5

crack size, mm

Figure 17.5. Calculated crack growth curves (tables 17.1-17.3)

Figure 17.5. Calculated crack growth curves (tables 17.1-17.3)

point a = 0. There is no retardation in this first cycle (</> = 1) and the "retarded" da/dN is equal to the linear da/diV. The crack extension is Aa= 0.00245 mm, so that the new crack size is 5.00245 mm.

At the new crack size, the next cycles at 12 kg/mm2 causes a plastic zone of 0 048. The extent of this current zone size is 5.00245 + 0.048 = 5.051 mm from the point a = 0. The extent of the largest previous plastic zone was 5.061 mm. This means that there will.be retardation, because the current plastic zone is completely contained in a previous plastic zone. The current plastic zone extends 0.048 from the current crack tip, whereas the previous plastic zone extends 0.059 mm from the current crack. Thus, the retardation factor is (48/59)' 3 = 0.76. The retarded growth rate is therefore 0.76 times the linear growth rate.

Continuing through the rest of the block, it appears that all later plastic zones are contained in the plastic zone of the first cycle. Due to continuing crack growth, the border of the overload plastic zone is almost reached when the last stress level is applied. At that time, the largest previous plastic zone extends to 5.061 mm, the current plastic zone to 5.061 mm. After 312 cycles of retarded growth, the crack has grown over 0.001 mm. This means that the current plastic zone has reached the border of the overload plastic zone. Therefore, the remaining 1688 cycles of the lower level will not be retarded. They appear separately in table 17.3.

During the first cycle of the second block, a large plastic zone is formed again. Retardation occurs during the 3 subsequent levels, but after 774 cycles of 5.8 kg/mm2 the border of current and previous plastic zone coincide. Thus, the remaining 126 cycles of this level are not retarded

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