatigue behaviour of welded joints in the VHCF regime is limited, particularly when compared to the low and high cycle fatigue regimes. The development of ultrasonic fatigue testing equipment has accelerated fatigue testing and allowed for the VHCF properties of welds to be investigated in a feasible timeframe. In the present review, the emerging research concerning the VHCF behaviour of welds of various steels and non-ferrous alloys are individually explored. Overall, it is observed that welded joints have significantly lower fatigue strength than the base metal in the VHCF regime and that welding defects have a considerable influence on fatigue strength. Through the discussion of the relevant literature, important findings concerning the effects of specimen geometry and fatigue improvement methods are underlined. Furthermore, the guidance provided within design standards is compared, and some examples of VHCF failures of in-service components are highlighted. Finally, perspectives on future directions of investigation are put forward with the aim of encouraging further research in the field of VHCF of welds.