Page 416 - Marine Structural Design

392 Part III Fatigue and Fracture

Maximum Tolerable Defect Size
A maximum tolerable defect size can be derived when the geometry and the fracture toughness
of the selected material are known. For statically loaded structures, the maximum tolerable
defect size must satisfy the fracture mechanics criteria. For dynamically loaded structures, the
maximum tolerable defect size represents the critical crack size in a fatigue failure event. It
may be used to minimise the risk of unstable fracture throughout the operating life of the
structure. The result also gives direct input to the calculation of fatigue crack growth period.
There are three levels of procedure that are applied in fracture assessment (Reemsnyder, 1997):
Level 1. Utilisation of the Crack-Tip Opening Displacement (CTOD) Design Curve
(explained in Section 2 1.2)
Level 2. The Normal Assessment or Design Safety Format that makes use of the Failure
Assessment Diagram (described in Section 21.3). No practical safety factors need
to be applied here.
Level 3. Utilisation of the Failure Assessment Diagram based on detailed information of
stress-strain curves of materials. Partial safety factors are applied to the defect size,
stress level, etc., see Section 21.4.
More information may be found from MI 579 (2001), Andersen (1991) and BSI (1999).

21.2 Level 1: The CTOD Design Curve
21.2.1 The Empirical Equations

The CTOD Design Curve may be used to evaluate the resistance against fracture of a wide
range of structures such as pipelines, pressure vessels, ship and offshore structures, buildings
and bridges. One of the most commonly used CTOD Design Curves is the one developed by
the British Welding Institute (TWI) that relates the CTOD at some critical event, the yield
strength cy, nominal strain at a notch E, and flaw size a (Burdekin and Dawes, 1971; Dawes,
1974). This Design Curve was initially included in the first edition of the BSI fitness for
purpose guidance (BSI PD 6493, 1980). The BSI (1980) CTOD design curve may be
expressed as:
E
for -50.5 (21.1)
a=(:)’ EY
and

(21.2)
EY
where the non-dimensionalised CTOD is @,

(21.3)

with the yield strain sy