PSI - Issue 13

Wei Song et al. / Procedia Structural Integrity 13 (2018) 2227–2232 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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These methods are in the elastic or elastic fracture mechanics regime. With regard of the low cycle fatigue (LCF) regime (locally exceeds yield strength of material), the determination of fatigue crack initiation point and cycle number for welded joints depends on the applied stress, weld geometry, material properties, and material and welding defects by T. Nykänen (2017). Therefore, it is more complicated to assess the LCF behavior of welded joints that incorporate geometry factors with the interaction of material plastic properties and cyclic loading conditions. Due to the introduction of material plasticity, it is difficult to evaluate LCF behavior by stress-based approaches. Instead, the strain-based or energy-based approaches are more suitable to illustrate the plasticity deformation behaviors under large cycle loadings for material fatigue. For welded joints and structures, Dong et al. (2006) firstly introduced a structural strain concept which extended from structural stress approach and proposed a corresponding LCF treatment procedure according to Neuber’s rule. Meanwhile, an analytical -based structural strain method is conducted to deal with the LCF results of girth-pipe component by Pei et al. (2017). The effective notch strain method was proposed by Saiprasertkit et al. (2012) to address the HCF-LCF fatigue behavior considering material mechanical heterogeneity and geometry variation of load-carrying cruciform joints. The corresponding analytical equations of weld root failure were given to estimate the notch strain values also by Saiprasertkit et al. (2012). The fatigue initiation location was also judged by elastic-plastic analysis using this method by Saiprasertkit et al. (2014). In addition, Benjamin et al. (2017) extended the S-N design curve to assess the high quality butt welds of LCF regime on the basis of fatigue class FAT 160 (k=5). The SWT fatigue damage parameter was employed to study the LCF behavior of butt welded joints under different notch radius. However, there is different failure locations on the load-carrying cruciform joints, which leads to variation of fatigue life. In low cycle fatigue regime, limited literature is focus on quantifying the fatigue parameter considering the materials mismatch, geometry variation and failure locations for load-carrying cruciform joints. In this paper, the fatigue initiation points were studied firstly considering material heterogeneity and geometry factors. Subsequently, the LCF-HCF life of load-carrying cruciform joints was investigated by traditional S-N curves and effective notch strain method.

Nomenclature LCF

Low cycle fatigue

HCF High cycle fatigue LCWJ Load-carrying Cruciform welded joints

Fig. 1. Overview of the fatigue test program for load-carrying cruciform joints.

2. Experimental procedures The 10CrNi3MoV high strength steel was used to fabricate load-carrying cruciform joints.Fig. 1 illustrates the procedure of specimens processing and low cycle fatigue tests. The 500mm welded plates were cut up into CLWJ specimens of 25mm width by wire-electrode method. The loading plates for low cycle fatigue were driven by INSTRON hydraulic cylinders with a maximum force of 250 kN, as shown in Fig.1(c). In details, the displacement