PSI - Issue 28

Wei Song et al. / Procedia Structural Integrity 28 (2020) 200–207 Author name / Structural Integrity Procedia 00 (2020) 000 – 000

201

2

reduce the loading capability and induce the low cycle fatigue failure of the localized plastic deformation area under cyclic loading. Notch effect in the structural fatigue analysis can exert local high stress concentration in engineering components. However, it is inaccurate to apply the maximum stress or strain around the notch area for multiaxial fatigue evaluations in engineering components with high stress concentrations, and often provide over-conservative life design or assessment in a low cycle fatigue (LCF) regime [1]. Multiaxial low cycle fatigue of notch components is relative complex compared with smooth specimens in high cycle fatigue regime, which needs to consider the variations of material properties, geometries and service loads. The fatigue characteristics of local locations is essential for notch multiaxial low cycle fatigue assessment. Various fatigue criteria have been proposed for multiaxial fatigue assessment of smooth specimens by Fatemi-Soice [2], on the basis of critical plane or specific fatigue characteristics, which have been utilized to analyzed the experimental data. Regarding to the notch multiaxial fatigue, different methods have been developed to illustrate the notch effect on fatigue characteristics under different loading conditions by combination of critical plane criteria or specific parameters, such as local stress, strain and energy. To provide a quantitative description for stress concentration severity of notched components, the concept of theoretical stress concentration factor K t is often incorporated into the multiaxial fatigue damage parameters. However, it is widely accepted that the use of K t yields nonconservative predictions during fatigue lifetime evaluation of ductile materials or structures with sharp notches [3]. On the other hand, the critical plane method is used to assess the multiaxial fatigue by combining the notch effect, which orientation may be connected with the principal stress directions by using both suitable weight functions and an off-angle, the latter depending on the shear-to-normal stress fatigue ratio [4]. In addition, the critical plane approach is f urther elaborated to consider the influence of stress/strain gradient on the fatigue strength. Particularly, specific critical plane-based models by combining with energy-based criteria have recently been proposed for notched structural components. Ellyin investigated the multiaxial fatigue by this approach based on a combination of both plastic and elastic Strain Energy Density (SED) [5]. Park and Nelson corrected the deviatoric strain energy density evaluated at the notch tip to assess the multiaxial fatigue behavior of blunt notch specimens [6]. A deviatoric interpretation of the Neuber’s rule and the SWT parameter has been presented by Kujawski in Refs [ 7], where the criterion is successfully applied to multiaxial fatigue assessment of different metallic materials. Moreover, Andrea Carpinteri conducted the multiaxial high cycle fatigue assessment of notch specimens by a strain-based multiaxial fatigue criterion by the extension from unnotched specimens [8]. To fully quantify the notch gradient distribution of damage factors, such as the local stress, strain or both, the multiaxial fatigue damage model by the combining the modified SWT model and energy concentration factor is proposed for notch fatigue assessment. Particularly, the fatigue tests of severely notched specimens under multiaxial low cycle loading is carried out by comparisons of the proposed model and other models. Experimental data of 10CrNi3MoV steel and its undermatched weldment notched specimens under different loading conditions are utilized for model validation and comparison. Nomenclature LCF Low cycle fatigue SWT Smith-Watson-Topper SED Strain energy density 2. Experimental procedures The multiaxial notch bar specimens were fabricated by 10CrNi3MoV high strength steel and its undermatched welds, and the geometry details were presented in Fig. 1. The notch specimens were manufactured from the welded plate 16mm width by wire-electrode method. The multiaxial low cycle loadings were driven under different force configurations by MTS 809 hydraulic cylinders with a maximum force of 250 kN. All tests were conducted under force-controlled loadings at a frequency from 5 to 10 Hz. It should be noted that two multiaxial loading paths, in phase proportional and 90  non-proportional loadings, were carried on for all notch specimens.