BULETINUL INSTITUTULUI POLITEHNIC DIN IAŞI - Universitatea ...

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Bul. Inst. Polit. Iaşi, t. LVI (LX), f. 2, 2010 201
1.
65
⎛ a ⎞ a
Q = 1+ 1,
464⎜
⎟ for ≤ 1.
⎝ c ⎠ c
Fig. 5 – Computation model for crack propagation.
As was previously mentioned, the mathematical simulation was done with
the program AFGROW, version 4.12.15 from 10.07.2008. The entrance data
were: the external diameter (Do): 1.20 m, the internal diameter (Di): 0.60 m, the
crack depth (A): 0.001 m, crack half length (C): 0.002 m
In conformity with the shaft disposition, for studying the crack propagation
there were taken into consideration two load situations with constant amplitude:
a) a bending loading with a symmetrical alternative cycle having σmax = - σmin
= 42 MPa, with an asymmetry coefficient of R = -1.
b) a composed load (bending plus elongation); considering that the static
elongation stress is superposed over the bending stress it result an approximate
pulsating cycle with σmax = 88,9 MPa, σmin = - 6,5 MPa , R = - 0,07 [1].
In order to introduce the material constants, characterizing the crack
propagation, the used steel was considered to be equivalent with the AISI 1020
for which the characteristic data are included in the data base of the AFGROW
program. So, the constants for the Walker law [5]: