Tham khảo tài liệu 'metal fatigue part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 134 Chapter 7 Table Size of nonmetallic inclusions at fracture origins 13 Diameter of inclusions rm Percentage 0-10 9 11-20 24 21-30 27 31-40 18 41-50 11 51-60 7 61-70 4 Figure Comparison between the inclusion size distributions obtained by optical microscopy of sections and those obtained by observation of fracture surfaces in rotating bending fatigue tests 27 a Inclusion size distributions obtained by optical microscopy of sections b Size distributions obtained by observation of fracture surfaces in rotating bending fatigue tests. Fig. 27 compares the size distribution of inclusions for bearing steels produced by processes A and B with the diameters of inclusions oxides appearing at fatigue fracture origins. The scale of size distribution in Fig. is logarithmic hence steel A contains more small inclusions than steel B. The inclusion rating by the ASTM B scale which is for steel A and for steel B indicates that the cleanliness of steel B is expected to be higher than that of A. However the sizes of inclusions appearing at fracture origins of steel B are larger than those of steel A and the fatigue limit is 830 Bearing Steels 135 Figure a 16 small inclusions b 4 large inclusions whose total volume is identical to a . MPa for steel A and 750 MPa for steel 3 Thus Fig. indicates that steels containing larger inclusions have lower fatigue strength regardless of high cleanliness measured by conventional inclusion rating methods. Thus conventional cleanliness ratings are not necessarily rational criteria for clean steels. Monnot et al. pointed out that conventional cleanliness ratings have no scientific or engineering rationale as scales relating to fatigue strength. Since the crucial inclusion which causes fatigue fracture of a specimen is the one present at the centre of a fish eye it is most important to predict the size of such an inclusion. Based on the fatigue mechanism described in Chapter 6 such a crucial inclusion should be .
