Crack Paths 2009

Considering current engineering practice of damage tolerance analysis for aircraft

structures is mainly based on the L E F Mapproach [20], in this study, crack driving force

is defined by two local stress fields at net-section to account for the notch effect. The

first stress field is local notch stress state dominated by NPZSwhile the other is gross

net-section stresses. It is postulated that the crack growth rate

is written as a

K

dNda/

function of the local notch stress intensity factor ()mpeakn K C d N d a _ / =

.

p_e a k n

(1)

C and are experimentally derived parameters that are the

where the constants

m

functions of local stress distributions at notches for given material and loading

condition. For spectrum loading in a block by block manner, the peak stress intensity

factor at notch is defined according to NPZSas following

π σ =

for

(2a)

K

a

NPZS a ≤

p e a k n _

o

for

(2b)

NPZS a >

a π β σ _ p e a k n

K

=

p e a k n _

plastic condition and

in which

o σ is the material flow stress for elastic-perfectly

is the peak net-section stress in the spectrum block loading. β

is the geometry

σ

peakn_

factor.

1.0E-01

1.0E-02

S F H )

1.0E-03

m m /

1.0E-04

t (

d a / d

1.0E-05

For High Kt

1.0E-06

For L o wKt

1.0E-07

1.0E-01

1.0E+00

1.0E+01

1.0E+02

1.0E+03

Kn_peak (MPa√m)

Figure 8. F C Grate vs

for all high and low specimen p e a k n K _ t K

Figure 8 is the F C G rate curves plotted against the crack growth driving

force, . It appears that the approach is able to explain the growth behavior of p e a k n K _

cracks for all high and low specimens. t K

Figure 9(a) presents the effect of peak net-section stress on the F C Gexponent for

m

t K

both low and high specimens by the linear regression of “Eq. 1”. It was found that

the exponent is approximately equal to two under spectrum loading, not only for small

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