Study On The Multiaxial Fatigue Strength Of Welded Structures Based On Critical Plane Method

Fatigue failure is one of the main failure modes of actual engineering structures. The components on ship and marine structures are subjected to multiaxial loading during working, so research on multiaxial fatigue is very significant for fatigue strength evaluation of ship and marine structures.. Compared with uniaxial fatigue, multiaxial fatigue is more complicated both on mechanical analysis and physical mechanics. So far, the multiaxial fatigue has been considered to be a hot and difficult problem in the field of fatigue research.First of all, based on the critical plane method, a new approach to predict multiaxial fatigue life called MWCM (Modified Wohler Curve Method) is proposed, and its principle and assessment process are illustrated in detail. Then this method is successfully applied to predict multiaxial fatigue life for welded joints.Secondly,3D FEM models of cruciform joint and T-joint are established to predict fatigue life under both uniaxial and multiaxial loads which include proportional and non-proportional loads. Load separation method is adapted to deal with the multiaxial non-proportional loads. The accuracy of shear stress on the plane experienced the maximum shear stress range under a kind of non-proportional load condition is believed totally depends on the dispersion degree of its loading path. A program of spline interpolation is used to get the accurate results of fatigue damage parameters on the plane called critical plane which experiences the maximum shear stress range. This method reduces the influence of dispersion degree of its loading path, weakens the man-made factors, and thus it makes the results more accurate.Finally, this paper adapted two methods to extract the stress of the fatigue assessment point. One is critical distance method, the other is hot stress method based on non-linear peak stress self-balance theory. With the combination of the theory of critical distance method and MWCM, the multiaxial fatigue life of cruciform joint is predicted under six kinds of multiaxial loading paths. Besides, with the combination of a new hot spot stress method and MWCM, the multiaxial fatigue life of T-joint is predicted under conditions often common multiaxial loading paths which include uniaxial tension and compression, torsion,45°and 90°non-proportional elliptic paths,90°non-proportional circular paths, etc.