45 60 75 90 0 100 200 300 400 500 Angle of intrusion, α (deg) Logarithmic growth rate of intrusion, b (nm) σa=173 MPa (Material A) σa=110 MPa (Material B) σa=115 MPa (Material B) Figure 7: Growth rate of intrusion as a function of intrusion angle. 0.5 1.0 1.5 0 100 200 300 400 500 600 σa=110MPa σa=115MPa 49 73 72 76 75 58 83 67 77 90 78 85 60 86o Length of slip line, l (mm) Logarithmic growth rate of intrusion, b (nm) o o o o o o o o o o o o o Figure 8: Growth rate of intrusion as a function of slip-band length. because the intrusion depth increased linearly with the logarithm of the number of cycles like Fig. 3. Therefore, the location and remaining life of fatigue crack initiation can be predicted by measuring the intrusion depth few times before the crack initiation. CONCLUSIONS The fatigue slip-band formation and the fatigue crack-initiation process in 70-30 brass were observed by means of AFM, and the following results were obtained. (1) The depth of an intrusion drastically increased with its outgrowth to a crack, and with coalescence of cracks, the width of cracks increased rapidly. (2) For the transgranular crack initiation, the intrusion depth at the crack initiation depended on the slip-band angle relative to the stress axis. At crack initiation, the slip distance in the slip direction, however, was constant independent of the slip-band angle, the stress amplitude, the mean stress, and the grain size. (3) Intergranular cracks were formed along grain-boundaries between highly deformed grains and grains without activating slip systems. For the intergranular crack initiation, the value of the intrusion depth (the grain boundary depth) at the crack initiation was not a unique function of the grain boundary angle relative to the stress axis. REFERENCES 1. Tanaka, K. (1987). JSME Int. J., 30, 1. 2. Nakai, Y., Hiwa, C., Imanishi, T., and Hashimoto, A. (1999). Proc. Asian-Pacific Conf. on Fracture and Strength ’99, SM22 (CD-ROM). 3. Nakai, Y., Fukuhara, S., and Ohnishi, K. (1997). Int. J. Fatigue, S223. 4. Nakai,Y., Ohnishi, K., and Kusukawa, T. (1999). Trans. Jpn Soc. Mech. Eng., 65A, 483. 5. Nakai, Y., Ohnishi, K., and Kusukawa, T. (1999). In: Small Fatigue Cracks: Mechanics and Mechanisms, pp. 343-352, Ravichandran, K. S., Ritchie, R. O., and Murakami, Y. (Eds). Elsevier, Oxford. 6. Nakai, Y., and Kusukawa, T. (2001). Trans. Jpn Soc. Mech. Eng., 67A, 476. 7. Tanaka, K., Nakai, Y., and Maekawa, O. (1982). J. Mat. Sci., Jpn, 31, 376. 8. Tanaka, K., Hojo, M., and Nakai, Y. (1983). In: Fatigue Mechanisms: Advances in Quantitative Measurement of Physical Damage, pp.207-232, Lankford, J., Davidson, D. L., Morris, W. L., and Wei, R. P. (Eds). ASTM STP 811.
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