Crack Paths 2006
From the above figures 6 and 7 it appears that at the first crack stage the crack growth
proceeds according to modes I and II. Next, the material is subjected to plastification
and the crack growth stops. After a break, the fatigue crack growth rate proceeds only
according to modeI for M = 45q and 60q. The greater the slot inclination angle M is, the
longer time of the crack growth according to mode I + II is. In Figs. 6 and 7, the rate of
fatigue crack growth for M = 60q is higher than for M = 45q in the FeP04 steel.
Moreover, in the mixed mode I + II, 'J for both mode I and mode II is higher for M =
45q than for M = 60q. For both considered angles, M = 45q and 60q, mode II is
dominating in the mixed modeI + II.
The 'J integral range for modeI and modeII is calculated from
(1)
'
V G '
,
IJ
Y S I
(2)
'
II Y S V G '
,
IIJ
E/K6.22/KYS2IIYS2II II V ' P G' , E –
E/KYS2I I G' V 'and
where, from [7]
Young’s modulus, Q - Poisson’s ratio, w – width, WYS – shear yield stress, P - shear
modulus,
'J = Jmax
-
Jmin,
a d cos Y K 2 I I S M V ' ' ,
a d S M M V '
cossin Y K 2 II '
- stress intensity factors ranges for mode I and mode
a d 2 w a d 2 1 . 0 1 Y a n Y¸
2 ¨ © § ¸ ¹ · ¨ © - correction coefficients for mode I and w ¹ ·
II,
1
2
modeII [8].
JI for modeI and plasticity was calculated by using the following relationship [9]
cH'V'
J
K
I
2I
2I
p
(3)
E Y ' ' S
a ,
n
where a – crack length, n’ = 0.22 - cyclic strain hardening exponent, 'V - ranges of
stress under tension in the notch root, 'Hp – ranges of plastic strains under tension in the
notch root.
The test results for cyclic mixed mode I+II include the error not exceeding 5 %at the
significance level D = 0.05 for the correlation coefficients r = 0.98 to 0.99.
For the controlled crack tip opening displacement, the J-integrals are constant, and their
values are: JmaxI+II = 0.0389 MPam.
C O N C L U S I O N S
The results of fatigue crack growth tests on notched specimens made of FeP04 steel,
all tested under tension loading, allow the following conclusions to be drawn:
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