表面质量对疲劳强度的影响(英语).ppt

1、Chen Junwei, Group 3 May 12 ,2011,Influence of Surface Quality on Fatigue Strength,The fatigue life of a machined part depends strongly on its surface condition. And the effect of residual stress on fatigue life is more pronounced than that of surface roughness. Some defects of surface quality have

2、a considerable effect on fatigue strength, such as strain hardening and residual compressive stresses . Stress concentration occurs easily at the troughs of surface roughness ,especially the groove and fillet corner ,which can lead to fatigue crack and accelerate the fatigue failure .,A brief introd

3、uction,Material and experimental set-up,The test specimens were machined by turning in wet condition. Residual stresses were measured by means of the blind hole drilling method.,Surface roughness,The surface roughness of the machined part is largely determined by the feed and tool nose radius. A lar

4、ge feed will give shorter cutting times but a poor surface quality. A large nose radius will generate a better surface quality. But an excessively large nose radius can lead to vibration tendencies, unsatisfactory chip breaking and shorter tool-life. A theoretical model for surface roughness is appr

5、oximated by the following equation: where f is feed rate (mm/r) and r is the tool nose radius(mm).,The Figure illustrates the effect of the nose radius on the surface roughness for the cutting condition employed for the fatigue test (f = 0.2, r = 0.2, 0.4, 0.8). The results present that the value of

6、 Rmax parameter increases with the decreasing of nose radius .,Residual stress,After the turning operation, the residual stresses were measured using the blind hole drilling method. The machining of round samples was carried out with different feed rates and nose radius .,These data are measured by

7、the blind hole drilling method. It shows residual stress increases with the depth .,We can see that an increase of feed rate causes an increase of compressive stresses in both directions. On the other hand, an increase of the nose radius of the insert causes a decrease of the compressive residual st

8、resses.,In the rotating bending fatigue tests, residual rotation speed was 3800 rev/min. When the number of rotation exceeded 107 , the test was stopped, with the test specimen not break under that condition. Although the surface roughness of specimens with re = 0.2 is higher than that with r= 0.4 a

9、nd 0.8, which shows the former has a higher fatigue life than others .That means a higher compressive residual stress causes a higher fatigue life and the effect of residual stress on fatigue life is more than the effect of surface roughness.,Finite element analysis,A 2*2 cm2 model of a forged Alumi

10、num component,This is just a model we abstract from the real component , and may not represent the actual surface. However , we still believe finite element analysis can be a valuable tool when studying the cause and effect of surface quality and fatigue strength . Common methods employing reduction

11、 factors and roughness parameters are inaccurate, leading to large safety factors and overly conservative designs. So with the new method proposed, we can solve these problems easily .,Summary,Fatigue cracks usually initiate at the surface in materials, thus the surface state has a high influence on

12、 fatigue life. The most important surface parameters are surface roughness, residual stress, and microstructure. The residual stress become more compressive as the feed rate increases. On the other hand, an increase of the nose radius of the insert causes a decrease of the compressive residual stresses. An increase of c