Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Numerical study on fatigue behavior of offshore structures under different stress triaxiality
Zheng-qing Cheng, Jia-cheng Shen, Fa-cheng Wang
Department of Civil Engineering, Tsinghua University
Beijing, China

structures. However, the applicability of such S-N curves to CFST
structures has not been properly solved. The steel tube of CFST is usually
under multi-axial stress state due to its composite action between steel
and concrete, and may further affect its fatigue performance adversely.
Many studies have shown that the brittleness of steel will increase underhigh stress triaxiality. Bai and Wierzbicki (2008) gave a fracture locus
which shows that the equivalent plastic strain of steel to fracture
decreases as stress triaxiality increases. The research of Pandey, Samant,
Singh and Mishra (2020) on fatigue crack propagation indicates that thecrack growth faster as stress triaxiality is higher and the damagemechanics model proposed in this research also indicates the damage
accumulation will accelerate with high stress triaxiality. Bartsch,Schaffrath, Di, Hoffmeister, Feldmann and Münstermann (2020) also
gave a similar conclusion. Therefore, the initiation and propagation of
fatigue cracks in steel tube of CFST under multi-axial stress state may
faster than that in traditional steel structures. Such research is worthy offurther study.

ABSTRACTThe steel tube of concrete-filled steel tubular (CFST) structures is undermulti-axial stress state due to its composite action between steel andconcrete, and may further affect its fatigue performance adversely. In thispaper, a numerical analysis method based on the extended finite element
method (XFEM) to describe the whole fatigue life of steel in CFST ispresented and a further exploration of the effects of stress triaxiality isconducted. The results show that the possibility of fatigue fracture in
CFST structures is greater than traditional steel structures. The common
S-N curves need to be corrected.

KEY WORDS: Fatigue; CFST; stress triaxiality; XFEM; S-N curves.
INTRODUCTION
Concrete-filled steel tubular (CFST) structures, consisting of outer steel
tube and core concrete, have been increasingly adopted in offshore
engineering such as jacket member repair and pile foundation due to its
significant mechanical properties derived from the interaction
mechanism between steel and concrete. The core concrete can suppress
the local buckling of steel tube, while the steel tube can improve theductility and strength of concrete.

This paper aims to study the influence of stress triaxiality on the S-N
curves of traditional steel structures when applying to CFST members.
A method for fatigue crack development analysis based on damage
mechanics and extended finite element method (XFEM) is adopted.

SIMULATION OF FATIGUE CRACK DEVELOPMENT

Because of the presence of frequent fluctuated loadings like wind, waves
and currents, fatigue behavior, which has the characteristic of sudden
failure, is one of the most threatening issue for offshore structures. For along time in the past, offshore structures were mainly built based on
traditional steel structures like the use of shaped steel or hollow steel tube,and existing literatures provide several methods to calculate the fatigue
life of steel members and joints including damage mechanics, Paris law,finite element method (FEM) and the S-N curves, which is the mostconvenient way. Number of codes have suggested S-N curves fordifferent member forms (BSI, 2005; BSI, 2015; DNV, 2010; GB, 2017).

Overview
In practice, it is generally considered that the crack development can bedivided into two stages, namely crack initiation stage and crack
propagation stage, as shown in Fig. 1. Generally, for the smoothlaboratory specimens, the life of the crack initiation stage is relativelylong, accounting for 90% of the total fatigue life, while the life of thelatter stage accounts for about 10% (Liu, Shi, Zhang and Fei, 2019; Shao,
Cui, Lu, and Li, 2019). When the fatigue life is calculated using the FEM,
the proportion of the two stages depends on the size of the initial crack.

In the present research on the fatigue problem of CFST structures still
adopts the same design method and S-N curves used in traditional steel

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