Experimental Study And Modeling Of Fatigue Crack Growth Profile And Cracking Path For 16MnR Steel

Many pressure vessels and pressure equipments used in the petrochemical, power generation and aircraft industries usually undergo complex multiaxial cyclic loading. It is very easy to produce the fatigue crack growth and result in catastrophic accidents in many engineering structures. Thus the systematical studies on the fatigue crack growth and the prediction approach of crack growth behaviour have not only the important significance, but also a practical value.In the dissertation, the method combined with the experiment and theory prediction is adopted. Lots of fatigue crack growth tests are conducted, and the new approach is used to predict the crack growth behaviour under mixed mode I/II step loading according to the Jiang multi-axial fatigue damage criterion.The fatigue crack growth tests are conducted by using different thichness notched compact specimens of 16MnR steel under mode I constant amplitude loading and mode I constant amplitude loading with a single tensile overload. The crack growth profiles for three kinds of thichness specimens are obtained by using the marking method. The results show that the fatigue crack is extended in terms of the ellipse curve crack profile under mode I constant amplitude loading, namely "crack tunneling" behaviour. Fatigue crack initiation life at the mid-plane is shorter than that of at the surface, the similar ellipse curve crack profiles are obtained in the stable growth stage, thus the crack growth rate of the mid-plane (plane strain condition) is similar to that of on the surface (plane stress condition). The subsequent crack growth rate is retarded significantly by the application of a single tensile overload. Delayed retardation and retardation stages are experienced. The fatigue crack profiles in the retardation zones are agreement with that of the stable crack growth for the thin specimen. There is a big difference between the "Omega" crack profiles in the retardation zones and that in the stable crack growth for the thick specimen. Based on the finite element analysis and the multi-axial fatigue damage, the three-dimensional characteristics of crack initiation and the evolution of crack profile under mode I constant amplitude loading is explained reasonably.Under mixed mode I/II step loading, the mixed mode I/II fatigue crack growth are experienced for the notched compact tension and shear specimen, and the crack growth path are changed at any moment. Particularly, when the loading direction is switched to a certain angle, the crack fronts in the RDZ are affected by the previous loading condition. The big change is happened for the crack growth path and the crack growth rate is retarded greatly at the moment. The crack growth directions have a tendency perpendicular to or spanning the current loading axis. The mixed mode load ratios are calculated by the linear finite element stress analysis, so the transient of mixed mode I/II fatigue crack growth path are explained reasonably.Combined Jiang-Sehitoglu cyclic plasticity model with Jiang multi-axial fatigue damage criterion, a model unifying the fatigue nucleation and crack growth based on the continuous accumulated damage is adopted. Mixed mode I/II fatigue crack growth behaviour for 16MnR steel are predicted, and the gradual change for the crack growth direction are also predicted accurately. Jiang-Sehitoglu cyclic plasticity model is used to describe the cyclic plasticity behaviour of 16MnR steel. Elastic-plastic stress and strain responses near the crack tip are evaluated accurately. On the basis of these stress and strain values, fatigue propagation accumulation damage and residual fatigue damage near the crack tip are well evaluated according to the Jiang multi-axial fatigue damage criterion. The predictions for the crack growth behavior including the crack growth rate and crack growth path are in agreement with the experimental data.