Analysis And Experimental Investigation Of Fatigue Stress Concentration Factor Of Milled Surface

The structure and motion of dynamic positioning rotary propeller suffered cyclic load of wind,wave and ocean current were complex. The whole machine suffered the thrust of nearly 80 tons, even eight sets of equipment operated simultaneously, each blade would suffer load of 10 tons. Hostile service conditions put forword the high requirements of service performance. One of the most important processes of propeller processing is milling. There will be an affected layer after milling.The affected layer is characterized by sets of surface integrity. The parameters of surface integrity are consisted of surface roughness, residual stress, hardness,metallography and so on. Inhomogeneity of these characteristic parameters will result in the stress concentration under the cycle load. Generally, the stress concentration will lead to the initiation of cracks.To improve the service life of copmponents,we need to investigate the characteristic parameters of milled surface,the effect of process parameters on surface integrity and the effect of surface integrity parameters on fatigue performance.The research may encounter the following questions:(1)On account of the big size and irregular surface of the propeller,it will be difficult to measure the parameters of surface integrity.(2)There are so many parameters of surface integrity,but many researchers described the effects of single parameter of surface integrity on fatigue performance qualitatively.There are few parameters to evaluate fatigue performance exclusively.Tasked with setting up evaluation index of fatigue performance of milled components.The parameters of surface topography of the propeller after milling were analyzed.Through the comparison for the parameters from propeller and the sample after milling,verifying the feasibility of using the surface of sample to simulate the milled surface of the propeller.Baesd on the principle of elastic-plastic fracture machanics,using the fatigue stress concentration factor to characterize the effects of surface topography,residual stress and material property on fatigue performance,and establishing the evaluation model;Using the ellipse notch to simulate the 2D profile of machined surface by FEM,the effects of surface topography and residual stress on stress concentration were studied.A series of milling experiments were conducted to comprehensively characterize surface integrity at various milling conditions,the effects of milling parameters on surface integrity were summarized; The fatigue performance of milled surface was evaluated by fatigue stress concentration factor,and the results of evaluation were verified by three points bending fatigue test.On the one hand, fatigue stress concentration factor will provide a reference for anti-fatigue design and the selection of process parameters.On the other hand,it can characterize the effect of machined surface quality on fatigue performance.It will act as a bridge to link the designing,manufacturing and evaluation of service performance.