Cyclic behavior and strain energy-based fatigue damage analysis of mooring chains high strength steel

Ershad P Zarandi,Bjørn H Skallerud

doi:10.1016/j.marstruc.2019.102703

Ershad P Zarandi, Bjørn H Skallerud

Open Access

https://doi.org/10.1016/j.marstruc.2019.102703

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Abstract

This study investigates the low-cycle fatigue behavior of mooring chains high-strength steel grade R4 under different strain amplitudes and strain ratios at room temperature. A fatigue test program has been carried out on small low cycle fatigue specimens cut from large mooring chains. The experimental results characterize the cyclic stress-strain relationship, the mean stress relaxation behavior, and the cyclic plasticity parameters of the material. Strain energy density is correlated with fatigue life through a simple power-law expression and very well represented by Basquin-Coffin-Mansion relationship. Further, a non-linear elastic-plastic material model is calibrated to the experimental stress-strain curves and used for the estimation of energy dissipation in the specimens under applied cyclic loads. The predicted fatigue life using the calibrated material parameters demonstrates a close agreement with the experimental fatigue life. Numerical simulations are carried out to analyze local plastic straining and assess crack initiation at the pit site of corroded mooring chains considering the multiaxial stress state. An energy-based approach is employed to estimate the number of cycles needed for a crack to initiate from an existing corrosion pit.

Highlights

In the last decades, energy sources at sea and under the seabed have captured engineering society’s attention
A similar S-shape trend in the reduction of stress amplitude is seen for all strain ranges
Cyclic plasticity and fatigue performance of the mooring chain high strength steel grade R4 has been investigated using the data obtained from uniaxial strain-controlled fatigue tests on low cycle fatigue (LCF) specimens

Summary

Introduction

Energy sources at sea and under the seabed have captured engineering society’s attention. The number of floating offshore production systems and facilities has increased [1] Continuous operation of these offshore facilities, is directly dependent on the integrity of their mooring systems that keep these facilities in position during an operation when subjected to cyclic loads from wave, wind, and current as well as a continuous exposure to a corrosive environment. The amount of plastic straining at the corrosion pit site in mooring chains depends on the applied load as well as the material response to it. Investigation of cyclic response and fatigue properties of mooring chain materials is of great importance to the prediction of the fatigue crack growth rate and the remaining fatigue life of pitted mooring chains

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Conclusion