Page 431 - Marine Structural Design

Chapter 22 Material Selections and Damage Tolerance Criteria 407

The calculation of stresses and crack driving forces may also significant influence the result of
the safety check for the cracked weld detail.
22.4.3 Residual Life Prediction Using Paris Law
The Paris Law may be used to calculate the crack growth due to cyclic loads of constant
amplitude or variable amplitude, see Part III Section 21.5. For the reliable prediction of crack
growth, it is important to accurately
predict the Paris parameters (C and m) used in the Paris equation
assess the initial crack size to be used in the Paris equation
calculate cyclic stresses and the stress intensity range
The outcome of integrating the Paris equation is the number of cycles from the time the crack is
inspected to the final fracture. The damage tolerance criterion requires that this predicted fatigue
life be longer than the sum of the time to the next inspection and the time required for repair or
replacement. If no damage is detected in the inspection, the minimum inspectable size for cracks
shall be used as the initial crack size.

22.4.4 Discussions
A damage tolerance analysis may be conducted during the design and fabrication stage, while
performing an in-service inspection or in the course of extending the design life of a structure.
BS 7608 (BSI, 1993) may be used for the damage tolerance analysis. It recommends to select
materials and to reduce stresses so that the crack growth rate is low and critical crack size is
large. Providing readily inspectable details and crack arresting details may also help.
The above discussions are made using fatigue and fracture as an example. Similar discussions
may be made on corrosion defects and wear out. In the evaluation of the tolerance criterion for
corrosion defects, it is necessary to predict
. the initial corrosion defect size,
. the residual strength of corroded member,
. the future growth of corrosion defects using an adequate corrosion rate model,
. the maximum loads that may occur for the period of interests or until the end of design life.
The dent damage caused by accidental loads will not grow and therefore its tolerance criterion
may be simply determined by comparing the residual strength with the maximum load expected
for the interval of interests.

22.5 Non-Destructive Inspection
Almar-Naess (1985) and Marshall (1992) outlined several methods for the inspection of cracks
in weld details, such as:
liquid penetrant (to reveal surface flaws, require a clean surface)
magnetic particles (to reveal surface flaws, dos not require a clean surface)
eddy currents (primarily for detecting surface flaws, magnetic field based)

radiography (for detecting internal cracks, using x or y radiation recorded in film )