Metal Fatigue Part 7

Tham khảo tài liệu 'metal fatigue part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 164 Chapter 8 106 5X105 -2X105 IO5 -5X10 -2x10 - 10 - 5X10 -2X103 103 5X102 12 9 14 13 11 10 99 98 Figure Cumulative frequency distribution of t areamaK for inclusions in steel SUP12. standard inspection area So mm2. 1 E a Ẹ y -In lnF F l-1 T T S So N 1 o8 some fatigue tests were carried out using specimens oiled with a machine oil every 3 X 107 cycles. The nominal sưess ƠQ was calculated for the minimum section of the hourglass specimen Fig. . The Vickers hardness Hy was measured before fatigue testing at 8 surface points using a force of N and the mean value was taken as the representative value. The hardness after fatigue testing was taken as the average value measured at three points near a fatigue fracture origin using a force of N. Fig. is the extreme value statistics plot see Appendix C of the maximum inclusion size -J area present in a standard inspection area So mm2. The distribution of y areamM in Fig. shows good linearity and may be regarded as obeying the distribution of extremes. There is no inclusion having yJareamM 10 pirn in 40 inspection areas indicating that this material is a very clean spring steel. However it does not follow that the maximum inclusion contained in one specimen is smaller than 10 . It implies rather that there is the possibility of the presence of quite large inclusions in the total volume of one specimen or of many specimens. The stress a at the fracture origins of all the fractured specimens obeyed the relationship ơ ơb- Therefore the control volume which may be regarded as containing the fracture origin was calculated as the part subjected to a stress greater than 95 of the applied nominal stress ƠQ at the minimum section. Thus the control volume calculated by considering the specimen curvature is mm3. Table shows the control surface 51 mm2 return period T S So the estimated maximum value of y areamm for the

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