Page 418 - Marine Structural Design

394 Part 111 Fatigue and Fracture

21.3 Level 2: The CEGB R6 Diagram

This Level 2 Assessment provides a simplified method of checking whether particular flaws
present in the structure may lead to fracture failure, or whether the flaws can be considered
safe without having to go through more complex assessment procedures. The approach
adopted in this preliminary assessment uses a variable safety factor on flaw size averaging
about 2. No additional partial safety factors should be used in Level 2 Assessment.
Two normalised parameters are specified and given as follows:
K
K, =- (21.5)
KM”,
and

s, =- ON (21.6)
~FLOW
where KR is the fracture ratio,
K = Stress-intensity factor (a function of net section stress cN, crack size a, and
geometry) at fracture of the component.,
KMAT = Linear elastic fracture toughness of the component,
SR = Collapse ratio,
cN = Net section stress in the component at fracture, and
crFLow = Flow stress that is defined as the average of yield stress and tensile stress in
BS 7910 (1997).

1.0

0.8
0.2 1 /
0.6 I / / I R6
KR / / /
0.4 / S.F. = OF/OW 1

o,/ //W
0.0
0.0 0.2 0.4 0.6 0.8 1.0
SR

Figure 21.2 CEGB R6 Curve