Fatigue Crack Propagation Behavior Of High Strength Steel Welded Joint Under I-ⅡMixed-mode Loading

Hull and ship structures are extremely easy to generate fatigue failure, because they are bearing alternating stress of water pressure for a long term. Since the underwater environment is complex, the crack is usually not under a single type of mode I, II or III loading but under mixed mode loading. So it’s necessary to study characteristics of the fatigue crack propagation under mixed mode loading.High strength steel is widely used as a basic and important material for shipping development because of its high strength and good toughness. Base metals and welded joints for a certain type of steel which are suitable for shipping construction are carried on fatigue tests under I-II mixed mode loading. On the basis of the experimental data, stress intensity factor(SIF) KI and KII, crack propagation direction and rate are received to analyze differences between base metals and welds. Crack closure is always along with crack growth, which has effect on the driving force of crack growth. Details are as follows:SIF of the crack tip is obtained by the finite element method(FEM). The s tress extrapolation method using 0.1mm mesh and quad 8 of FEM is adopted, which can reduce calculation errors to get more accurate results and assure computational efficiency. The compact tension shear(CTS) specimens and composite load device by Richard are used. The propagation directions of the cracks for base metals and welds from test results follow the maximum stress criterion. The directions basically don’t change except that the cracks of welded joints meet the boundary of base metals and welds. When the crack grows KI increases while KII meets a sharp decrease, which is much less than KI. The crack growth rates of base metals are higher than those of welds. The rates are influenced by material properties and the microstructure. Martensite hinders crack propagation. The stable stage of the fatigue crack growth rate(FCGR) curve appears a turning point because the mechanism of the crack extension changes near the point. The fracture mechanism changes from single fatigue fracture mechanism to mixture of cleavage and fatigue fracture by analyzing the micro fracture with the scanning electron microscope(SEM).The equivalent SIF ΔKv under mixed mode loading is expressed with Tanaka and Richard formula. The curves of da/d N-ΔKv for different loading angles of base metals and welds don’t coincide. This shows the two formulas are not effective to describe the driving force of crack propagation. Crack closure exists in the process of crack propagation, making the real payload of crack extension change. Analyses found that the level of crack closure in base metals is low and less than that in welds. Considering the influence of the closure effect on crack growth driving force, it proposes an effective stress intensity factor ΔKeff = UΔK to characterize the real driving force of the crack propagation. However, it seems that ΔKeff is not good enough to express the real driving force from judging the da/d N-ΔKeff curves. So a new formula is proposed to describe the real equivalent SIF, which takes the loading angle, cracking angle and crack closure into account. It proves that the new formula well describes the driving force of the crack propagation.