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Abstract
The primary focus of this work is to investigate the sensitivity of cyclic waveform, frequency (f), load level and microstructure on the corrosion-fatigue crack growth rate (CFCGR) in modern normalised-rolled (NR) and thermomechanical control process (TMCP) ferrite-pearlite steels in the Paris Region of the da/dN vs. ΔK log-log plot. Constant amplitude sinewave (si) and trapezoid waveform (generally referred to here as hold-time (h-t)) were used under frequencies of 0.2 Hz, 0.3 Hz and 0.5 Hz and stress ratio of 0.1. Comparison is also made between the crack path in the S355 TMCP steel under si and h-t in seawater (SW). The role of microstructure in retarding or accelerating fatigue crack growth in SW is also discussed. Experimental results showed that the CFCGR corresponding to the si is higher than that of the h-t for all the load levels and frequencies examined. It was observed that reduction in the f and fatigue load level increased the CFCGR for the h-t but had little effect on the si. Generally, f in the range 0.2–0.5 Hz had little effect; and for a given f an increase in load led to a reduction in the CFCGR, in the Paris Region (PR) for both si and h-t in SW. Under both si and h-t, the CFCGR in the TMCP steels (e.g. S355G8 + M, S355G10 + M) is lower than that of the normalised steels (e.g. S355J2 + N). Metallurgical analyses on the fractured surface of corrosion-fatigue specimens show that the main active crack tip blunting process is the primary factor controlling the CFCGR of steel at high stress intensity factor range (SIFR) and low f in SW. The results obtained from this study have been discussed in terms of the potential impact on the structural design and integrity of offshore wind turbine foundations.
Highlights
Steel structures in a dynamic service environment degrade with time due to fatigue
The waveforms considered in this study are sinewave and trapezoid with hold-time and the experiments were conducted under frequencies of 0.2 Hz, 0.3 Hz, 0.5 Hz at the fixed stress ratio of R = 0.1 for comparison purposes
The current study investigated the sensitivity of the corrosion-fatigue crack growth rate (CFCGR) of the S355 steel subgrades S355G8 + M, S355G10 + M and S355J2 + N to waveform, f and load level in SW
Summary
Steel structures in a dynamic service environment degrade with time due to fatigue Offshore engineering structures, such as wind turbines (WTs) are subjected to cyclic loading conditions due to constant exertion of wind, wave and currency forces. E., the combination of a corrosive environment and fatigue phenomenon is found to be more detrimental than each of them acting separately This simultaneous action of cyclic load and chemical attack is commonly referred to as corrosion-fatigue (CF) and it is found to be an important failure mechanism in offshore structures [1,2]. The structural integrity design for such structures aims to ensure that they carry the applied loads during operation without any macro-scale failure throughout the lifespan For this reason, it has become a common research subject to experimentally measure the FCG under a given stress condition
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