Page 430 - Marine Structural Design

406 Part Ill Fatigue and Fracture

Review of grinding, re-melting and peening techniques indicate substantial scatter of
fatigue strength improvements. Typically, the best fatigue strength improvements are
achieved when using TIG dressing and hammer peening. Toe disc grinding is the least
effective technique.

22.4 Damage Tolerance Criteria

22.4.1 General
Marine structures are subjected to various sources of cyclic loading that may cause fatigue
cracks to propagate at welded details. The propagation of these cracks may eventually threaten
the structural strength and stability. Therefore, severe fabrication flaws and cracks detected in
service are to be repaired. Similarly corrosion defects and dent damages also need to be
inspected and repaired. In order to optimize the life-cycle inspection and maintenance costs,
there is a need for a rational criterion to determine the acceptability of damages.
Damage tolerance is the ability of structure to sustain anticipated loads in the presence of
fatigue cracks, corrosion defects, or damages induced by accidental loads until such damage is
detected through inspection or malfunctions and repaired. In this Section, focus will be
devoted to fatigue cracks. A damage tolerance analysis for fatigue cracks makes use of
fracture mechanics to quantitatively assess the residual strength and residual life of a cracked
weld detail.
Yee et a1 (1 997) and Reemsnyder (1 998) presented detailed guidance on the application of
damage tolerance analysis to marine structures. The damage tolerance analysis consists of the
following essential elements:
the use of Failure Assessment Diagrams to assess the local residual strength of a cracked
structural detail,
the use of linear elastic fracture mechanics models for fatigue crack growth to predict the
residual life of a cracked structural member,
the estimation of peak stress and cyclic loads over the assessment interval of interest, and
the inspection to detect damages and its accuracy.
Some of the above items will be discussed in the following sub-sections.
22.4.2 Residual Strength Assessment Using Failure Assessment Diagram
The failure assessment diagram (FAD) may be used to predict residual strength of a cracked
member for a given set of fracture toughness and defect size, see Part 111 Section 21.1.2. If the
peak stress exceeds the residual strength derived through FAD, failure may occur. For the
accurate prediction of residual strength, it is important to properly
assess the maximum defect size, considering damage detachability for the inspection
prograams,
determine the material toughness and the appliedhesidual stresses
select an appropriate failure assessment diagram and define its net-section stress and stress
intensity factor
While the residual strength represents the “capacity” of the damaged member, the “load” is the
peak stress that may be applied to the cracked member over the assessment interval of interest.