Tham khảo tài liệu 'metal fatigue part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 314 Chapter 16 Axial Direction L 100 xm I pm Figure a Non-propagating crack observed at the notch root of a specimen with artificial surface roughness 200A. ơa 226 MPa. b A higher magnification view of the area indicated by arrow B in a . 300 S45C Annealed 250 o 200- w w 2 Ỡ5 ofo E p Notch 100A 150A 200A HV 170 10 Surface Roughness HV 180 10 105 106 107 Number of cycles Nf 108 Figure S-N curve annealed specimens. Effect of Surface Roughness on Fatigue Strength 315 Figure Notches and their equivalent cracks. X - aP Jo V--y y I ơn F K ff aJTFa 0 1 a 2b Figure Stress intensity factor for periodic surface cracks. as a crack problem rather than as a notch problem. Murakami et al. 10 applied the ffarea parameter model to the problem of periodic surface notches simulating surface roughness. In this chapter the same evaluation method Eq. is applied to surface roughness with irregular depth. Evaluation of Equivalent Defect Size for Roughness JareaR The initial value of ựứr ứR of a defect is the crucial geometrical parameter that controls the fatigue limit. For a single shallow circumferential notch ffarea is given by the following equation -Jarea ựĩõ X a where a depth of notch . Murakami et al. 10 proposed an evaluation method for the value of ffarea for periodic notches on the assumption that a periodic roughness notch is equivalent to periodic cracks as shown in Fig. . The method of evaluation of Jarea for periodic notches follows. Fig. shows the stress intensity factor Ki for periodic surface cracks in a semi-infinite body 13 . The term F in Fig. is a geometric correction factor which depends upon the depth and pitch of cracks and is defined by the following equation Kị Fatíự When the depth a is kept constant and the pitch 2b is decreased Ki decreases due to the effect of interference between cracks. The maximum value of stress intensity factor along a surface .
