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Metal Fatigue Part 3

Tham khảo tài liệu 'metal fatigue part 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 44 Chapter 4 Figure 4.7 Non-propagating cracks at an artificial hole annealed medium carbon steel diameter of hole 1 100 pm depth of hole h 200 pm ơ 191 MPa . the characteristic dimension for the evaluation of the effects of defects of various sizes and shapes on fatigue strength as in Tables 4.1 and 4.2. The author wishes to reaffirm here the point that this approach is based on two important findings. These are that the fatigue limit of materials containing defects is essentially a crack problem and that there is a very strong correlation between -Jarea and Ajnuix. This concept is illustrated in Fig. 4.8. The data shown in Tables 4.1 and 4.2 are discussed below from this point of view. Figs. 4.9 and 4.10 show the relationships between the rotating bending fatigue limits of 0.46 c steel and 0.13 c steel and the square root of defect projected area J area. The value of Jarea for the holes is calculated using J dh d- j3. This assumes that the flank angle at the bottom of a hole is 120 which is approximately true. As can be seen in Figs. 4.9 and 4.10 J area seems to be a relevant controlling parameter over a wide range of dimensions h d 0.5-2 d 40-500 p.111. Therefore we can anticipate that J area will also be useful for the prediction of fatigue strength in the presence of natural defects.2 Here il must be noted that the value of yjarea is defined as the square root of the initial defect projected area. The area of a newly produced crack at the initial defect is not added. As was pointed out on page 39. in rotating bending tests on 0.13 c steel specimens containing 12 artificial small holes either 40 pm or so pm in diameter various crack behaviours were observed at the fatigue limit. At some holes there were non-propagating cracks on opposite sides of a hole corner some holes had a crack on one side only and other holes had no cracks. Thus if we were to add in the area of newly initiated cracks when calculating dureu. then several different values of dorea would be