Page 427 - Marine Structural Design

Chapter 22 Material Selections and Damage Tolerance Criteria 403

D, and E grades are to be tested at O°C, -1OOC and 4O0C respectively. The energy average for
standard Charpy test specimens is required to be higher than 27 J (or 2.8 kgf-m). As steels for
hull structures, the E grades have the highest toughness, and may be used as crack arrestor to
stop the propagation of brittle fracture. They are used in location for primary members that are
critical for longitudinal strength. In many cases, the toughness criteria for secondary members
may be relaxed.
In order to present fatigue cracks in welded details, allowable stress criteria have been defined
in ship design Rules based on simplified fatigue analysis (see Part I11 Chapter 19) and an
assumed design life of 20 years. The allowable stress criteria shall be satisfied in the
determination of net wall-thickness.
For quality control purpose, the materials are inspected when the steel is delivered from the
steel makers. The inspection requirements are given in classification Rules. For ships in
operation, surveys are conducted by classification societies, the reduction of wall-thickness
due to corrosion is measured, fatigue cracks and dent damages are also given due attention in
the survey process. The causes for damages are investigated, and damages are repaired or weld
details are modified when necessary. The damage tolerance criteria are discussed in Section
22.4 of this Chapter. The feedback from the process of inspection, causes investigation, repair
and modification is given to design through Rule changes and development of design guidance,
such as fatigue resistant details, see Sub-section 22.3.2 of this Chapter.

22.3 Weld Improvement and Repair

22.3.1 General
In many cases, the fatigue performance of severely loaded details can be design to be fatigue
resistant details, and improved by upgrading the welded detail class to one having higher
fatigue strength. In some cases, procedures that reduce the severity of the stress concentration
at the weld, remove imperfections, andor introduce local compressive stresses at the weld can
be used for improvement of the fatigue life. Similarly these fatigue improvement techniques
can be applied as remedial measures to extend the fatigue life of critical weld details that have
cracked.
In the following sub-sections, discussions will be made to the welding improvements through
modification of weld toe profile and modification of residual stress distribution (Almar-Naess,
1985, Kirkhope, 1997).
22.3.2 Fatigue-Resistant Details

Fatigue strength of weld details is based on "good" fabrication practice in terms of
. design: to minimize the restraint and geometrical discontinuity in the design of cruciform
joint misalignment, lap connection and fillet welds.
welding practice: fillet weld fit up, weld shape and continuity