| Type
number, description and notes on mode of failure
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Class explanatory comments
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Examples,
including failure modes
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TYPE 1 MATERIAL FREE FROM WELDING
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Notes on potential modes of failure:
In plain steel, fatigue cracks initiate at the
surface, usually either at surface irregularities or at
corners of the cross-section. In welded construction,
fatigue failure will rarely occur in a region of plain
material since the fatigue strength of the welded joints
will usually be much lower. In steel with rivet or bolt
holes or other stress concentrations arising from the shape
of the member, failure will usually initiate at the stress
concentration.
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| 1.1 Plain steel
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| (a) In the
as-rolled condition, or with cleaned surfaces but with no
flame-cut edges of re-entrant corners.
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B Beware of using Class B for a member
which may acquire stress concentration during its life, e.g. as
a result of rust pitting. In such an event Class C would be more
appropriate.
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| (b) As (a)
but with any flame-cut edges subsequently ground or machined to
remove all visible sign of the drag lines.
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B Any re-entrant corners in flame-cut
edges should have a radius greater than the plate
thickness.
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| (c) As (a) but
with the edges machine flame cut by a controlled procedure to
ensure that the cut surface is free from cracks.
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C Note, however, that the presence of a
re-entrant corner implies the existence of a stress
concentration so that the design stress should be taken as the
net stress multiplied by the relevant stress concentration
factor.
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TYPE 2 CONTINUOUS WELDS ESSENTIALLY PARALLEL TO THE DIRECTION
OF APPLIED STRESS
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Notes on potential modes of failure:
With the excess weld metal dressed flush, fatigue
cracks would be expected to initiate at weld defect
locations. In the as-welded condition, cracks might initiate
at stop-start positions or, if these are not present, at
weld surface ripples.
General comments:
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(a) Backing strips:
If backing strips are used in making these
joints: (i) they must be continuous; and (ii) if they are
attached by welding those welds must also comply with the
relevant Class requirements (note particularly that tack
welds, unless subsequently ground out or covered by a
continuous weld, would reduce the joint to Class F,
see joint 6.5).
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(b) Edge distance:
Edge distance: An edge distance criterion exists
to limit the possibility of local stress concentrations
occurring at unwelded edges as a result for example, of
undercut, weld spatter or accidental overweave in manual
fillet welding (see also notes on joint Type 4).
Although an edge distance can be specified only for the
‘width’ direction of an element, it is equally important to
ensure that no accidental undercutting occurs on the
unwelded corners of, for example, cover plates or box girder
flanges. If it does occur it should subsequently be ground
smooth.
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2.1 Full or partial penetration butt welds, or
fillet welds.
Parent or weld metal in members, without
attachments built up of plates or sections, and joined by
continuous welds.
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| (a) Full
penetration butt welds with the weld overfill dressed flush with
the surface and finish-machined in the direction of stress, and
with the weld proved free from significant defects by
non-destructive examination.
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B The significance of defects should be
determined with the aid of specialist advice and/or by the use
of fracture mechanics analysis. The NDT technique must be
selected with a view to ensuring the detection of such
significant defects.
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| (b) Butt or
fillet welds with the welds made by an automatic submerged or
open arc process and with no stop-start positions within the
length.
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C If an accidental stop-start occurs in
a region where Class C is required remedial action should be
taken so that the finished weld has a similar surface and root
profile to that intended.
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| (c) As (b) but
with the weld containing stopstart positions within the
length.
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D For situation at the ends of flange
cover plates see joint Type 6.4.
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TYPE 3 TRANSVERSE BUTT WELDS IN PLATES
(i.e. essentially perpendicular to the direction of applied
stress)
Notes on potential modes of failure:
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With the weld ends machined flush with the plate
edges, fatigue cracks in the as-welded condition normally
initiate at the weld toe, so that the fatigue strength
depends largely upon the shape of the weld overfill. If this
is dressed flush the stress concentration caused by it is
removed and failure is then associated with weld defects. In
welds made on a permanent backing strip, fatigue cracks
initiate at the weld metal/strip junction and in partial
penetration welds (which should not be used under fatigue
conditions), at the weld root.
Welds made entirely from one side, without a
permanent backing, require care to be taken in the making of
the root bead in order to ensure a satisfactory profile.
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Design stresses:
In the design of butt welds of Types 3.1 or 3.2
which are not aligned, the stresses must include the effect
of any eccentricity. An approximate method of allowing for
eccentricity in the thickness direction is to multiply the
normal stress by ( 1 + 3  ), where
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e is the distance between centres of thickness of the two
abutting members: if one of the members is tapered, the centre
of the untapered thickness must be used; and
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t is the thickness of the thinner member.
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| With connections which are supported
laterally, e.g. flanges of a beam which are supported by the
web, eccentricity may be neglected.
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| 3.1 Parent
metal adjacent to or weld metal in full penetration butt joints
welded from both sides between plates of equal width and
thickness or where differences in width and thickness are
machined to a smooth transition not steeper than 1 in 4.
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Note that this includes butt welds which
do not completely traverse the member, such as circular welds
used for inserting infilling plates into temporary
holes.
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| (a) With the
weld overfill dressed flush with the surface and with the weld
proved free from significant defects by non-destructive
examination.
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C The significance of defects should be
determine with the aid of specialist advice and/or by the use of
fracture mechanic analysis. The NDT technique must be selected
with a view to ensuring the detection of such significant
defects.
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| (b) With the
welds made, either manually or by an automatic process, other
than submerged arc, provided all runs are made in the downhand
position.
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D In general, welds made by the
submerged arc process, or in positions other than downhand, tend
to have a poor reinforcement shape, from the point of view of
fatigue strength. Hence such welds are downgraded from D to
E.
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| (c) Welds made
other than in (a) or (b).
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E In both (b) and (c) of the corners of the
cross-section of the stressed element at the weld toes
should be dressed to a smooth profile.
Note that step changes in thickness are in
general, not permitted under fatigue conditions, but that
where the thickness of the thicker member is not greater
than 1,15 x the thickness of the thinner member, the change
can be accommodated in the weld profile without any
machining. Step changes in width lead to large reductions in
strength (see joint Type 3.3).
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| 3.2 Parent
metal adjacent to, or weld metal in, full penetration butt
joints made on a permanent backing strip between plates of equal
width and thickness or with differences in width and thickness
machined to a smooth transition not steeper than 1 in 4.
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F Note that if the backing strip is
fillet welded or tack welded to the member the joint could be
reduced to Class G (joint Type 4.2).
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| 3.3 Parent
metal adjacent to, or weld metal in, full penetration butt
welded joints made from both sides between plates of unequal
width, with the weld ends ground to a radius not less than 1,25
times the thickness t.
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F2 Step changes in width can often be avoided by
the use of shaped transition plates, arranged so as to
enable butt welds to be made between plates of equal
width.
Note that for this detail the stress
concentration has been taken into account in the joint
classification.
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TYPE 4 WELDED ATTACHMENTS ON THE SURFACE OR EDGE OF A
STRESSED MEMBER
Notes on potential modes of failure:
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| When the weld is parallel to the
direction of the applied stress, fatigue cracks normally
initiate at the weld ends, but when it is transverse to the
direction of stressing they usually initiate at the weld toe;
for attachments involving a single, as opposed to a double, weld
cracks may also initiate at the weld root. The cracks then
propagate into the stressed member. When the welds are on or
adjacent to the edge of the stressed member the stress
concentration is increased and the fatigue strength is reduced,
this is the reason for specifying an ’edge distance’ in some of
these joints (see also note on edge distance in joint
Type 2).
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| 4.1 Parent
metal (of the stressed member) adjacent to toes or ends of
bevel-butt or fillet welded attachments, regardless of the
orientation of the weld to the direction of applied stress and
whether or not the welds are continuous round the
attachment.
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Butt welded joints should be made with
an additional reinforcing fillet so as to provide a similar toe
profile to that which would exist in a fillet welded
joint.
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(a) With attachment length (parallel to the
direction of the applied stress)
≤ 150 mm and with edge distance
≥ 10 mm.
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F The decrease in fatigue strength with
increasing attachment length is because more load is transferred
into the longer gusset giving an increase in stress
concentration.
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(b) With attachment length (parallel to the
direction of the applied stress)
> 150 mm and with edge distance
≤ 10 mm.
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F2
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| 4.2 Parent
metal (of the stressed member) at the toes or the ends of butt
or fillet welded attachments on or within 10 mm of the edge or
corners of a stressed member and regardless of the shape of the
attachment.
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G Note that the classification applies
to all sizes of attachment. It would therefore include, for
example, the junction of two flanges at right angles. In such
situations a low fatigue classification can often be avoided by
the use of a transition plate (see also joint Type 3.3).
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| 4.3 Parent
metal (of the stressed member) at the toe of a butt weld
connecting the stressed member to another member slotted through
it.
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Note that this classification does not
apply to fillet welded joints (see joint Type 5.1b).
However it does apply to loading in either direction (L or T in
the sketch).
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| (a) With the
length of the slotted-through member, parallel to the direction
of the applied stress, ≤150 mm and with edge distance ≥10
mm.
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F
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| (b) With the
length of the slotted-through member, parallel to the direction
of the applied stress, >150 mm and with edge distance ≥10
mm.
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F2
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| (c) With edge
distance <10 mm.
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G
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TYPE 5 LOAD-CARRYING FILLET AND T BUTT WELDS
Notes on potential modes of failure:
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| Failure in cruciform or T joints with
full penetration welds will normally initiate at the weld toe,
but in joints made with load-carrying fillet or partial
penetration butt welds cracking may initiate either at the weld
toe and propagate into the plate or at the weld root and
propagate through the weld. In welds parallel to the direction
of the applied stress, however, weld failure is uncommon, cracks
normally initiate at the weld end and propagate into the plate
perpendicular to the direction of applied stress. The stress
concentration is increased, and the fatigue strength is
therefore reduced, if the weld end is located on or adjacent to
the edge of a stressed member rather than on its
surface.
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| 5.1 Joint
description Parent metal adjacent to cruciform joints or T
joints (member marked X in sketches).
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Member Y can be regarded as one with a
non-load-carrying weld (see joint Type 4.1). Note that in
this instance the edge distance limitation applies.
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| (a) Joint made
with full penetration welds and with any undercutting at the
corners of the member dressed out by local grinding.
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F
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| (b) Joint
made with partial penetration or fillet welds with any
undercutting at the corners of the member dressed out by local
grinding.
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F2 In this type of joint, failure is
likely to occur in the weld throat unless the weld is made
sufficiently large (see joint Type 5.4).
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| 5.2 Parent
metal adjacent to the toe of load-carrying fillet welds which
are essentially transverse to the direction of applied stress
(member X in sketch).
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The relevant stress in member X should
be calculated on the assumption that its effective width is the
same as the width of member Y.
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| (a) Edge
distance ≥10 mm.
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F2 These classifications also apply to
joints with longitudinal weld only.
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| (b) Edge
distance <10 mm.
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G
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| 5.3 Parent
metal at the ends of load-carrying fillet welds which are
essentially parallel to the direction of applied stress, with
the weld end on plate edge (member Y in sketch).
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G
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| 5.4 Weld
metal in load-carrying joints made with fillet or partial
penetration welds, with the welds either transverse or parallel
to the direction of applied stress (based on nominal shear
stress on the minimum weld throat area).
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W This includes joints in which a
pulsating load may be carried in bearing, such as the connection
of bearing stiffeners to flanges. In such examples the welds
should be designed on the assumption that none of the load is
carried in bearing.
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TYPE 6 DETAILS IN WELDED GIRDERS
Notes on potential modes of failure:
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| Fatigue cracks generally initiate at
weld toes and are especially associated with local stress
concentrations at weld ends, short lengths of return welds, and
changes of direction. Concentrations are enhanced when these
features occur at or near an edge of a part (see notes on
joint Type 4).
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General comment:
Most of the joints in this section are also
shown, in a more general form in joint Type 4, they are
included here for convenience as being the joints which
occur most frequently in welded girders.
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| 6.1 Parent
metal at the toe of a weld connecting a stiffener, diaphragm,
etc. to a girder flange.
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Edge distance refers to distance from a
free, i.e. unwelded edge. In this example, therefore, it is not
relevant
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| (a) Edge
distance ≥10 mm (see joint Type 4.2).
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F as far as the (welded) edge of the web
plate is concerned. For reason for edge
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| (b) Edge
distance <10 mm.
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G distance see note on joint Type
2.
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| 6.2 Parent
metal at the end of a weld connecting a stiffener, diaphragm,
etc. to a girder web in a region of combined bending and
shear.
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E This classification includes all
attachments to girder webs.
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| 6.3 Parent
metal adjacent to welded shear connectors.
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| (a) Edge
distance ≥10 mm.
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F
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| (b) Edge
distance <10 mm (see Type 4.2).
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G
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| 6.4 Parent
metal at the end of a partial length welded cover plate,
regardless of whether the plate has square or tapered ends and
whether or not there are welds across the ends.
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G This Class includes cover plates which
are wider than the flange. However, such a detail is not
recommended because it will almost inevitably result in
undercutting of the flange edge where the transverse weld
crosses it, as well as involving a longitudinal weld terminating
on the flange edge and causing a high stress
concentration.
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| 6.5 Parent
metal adjacent to the ends of discontinuous welds, e.g.
intermittent web/flange welds, tack welds unless subsequently
buried in continuous runs.
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E This also includes tack welds which are not
subsequently buried in a continuous weld. This may be
particularly relevant in tack welded backing strips.
Note that the existence of the cope hole is
allowed for in the joint classification,
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| Ditto,
adjacent to cope holes.
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F it should not be regarded as an
additional stress concentration.
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TYPE 7 DETAILS RELATING TO TUBULAR MEMBERS
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| 7.1 Parent
material adjacent to the toes of full penetration welded nodal
joints.
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T In this situation design should be
based on the hot spot stress as defined in Pt 4, Ch 5, 5 Fatigue design (see also this Section for guidance
on partial penetration welds).
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| 7.2 Parent
metal at the toes of welds associated with small (≤150 mm in the
direction parallel to the applied stress) attachments to the
tubular member.
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F
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| As above, but
with attachment length >150 mm.
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F2
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| 7.3 Gusseted
connections made with full penetration or fillet welds. (But
note that full penetration welds are normally required).
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F Note that the design stress must
include any local bending stress adjacent to the weld
end.
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W For failure in the weld throat of
fillet welded joints.
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| 7.4 Parent
material at the toe of a weld attaching a diaphragm or stiffener
to a tubular member.
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F Stress should include the stress
concentration factor due to overall shape of adjoining
structure.
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| 7.5 Parent
material adjacent to the toes of circumferential butt welds
between tubes.
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In this type of joint the stress should
include the stress concentration factor to allow for any
thickness change and for fabrication tolerances.
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| (a) Welds
made from both sides with the weld overfill dressed flush with
the surface and with the weld proved free from significant
defects by non-destructive examination.
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C The significance of defects should be
determined with the aid of specialist advice and/or by the use
of fracture mechanics analysis. The NDT technique should be
selected with a view to ensuring the detection of such
significant defects.
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| (b) Weld made
from both sides.
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E
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| (c) Weld made
from one side on a permanent backing strip.
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F
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| (d) Weld made
from one side without a backing strip provided that full
penetration is achieved.
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F2 Note that step changes in thickness
are, in general, not permitted under fatigue conditions, but
that where the thickness of the thicker member is not greater
than 1,15 x the thickness of the thinner member, the change can
be accommodated in the weld profile without any
machining
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| 7.6 Parent
material at the toes of circumferential butt welds between
tubular and conical section.
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F2 Class and stress should be those
corresponding to the joint type as indicated in 7.5, but the
stress must also include the stress concentration factor due
to overall form of the joint.
C
E
F
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| 7.7 Parent
material of the stressed member adjacent to the toes of bevel
butt or fillet welded attachments in a region of stress
concentration.
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F Class depends on attachment length (see
Type 4.1) but stress should include the stress concentration
factor due to overall shape of adjoining structure.
or
F2
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| 7.8 Parent
metal adjacent to, or weld metal in, welds around a penetration
through the wall of a member (on a plane essentially
perpendicular to the direction of stress). Note that full
penetration welds are normally required in this
situation.
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D In this situation the relevant stress
should include the stress concentration factor due to the
overall geometry of the detail.
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| 7.9 Weld
metal in partial penetration or fillet welded joints around a
penetration through the wall of a member (on a plane essentially
parallel to the direction of stress).
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W The stress in the weld should include an
appropriate stress concentration factor to allow for the overall
joint geometry.
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