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Through Optimum Use and Innovation of Welding and Joining Technologies
Improving Global Quality of Life
The 21
st
Century for pressure equipment is the century of design by analysis (DBA) to permit much higher
allowable stresses for much higher strength materials than were acceptable for design by rule Codes. This
quantum leap must be done with assurances of high reliability including documentation with a defensible
paper trail of good engineering and quality control. Truly, major challenges and burdens are falling on today’s
welding engineers.
The increases in allowable stresses may be 50% or more. This more efficient use of materials developed
with emphasis on cost effective alloying and controlled processing is a particular problem for the welding
specialist. In the last half of the 20
th
century, welding specialists could overmatch strength properties and
provide a reliable margin of safety against many failure modes. When justification was required, related
experience could always be cited. Now, there is no relevant experience and past margins achieved by
overmatching are more difficult to obtain. DBA requires elastic–plastic, multi-axial strain limits for weld
regions as well as fracture mechanics and time dependent properties of welds and heat affected zones that
presently do not exist and must now be obtained through creative specimen design and testing. Design and
fabrication cannot be improved after a failure. They must be done right the first time.
The objectives of pressure vessel engineers today are remarkably different from those barely 30 years ago
whenequipmentwas tobebuilt using proven technologies and very forgiving, provenmaterials. Theemphasis
then was on economies of scale i.e. bigger was more efficient (economical). There was the expectation that
operating lifetimes would be limited. Replacement due to obsolescence would be in a generation. Today,
the pressure vessel engineer must be concerned with prolonging the life of that equipment after it has been
operating (and degrading) for 30, 40 or even 50 years. Fitness for service (FFS) or engineering critical (ECA)
assessments must be done with assurances of high reliability and with a defensible paper trail. Truly, major
challenges and burdens are apparent for today’s pressure vessel and welding specialists that support them.
These challenges are occurring in a global economy. This means that a component may be designed in
Country A, for installation in Country B, to be fabricated in Country C, of a material developed in Country D,
but produced in Country E and welded using consumables produced in Country F. All of this must usually be
in compliance with international Codes and regulations by engineers in every country along the way. There
must be better understanding and control of essential variables associated with welding and weld materials.
An important part of both design and post-construction assessments of suitability for continuing service
of pressure vessels is predicting fatigue behaviour of weldments. Fatigue of welds has been one of the
most common failure modes. A master S-N curve approach has recently been accepted by ASME and by
API for fatigue assessment of welded steel and aluminium vessel construction, and post-construction life
prediction in service at ordinary temperatures. Extension of this method to other materials and higher
temperatures requires more data and documentation of service experience. Additional work is needed to
validate proposed approaches for accounting for environmental effects, methods to improve weld profiles
and surface stresses, assigning quality factors, and analysing thermal stress cycles.
The master S-N curve approach is based on two key technical advances in fatigue analysis over the last
decade:
1.
A robust structural stress method that offers good mesh-insensitivity in stress concentration calculation.
2.
Definition of an “equivalent” structural stress so that the master curve can represent a large amount of
fatigue test data for various joint types, loading modes, plate thicknesses, etc with a relatively tight scatter
band. Based on the statistical characterisation of over 1,000 weldment fatigue tests, the design master
S-N curve shown below provides both a technically sound and reasonably conservative basis for fatigue
evaluation for pressure vessel and piping components (
Figure 9.10
).
9
Needs and challenges of major industry sectors for future applications
