COMPUTATIONAL
MECHANICS AUSTRALASIA – Cracks in Process
Pressure Vessel Piping
Computational
Mechanics Australasia was recently engaged
in a project conducted by local engineers in close
collaboration with Computational Mechanics BEASY.
Richard Rudas, Managing Director of Computational
Mechanics Australasia was instrumental in implementing
BEASY’s fracture mechanics analysis capabilities
in conjunction with the engineering critical assessments
(ECA) procedures described by the well known British
Standard PD6493.
These ECA
procedures, which provide guidelines to use when
evaluating fracture, fatigue, corrosion, and other
modes of failure, are routinely used in the power,
oil and gas industries world wide. Although British
Standard PD6493 provides fairly explicit guidelines
to incorporate when evaluating failure modes it also
supports other novel methods that can be used to
improve the quality of engineering analysis. Engineers
from Computational Mechanics Australasia saw this
as an opportunity for the use of BEASY’s Fatigue
and Crack Growth software.
BEASY was
used in conjunction with the existing ECA procedures
to predict the limiting fatigue life of welded joints
on the level switches piping associated with various
process pressure vessels. Non-destructive testing
(NDT) methods were used to determine the general
location and extent of cracks at pipe weld connections.
Radiographs produced during the NDT program provided
details such as material property zones, joint misalignments,
and the location and geometry of multiple cracks
both on the surface and embedded in the weld material.
BEASY was used to model the exact geometry of the
welds and simulate fatigue crack growth under cyclic
loading for a critically located weld crack.
 The weld
cracks, subject to cyclical loading, propagate at
a rate that is a function of the range of stress
intensity factors. An accurate prediction of this
range of stress intensity factors is critical to
compute the crack growth rate (da/dN). Prediction
of the stress intensity factors is not a trivial
task because the computed stress intensity factors
are a function of the cracked component geometry
and this geometry is continually changing as the
crack propagates.
The BEASY
Fatigue and Crack Growth software was used to accurately
predict fatigue crack growth. Measured crack geometry
and pipe loads were input using the BEASY preprocessor
environment to develop appropriate crack growth models.
Crack growth was modelled using the Paris equation
which is one of the fatigue crack growth criteria
options available with BEASY. Postprocessing options
in BEASY allowed rapid manipulation of the analysis
results allowing engineers to produce contour plots
of the von Mises stress distribution at the welded
joint as well as graphs of the stress intensity factor
versus number of load cycles.
BEASY provided
an extension to the analysis capabilities currently
described in British Standard PD6493 and demonstrated
its effectiveness as a complimentary tool that can
be combined with NDT measurements. Applied in this
way BEASY is not only an analysis tool but also a
useful resource to consider when planning and implementing
remedial design or monitoring projects where crack
growth is an issue. |
