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Steel Designers' Manual - 6th Edition (2003)
272 Fracture and fatigue
increase in fatigue life by a factor of 2.2 can be taken if controlled local machining
or grinding is carried out.
7.6.8.3 Weld toe remelting
Weld toe remelting by TIG and plasma arc dressing are performed by remelting the
toe region with a torch held at an angle of 50° or 90° to the plate (without the addition
of filler material). The difference between TIG and plasma dressing is that the
latter requires a higher heat input.
Weld toe remelting can result in large increases in fatigue strength due to the
effect of providing low contact angle in the transition area between the weld and
the plate and by the removal of slag inclusions and undercuts at the toe.
7.6.8.4 Hammer peening
Improved fatigue properties of peened welds are obtained by extensive cold
working of the toe region. These improved fatigue properties are due to:
(1) introduction of high compressive residual stresses,
(2) a flattening of crack-like defects at the toe,
(3) an improved toe profile.
It can be shown that weld improvement techniques greatly improve the fatigue life
of weldments. For weldments subject to bending and axial loading, peening appears
to offer the greatest improvement in fatigue life, followed by grinding and TIG
dressing.
7.6.9 Fatigue-resistant design
The nature of fatigue is well understood and analytical tools are available to calculate
the fatigue life of complex structures. The accuracy of any fatigue life calculation
is highly dependent on a good understanding of the expected loading sequence
during the whole life of a structure. Once a global pattern has been developed then
a more detailed inspection of particular areas of a structure, where the effects of
loading may be more important, due to the geometries of joints for example, should
be carried out.
Data have been gathered for many years on the performance of bridges, towers
cranes and offshore structures where fatigue is a major design consideration. Codes
of practice, such as BS 5400-10 and BS 7608, give details for the estimation of fatigue