Page 408 - Marine Structural Design

384 Part III Fatigue and Fracture

Joint Classification
Guidelines on joint classification may be found from the UK DEn (1990). Note that the S-N
curves in the UK DEn (1 990) was modified by HSE( 1995).
The UK DEn (1990) guidelines apply only to welded joints that are free from serious defects
or discontinuities. Factors such as undercut at the toe, internal or surface breaking defects or
cracks, and geometric irregularities may cause a reduction in fatigue strength and should be
evaluated separately.
The UK DEn (1990) guidelines allocate various types of welded joints into one of nine joint
classes. To determine the correct classification for a particular weld detail, it is necessary to
identify the weld type, the direction of the applied loading, and to consider all potential
cracking locations. For most types of joint, the weld toes, weld ends, and weld roots are
considered the most important locations.
The joints with the highest classifications are those that are stressed in a direction parallel to
the weld. Fillet or butt weld joints fall into Class C or B in the UK DEn (1990) guidelines
depending on whether the manufacturing process is manual or automatic. Such joints seldom
govern the fatigue strength of a welded details since other joints are likely to fall into lower
joint classes.
The classification of transverse butt welds is more complex. They can fall into Class D or E,
depending upon the details of the manufacturing process, position, and location, all of which
may influence the weld profile. Class C may be justified if the weld overfill is removed by
grinding or the weld is shown to be free from significant defects by using non-destructive
testing. However, if access is limited and the weld must be made from one side only, a lower
fatigue strength should be assumed.
The UK DEn (1 990) guidelines downgrade butt welds, made onto a permanent backing strip,
to Class F. The guidelines also warn against the use of tack welds within small distances of
the plates edge, in which case, the classification is lowered to Class G.
Tack welds are a controversial topic. A number of studies have been conducted for different
methods of attaching the backing to the plates prior to making the butt weld. Tacking the
backing strip to the root preparation, and incorporating this into the final weld, gives small
improvement in fatigue strength over joints in which the backing strip is fillet welded to one of
the plates. However, the increase is not sufficient to warrant a higher joint classification. In
both cases, failure may initiate at the root of the butt weld.
Currently butt welds made onto temporary backing such as glass or ceramic backing strips are
not classified and require further research. The availability of electrodes designed specifically
for root runs has resulted in an improvement in the quality of single-sided welds made without
backing. In recognition of this welding quality improvement, such joints can be considered as
Class F2 if full penetration is achieved. This classification should be used with caution,
because fatigue strength in some areas may be much lower due to lack of penetration at the
root.
The fatigue strength is seldom governed by butt welded joints, because these joints in general
posses a superior strength over fillet welded joints. Fillet welds fall into Class F, F2, or G
depending on their size, orientation, and location in relation to a free plate edge. However,
recent studies have shown that fillet welds posses a fatigue strength lower than that predicted
by Class G, if the weld is continued over the comer of the plate.