130 3 Fatigue Fracture z=fx),<Z>=0 Figure 3.1 Graphical definition of basic extreme vertical parameters:the maximum hight Rp,the maximum depth Re and the vertical range R=.For comparison,the arithmetic roughness Ra is also depicted.The grey area marks the first element of the profile Length Parameters Length parameters describe the distribution of specific altitudinal levels of the surface in the horizontal plane z-y.These parameters are particularly useful for quality control in manufacturing processes.Their application in the fractography is rather rare.The most commonly used length parameters are,for example,the average spacing of profile elements on the mean line, Sm,the number of profile intersections with the mean line no or the number of peaks per the length unit mo. By evaluation of the parameter mo,for example,the peak is counted only when its horizontal distance from the previously counted peak is higher than of the vertical range R[251].Consequently,the relationR must be fulfilled,where r is the z-coordinate of the i-th peak and is that of the previously counted peak. Hybrid Parameters Hybrid parameters can be understood as a combination of altitudinal and length characteristics 250,252.In quantitative fractography,the linear roughness RL and the area roughness RA have been used for some time. These dimensionless characteristics,sometimes respectively called the rela- tive profile length and the relative surface area,are defined as L R= S RA=130 3 Fatigue Fracture Figure 3.1 Graphical definition of basic extreme vertical parameters: the maximum hight Rp, the maximum depth Rv and the vertical range Rz. For comparison, the arithmetic roughness Ra is also depicted. The grey area marks the first element of the profile Length Parameters Length parameters describe the distribution of specific altitudinal levels of the surface in the horizontal plane x–y. These parameters are particularly useful for quality control in manufacturing processes. Their application in the fractography is rather rare. The most commonly used length parameters are, for example, the average spacing of profile elements on the mean line, Sm, the number of profile intersections with the mean line n0 or the number of peaks per the length unit m0. By evaluation of the parameter m0, for example, the peak is counted only when its horizontal distance from the previously counted peak is higher than 1 10 of the vertical range Rz [251]. Consequently, the relation xp i −xp i−1 > 1 10Rz must be fulfilled, where xp i is the x-coordinate of the i-th peak and xp i−1 is that of the previously counted peak. Hybrid Parameters Hybrid parameters can be understood as a combination of altitudinal and length characteristics [250, 252]. In quantitative fractography, the linear roughness RL and the area roughness RA have been used for some time. These dimensionless characteristics, sometimes respectively called the rela￾tive profile length and the relative surface area, are defined as RL = L L , RA = S S