Study On The Effect Of Machined Surface Topography On Structure Fatigue Behaviors

Generally,fatigue strength decreases dramatically with increasing surface roughness.To quantitatively evaluate the effect of machined surface topography on structure fatigue behaviors,investigation on stress concentration factors?SCFs?of surface topography,fatigue notch factors?FNFs?of machined surface topography,stress-driven surface instability and fatigue crack initiation was carried out.The main body of the works are as follows:1)A new method was proposed to build the finite element model?FEM?of complex surface topography.Based on linear elastic mechanics,the SCFs of surface topographies constructed by W-M fractal function were investigated under different cut-off frequencies.The relations between fractal parameters,profile moments and SCF were analyzed.Results show that the SCF of surface topography highly depends on the cut-off frequency of surface profile.The second profile moment can reflect the intensity of stress concentration induced by surface topography.2)The machined surface topography was considered as a superposition of numerous cosine waves by means of Fourier transform.The cut-off frequency was defined for 20machined surface topographies with different roughness grades.SCFs induced by machined surface topographies were calculated by FEM.Effective SCF was proposed to represent the intensity of stress concentration induced by machined surface topography.The effect of each roughness parameter of machined surface topography under defined cut-off frequency on effective SCF was analyzed.Results show that the slop parameters of surface topography play a more dominant role in determining the effective SCF than other roughness parameters.3)Based on the method of Airy stress function and the boundary perturbation approach with complex variable functions,an analytical solution with first order accuracy for the SCFs of slightly perturbed superposed surfaces was derived and validated by Digital Image Correlation experiment as well as finite element analysis?FEA?.It is found that the perturbations of stress components obey the superposition principle under the limitation that the surface topography is shallow.4)The proposed analytical expression for the SCFs of slightly perturbed superposed surfaces was applied to predict the SCFs of real machined surface topographies,and the prediction was validated by FEA.The comparisons of results between obtained from the expression and from FEA show that the proposed analytical expression is feasible in calculating the SCFs of real machined surface topographies.Based on the proposed analytical expression,a quantitative relation between the root-mean-square?RMS?value of SCFs and the RMS of the surface profile slop was established.5)Based on the analytical expressions for the stress distribution of slightly perturbed superposed surfaces under tension,analytical expressions to predict the FNFs of machined surface topography were derived by employing the point method?PM?and line method?LM?of the theory of critical distance?TCD?.Based on the PM of TCD,the high frequency cut-off of machined surface topography was defined.6)Based on the proposed analytical expressions to predict SCFs and FNFs of machined surface topographies,the effect of cut-off frequency on SCFs and FNFs of machined surface topography was analyzed.Results show that the maximum SCF and effective SCF of machined surface topography increase with increasing cut-off frequency,while the maximum FNF and effective FNF tend to converge when cut-off frequency is sufficiently high.7)The proposed analytical expressions were applied to predict the SCFs and FNFs of three machined surface topographies with varying roughness grade,and the prediction was also validated by FEA.Predictions for SCFs using the analytical solution are within 10%of that by FEA.The difference between FNFs based on PM from the analytical solution and that by FEA are within 15%,while the difference between FNFs based on LM from the analytical solution and that by FEA are within 10%.8)Considering metals surface diffusion,cyclic stress-assisted surface evolution under high temperature were quantitatively evaluated from linear stability analysis and FEA.Results from linear stability analysis show that frequency components vanish away during surface evolution when their frequencies are higher than the first critical frequencyf_cr.The amplitude of frequency components increases during surface evolution if their frequencies are lower thanf_cr.Frequency components evolution mainly occurs around the most unstable frequencyf_m.Stress-driven surface evolution and fatigue crack initiation process were simulated by using MATLAB scripts regarding surface diffusion and ABAQUS software.Results show that the local notch evolves from blunt to sharp,and finally become a cusp notch,which indicates fatigue crack initiation.