Fatigue Characteristic of Designed T-Type Specimen under Two-Step Repeating Variable Amplitude Load with Low-Amplitude Load below the Fatigue Limit

Jin Gan,Li Yao,Hui Deng,Xiaoli Wang,Zhou Wang,Weiguo Wu,Di Sun

doi:10.3390/jmse9020107

Jin Gan, Li Yao + Show 5 more

Open Access

https://doi.org/10.3390/jmse9020107

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Abstract

In order to investigate the non-linear fatigue cumulative damage of joints in ocean structural parts, one type of low carbon steel Q345D was employed to prepare designed T-type specimens, and a series of fatigue experiments were carried out on the specimens under two-step repeating variable amplitude loading condition. The chosen high cyclic loads were larger than the constant amplitude fatigue limit (CAFL) and the chosen low cyclic loads were below the CAFL. Firstly, the S-N curve of designed T-type specimen was obtained via different constant amplitude fatigue tests. Then, a series of two-step repeating variable load were carried out on designed T-type specimens with the aim of calculating the cumulative damage of specimen under the variable fatigue load. The discussions about non-linear fatigue cumulative damage of designed T-type specimens and the interaction effect between the high and low amplitude loadings on the fatigue life were carried out, and some meaningful conclusions were obtained according to the series of fatigue tests. The results show that fatigue cumulative damage of designed T-type specimens calculated based on Miner’s rule ranges from 0.513 to 1.756. Under the same cycle ratio, the cumulative damage increases with the increase of high cyclic stress, and at the same stress ratio, the cumulative damage increases linearly with the increase of cycle ratio. Based on the non-linear damage evaluation method, it is found that the load interaction effect between high and low stress loads exhibits different damage or strengthening effects with the change of stress ratio and cycle ratio.

Highlights

Fatigue failure is a common phenomenon in steel structures of the ships and ocean assemblies, which are always subjected to cyclic loading during service [1,2]
It can be seen that the load interaction effects of high-low cyclic stress have different damage or strengthening effects on fatigue performance with the change of stress ratio and cycle ratio
This paper investigated the fatigue characteristics of designed T-type specimen under the two-step repeating load block

Summary

Introduction

Fatigue failure is a common phenomenon in steel structures of the ships and ocean assemblies, which are always subjected to cyclic loading during service [1,2]. The actual predicted result is usually unsatisfactory, and the experimental results showed that the good agreement only occurred in some cases and some materials [18] Another approach is making use of the concept of fatigue limit reduction to measure the damage accumulation, when a multi-level load is applied and many rules based on the S-N curve modification have been developed [19]. A large number of experiments [9,25,26,27,28,29,30,31] show that the actual fatigue damage is larger than in the assumption method It was reported [12] that the use of a 2-slope S-N curve with the slope change at a stress range above 10.1 N/mm (corresponding to about 5.5 × 108 cycles) was potentially unsafe, for loading spectra containing a large number of small cyclic stresses. The nonlinear damage parameter was introduced to evaluate the load interaction effect and some conclusions were drawn

Experimental Setup

Constant Amplitude Fatigue Tests

Damage Accumulation of Variable Fatigue Experiment Analysis

Non-Linear Damage Evaluation

Conclusions