14
Improving Global Quality of Life
Through Optimum Use and Innovation of Welding and Joining Technologies
An example of the value of welding to a small economy is shown by this example from New Zealand (NZ)
(
population 4.4 million) undertaken by an industry-based research organisation the NZ Heavy Engineering
Research Association (HERA).
The study has produced some interesting results, such as:
An estimated 6,500 people’s work involves welding, and a further 800 people are involved as
welding supervisors, engineers, or inspectors.
The value added of welding and joining technology in 2007 was estimated as NZ$813 million.
In 2008, around 4,000 tonnes of welding consumables (imported and locally manufactured for the
local market) with an estimated total value of NZ$15 million was available on the NZ market.
3.5
Failures of welded structures
Structural performance of the welded components strongly depends on the local joint quality. The
welded structures are often fabricated to satisfy conflicting requirements (low cost and weight, long life
and performance, limits of the technology) of material/design/technology. Additionally, welded or joined
components inherently contain micro- and macro heterogeneities and stresses at the joint areas, which may
contribute to the failures of the components. Prevention of failures and ensuring long and safe life of welded
structures (such as offshore platforms, pipelines, steel constructions, bridges etc.) also depends on the
effectiveness of the corrosion protection. Fatigue is a serious problem (
see Figure 3.3
)
for welded structures
subjected to repeated or fluctuating loading in service. Advanced design and flaw assessment guidelines
are now available, however, to prevent failure of a component and ensure structural safety. Regarding the
corrosion resistance of welded structures, the current guidelines and standards are predominantly related
to base materials. This provides future challenges and tasks to adopt the respective practices to welded
components. Furthermore, there is a need to describe the effects of the various weld microstructures and
residual stresses on the resistance of components against corrosion damage and cracking.
Failures of the welded components are usually due to the deterioration of the material/weld properties
(
development of a crack or local damage under fatigue, creep, corrosion or their combinations etc.) and/
or an increase in loading and/or localised external damage and/or change in function of the component.
A satisfactory life-cycle of the welded structures requires a correct assessment of the stress levels and cycles
as well as correct determination of the weld joint local properties. When weld joint failures are experienced
in service (due to fatigue), it is often in cases where the acting stress levels (and overloads) are not known
or have increased during the service life by far more than was foreseen at the design and fabrication stages.
Strength mismatch (
see Figure 3.4
)
between weld deposit and base metal as well as weld width (2H) play
significant roles on crack tip deformation and failure behaviour of welds with respect to residual strength.
This feature has now been taken into account in new codes and standards dealing with structural integrity
assessment of welded structures.
Figure 3.3
Fatigue crack propagation
in laser welded aerospace Al-alloy
(
Reproduced courtesy: GKSS, M. Kocak)
