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 MPa˜m.

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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