techniques have been developed for industrial application: one is based
on the whole cross-section thermal tensioning effect (Ref. 9), the other is
based on the localized thermal tensioning effect (Ref. 10).
The nature of buckling is mostly a phenomenon of loss of stability of
thin elements under compressive stresses. Buckling distortions caused by
longitudinal welds either in plates, panels or in shells are mainly dominated
by longitudinal compressive residual stresses induced in areas away from
the weld. The mechanism of buckling in weldments lies in the action of
inherent residual plastic (incompatible) strains formed during welding.
Losing stability, the buckled plate is released from an unstable flat
position of high potential energy with the maximum level of residual stress
distribution after conventional welding and takes a stable warped shape.
Losing stability, the buckled plate reaches a state of minimum potential
energy . In other words, any forced change of the stable curvature of the
buckled plate will cause increase in potential energy and once the force is
removed, the buckled plate will be restored to its stable position minimizing
the potential energy.
For plates of thickness less than 4 mm as widely used in aerospace and
modern vehicle welded structures, the value of
σ
cr
at which buckling occurs
is much lower than the peak value of residual compressive stress
σ
comp-max
after conventional gas tungsten arc welding (GTAW). However, the actual
value
σ
cr
for a welded element is difficult to be solely determined either by
the linear stability theory of small deformations or by the non-linear theory
of large deformations in theory of plates and shells.
In principle, all efforts either “passive” post-weld correction measures
or ’active’ in-process control methods of Low Stress No-Distortion (LSND)
welding to eliminating buckling aim at adjusting the compressive residual
stresses to achieve
σ
comp-max
< σ
cr
by means of reduction and redistribution
of the inherent residual plastic strains.
Buckling can be controlled by a variety of methods applied before
welding, during welding and after welding for its removal, mitigation or
prevention.
Low Stress No-Distortion (LSND) results could be achieved during
the welding process based on the thermal tensioning (temperature gradient
stretching) effect which is produced by establishing a specific temperature
gradient either in whole cross section of the plate to be welded or in a
localized area in the near-arc zone. Simultaneously, restraining transient
out-of-plane warpage movements of the workpiece is necessary. Differing
from the ’passive’ methods which have to be applied after welding once
buckling is in existence, LSND welding techniques can be classified as
’active’ methods for in-process control of buckling distortions with no
need of reworking operations after welding (Refs. 8–10).
ISSN 0236-3941. Вестник МГТУ им. Н.Э. Баумана. Сер. “Машиностроение”. 2005. № 4 93
