## Part Functionality as Effected by Surface Topography
### Fatigue Strength and Wear Resistance as Effected by Surface Topography  
Milling process is generally applied to conduct finishing machining of sculptured surfaces, therefore the machined component have very demanding specifications in surface topography for its performance. The smoothest surfaces are desired in most milling processes, especially when the fatigue life of the part being machined is high [38]. However, in some biomedical fields, it is desirable to have rough surface morphology [39]. The surface structure is responsible for the mechanical functionality of the component. Sometimes, even if the surface dimension and surface finish of a component are well within the tolerance, there remains the possibility of lack of surface quality for a milled surface. The reason is surface topography influences not only the mechanical and physical properties of contacting parts, but also optical and coating properties of some no contacting components. Because of the stress concentration caused by pits and groove, the characteristics of milling morphology have important influence on its performance. Generally, rougher surfaces are expected to encourage fatigue crack initiation. It is suggested that parameters such as Rt and Rz are more appropriate than Ra in respect of fatigue strength, as they equate with adverse component surface features [38]. [PERSON] et al. [40] used surface roughness to estimate the effective stress concentration coefficient of high strength low alloy steel workpiece. It was found that the fatigue strength decreased with the increase of surface roughness at low stress level. [PERSON] et al. studied the factors that affect the fatigue life of titanium alloys. They found that the surface roughness does not affect the fatigue life of the workpiece [41]. [PERSON] et al. found that the surface roughness has a slightly influence on four-point bending fatigue life for end milling process of AISI H13 [42]. [PERSON] et al. [43] found that surface roughness has little influence on fatigue life due to the small ratio between the ten-point surface height  
Figure 4: Three kinds of tearing surface [34]. **(a)** Microvoids and scratches; **(b)** microvoids and scratches; and **(c)** groove and pitting corrosion. and the bottom curvature radius of surface gaps. [PERSON] et al. [44] found that high surface roughness leads to a high stress concentration coefficient, which reduces the fatigue life of the workpiece. The machined surface morphology affects the fatigue performance of the final assembly, especially when the crack initiation life is noteworthy [45; 46]. Therefore, many works have been done to explore the effect of surface topography on part performance. The traditional two-dimensional surface roughness does not adequately characterize the effect of surface properties on the fatigue performance, while the three-dimensional topography can provide a more accurate correlation with fatigue behavior [47; 48; 49]. Performance and reliability of engineering components can be enhanced by selecting appropriate 3D topographic characteristics [50]. [PERSON] et al. investigated the effected of surface and subsurface condition on the fatigue life of Ti alloy workpiece. Their research results indicated that the surface topography, in particular texture direction, showed a strong correlation with the fatigue life of workpiece [51]. [PERSON] et al. [52] studied the effect of progressive milling process on the surface morphology and fatigue properties of 7475-T7351 high-strength aluminum alloy. The results show that the surface topography is not the right factor affecting the fatigue properties of aluminum alloys. When the roughness level is the same, the milling surface has a higher fatigue limit than the grinding surface. The reason is that the surface grooves produced by milling process are more random [53]. [PERSON] et al. [54] studied the effect of different tool paths on the fatigue life of AISI H13 high speed milling process.