Page 428 - Marine Structural Design

404 Part III Fatigue and Fracture

be used for designers as guidance, while the criteria made by Ma et a1 (2000) may be used to
assess the acceptability of a particular design (see Part Ill Section 19.6).
22.3.3 Weld Improvement
Both contour grinding of the weld profile and the local grinding of the weld toe area, are
recommended to modify the weld profile and improve fatigue strength. When modifying the
weld toe profile, the essential objectives are:
. Remove defects at the weld toe
Develop a smooth transition between weld material and parent plate
The fatigue life can be increased by applying local grinding or re-melting techniques to remove
defects and discontinuities.

Grinding
Full-profile burr grinding, toe burr grinding or localized disc grinding are widely used grinding
methods. Considering the time required for grinding, local weld toe grinding has become one of
the best grinding methods. Careful and controlled local grinding of the weld toe improves the
fatigue strength of a specimen in air by at least 30%, this is equivalent to an increase in fatigue
life by a factor greater than 2. However, in order to obtain such a benefit the grinding should
extend about 0.04 inch (1 mm) beneath the plate surface.

Controlled Erosion
An alternative weld toe modification technique uses a high-pressure water jet. Under carefully
controlled conditions, the weld toe area can be eroded as if it were ground. Early research
indicates that fatigue life improvement due to Abrasive Water Jetting (AWJ) erosion and toe
grinding are comparable. The advantage of controlled erosion is that it does not require heat
input and it can be carried out quickly.
Remelting Techniques
Re-melting weld material to a shallow depth along the weld toe results in removal of inclusions
and helps achieve a smooth transition between the weld and the plate material. Tungsten-hert-
Gas (TIG) and plasma welding are not practical techniques for routine use, but TIG and plasma
dressing can be used to improve the fatigue strength of selected hotspot areas.
TIG welding is based on a stringer bead process. TIG dressing is performed on welds made by
other processes where the toe region is melted to a shallow depth without the use of filler
material. Slag particles in the re-melted zone are brought to the surface, leaving the weld toe
area practically defect fiee. High heat input should be maintained to obtain a good profile and a
low hardness. A low hardness in the heat-affected zone (HAZ) may also be achieved by a
second TIG application.
Plasma dressing requires re-melting the weld toe using the plasma arc welding technique. It is
very similar to TIG dressing, but plasma dressing uses a wider weld pool and higher heat input.
This technique is relatively insensitive to the electrode position, because fatigue strength
improvements using plasma dressing, are better than those obtained when using TIG.
Although overall weld profiling is considered desirable for fatigue strength improvement, rules
and recommendations, other than API (2001), do not allow improvement in fatigue strength due
to weld profiling unless weld profiling is accompanied by weld toe grinding. It should also be
noted that the data associated with weld profiling and weld toe grinding is limited. Therefore,