PSI - Issue 28

Wei Song et al. / Procedia Structural Integrity 28 (2020) 200–207 Author name / Structural Integrity Procedia 00 (20 0) 0 0 – 00

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Fig. 1. Geometries of notch specimens for 10CrNi3MoV high strength steel and its undermatched welds. (a) Notch specimens; (b) Geometry of notch specimens for base metal; (c) Geometry of notch specimens for undermatched welds; (d) Details of notch specimens for different radius. 3. Experimental results 3.1. Multiaxial fatigue tests data Fatigue test data for base metal and undermatched weldments under fully reversed in-phase proportional and 90  non-proportional multiaxial loading is displayed in Fig. 2. Specifically, Fig. 2(a)-(b) show comparisons of N f under different notch radius. specimens under multiaxial fatigue loading. The fatigue lives are affected by the multiaxial loading modes, which are reduced due to the existing of non-proportional loading. Fig. 2(c) summaries the fatigue life data of base metal for different notches and loading conditions. Regarding to the effect of notch radius, the results show slight influence on fatigue life 90  non-proportional multiaxial loading for base metal, while it demonstrates obvious discrepancy under in-phase proportional loading. Again, for the undermatched weldments, it is clear that the fatigue life under 90  out-of-phase fatigue loading has negligible influence of specimen notch radius. Finally, the fatigue data of base metal and undermatched welds are summarized in Fig. 2(d). 3.2. Notch gradient analysis and calculation procedure Notch gradient of fatigue characteristic parameters around the notch root for multiaxial low cycle fatigue depends on the notch geometry, loading conditions, and material properties. Therefore, it should explicitly evaluate the specific stress, strain or energy parameters by notch mechanics theory. In our study, the notch gradient concept was introduced to analyze the model mechanical parameters considering the notch effect. The energy gradient is a significant part of evaluating fatigue strength of notch parts, which can be referenced the function of the relative stress gradient  with distance from the notch tip. The illustration of the notch gradient is expressed as follows, and is shown in Fig. 3: (1) where x=0 is the coordinate point in the structure/component with maximum stress value. According to the definition of K t , the energy gradient mainly evaluates the effect of peak values at the notch root by taking into account of material elastic-plastic material properties. Furthermore, the gradient parameter can account for notch effect within the damage zone by combining with a weight function. In order to overcome some shortcomings of some criteria, a new damage parameter on the bases of the SWT criterion by the critical plane approach is proposed, which incorporates the stress concentration factor and loading non-proportional effect. The base fatigue material properties about base metal and undermatched welds obtained from previous investigations were used as initial input conditions. The multiaxial fatigue analysis for notch specimens were conducted considering the geometric variations in the ABAQUS and MATMAB software. The final output results of ( ) ( dx   ) , 0 1 ( ) r ,   0 r r   = =  =