Study On Low-Cycle Biaxial Fatigue At Butt Joint Of HTS-A Steel

In the engineering, welded structures usually bear complicated multiaxial cyclic loading, and weld zone is the hot spot of fatigue failure. Consequently it is full of valuable significance to study the multiaxial fatigue of the welded structures under complicated stress or strain state. The welded joint of the cone-cylinder is a hot fatigue spot of submarine, which bear longitudinal bending, longitudinal compressing and vertical compressing. Combining with actual complicated stress situation and discarding the longitudinal compressing load, the paper studies the biaxial fatigue life at butt joint of HTS-A steel under longitudinal cyclic bending and vertical cyclic compressing load. The paper decomposes fatigue life into the engineering crack-initiation life and the crack-propagation life, and studies fatigue life of butt joints of HTS-A steel from experimental and theoretical analysis.Adopt 35mm thick butt joints of HTS-A steel to carry on axial and biaxial fatigue test. Axial loading is longitudinal bending, and biaxial loading are longitudinal bending and vertical compressing. MTS structural test and analysis system and facility designed by ourselves can carry out axial fatigue test and multiaxial fatigue test. Because of the complicated facility, there is no report on the test, and it is first experimental study. "Reagent observed method" and "rust mark method" are proposed, which is scanning crack-initiation and recording crack propagation during test. They make certain betterment to the traditional methods, so they have more practicability. The fracture form and fatigue life of test specimens is obtained under axial and biaxial loading. It is shown that the engineering crack-initiation life can't be negligible. Vertical compressive cyclic loading shortens the fatigue life, and accelerates the fatigue failure of test specimens. Besides, the vertical compressive cyclic loading has greater influence on crack-initiation life than the crack-growth life. With the ratio of vertical compressing stress to longitudinal bending stress growing, the ratio of the crack-propagation life to total life is larger and larger, but the ratio of the crack-initiation life to total life is opposition.Estimating crack-initiation life adopts ANSYS to calculate the axial stress concentration factors, and adopts multiaxial Neuber's law to calculate local stress-strain and the cyclic stress-strain amplitude under biaxial loading state. Then combine with different strain-life model, and consider the influence of the mean stress and the residual stress. Contrast with the test result, it is thought that the critical plane theory is well used for estimating the crack-initiation life of butt joint under longitudinal cyclic bending and vertical cyclic compressing. Besides, it is essential to consider the influence of the mean stress and the residual stress while estimating the crack-initiation life of welded joint.Fracture forms of biaxial fatigue test are mixed-mode and predominated by ModeⅡ, therefore a new equalΔK is proposed based on maximum shear strain and the normal strain to maximum shear strain plane. The equalΔK considers predominated effect of maximum shear strain and side effect of normal strain to maximum shear strain plane. It has certain physical significance. Estimating crack-propagation life adopts new equalΔK combined with Paris relationship of axial bending fatigue. The estimated result is ideal while taking suitable material parameter. And along with biaxial stress ratio changing, its trend is also close with test. As a whole, it shows that the method have certain practicability.