Page 420 - Marine Structural Design

396 Part III Fatigue and Fracture

Level 3 is the most sophisticated one among of the three levels and will normally be used in
the assessment of high strain-hardening materials andlor stable tearing where the Level 2
approach would prove too conservative. In PD 6493 (now BS 7910), Level 3 FAD consists of
two alternative criteria: 1) a general FAD and 2) a material specific FAD in which material
stress-strain curves are also input data to the FAD assessment.
CTOD is popular in the UK nd European countries, while J-integral is used in the USA, e.g. by
the nuclear engineering industry.

21.5 Fatigue Damage Estimation Based on Fracture Mechanics
21.5.1 Crack Growth Due to Constant Amplitude Loading

The total number of cycle to final fracture is the sum of the number of cycles for the crack
initiation phase and crack propagation phase. The number of cycles for crack propagation
phase, Np, my be estimated using,
(21.8)

where a, and aCR are crack depth (or length) at crack initiation and final fiacture respectively.
The value of acR may be determined using methods for the assessment of fmal fracture, as
discussed in Section 21.1 thru Section 21.4. The crack propagation may be predicted using
Paris Law. Substituting the Pans Law into the above equation, we may obtain that

(21.9)

where F is so-called crack shape factor and S denotes the stress range. When the stress range S
is of constant amplitude, the above equation may be re-written as:

(2 1.10)

If F does not dependent on a, the above equation may lead to (Almar-Naess, 1985):

I-n/2 1-m/2
N, = acR -a0 for m#2 (21.11)
( Y
c SA (1-rn12)
F
The Paris parameters C and m may be found from Gurney (1 979), IIW( 1996), BS 791 0 (1999)
and API 579 (2001). The values of C and m depend on the material, service environment and
stress ratio. The value of C may also be determined by mechanical tests and the chosen value
is to be the mean value plus two standard deviation of log dddN.
The size of initial crack ao, should be determined considering the accuracy of the non-
destructive testing which is used to inspect the defects during fabrication.