oct ewj 24 online - Flipbook - Page 95
Whilst these techniques are important, they can only
be used once a failure location or initiation site has
been found and separated from what may be a large
equipment item e.g. a sub-sea hv cable or high voltage switchgear. Facilities allowing inspection, dismantling, separation and preparation of these samples,
often with multiple parties present, play as important
a role in an investigation as the advanced technique
used in the final stages confirming the contaminant is
present or the failure mode.
Figure 5: The investigation cycle used in root cause analysis of
engineering equipment
Figure 3: Large scale laboratory suitable for dismantling and
examination of large equipment items
Figure 7: Silver migration across an insulator within a
microscale potentiometer due to the presence of moisture within
its housing
Having a range of experts and equipment on hand can
also aid complex investigations where the position
from which a sample is taken or maintaining the sample condition prior to its analysis is critical. The analyst
being able to see the sampling location and understanding the context of the failure can also be beneficial to ensure the samples are taken appropriately to
show the contaminant or degradation mechanism.
The analyst can also give a realistic opinion of whether
the technique the investigator is requesting will be able
to provide the information they need or whether
another technique would be more appropriate.
Figure 4: Sample preparation laboratory for preparation of metallographic cross sections
Direct and continuous access to the laboratory also
provides the expert with the ability to develop and
evaluate a failure hypothesis, experiment with alternatives or confirm the hypotheses through the application of multiple techniques or the re-examination of
a failed part or fracture surface.
Often as an investigation continues new technical and
operational information becomes known that may
affect a failure hypothesis. Being able to quickly return
to the laboratory to look again at the dismantled
equipment, take further samples or check the composition or properties of a material, now considered to
have contributed to the failure, allows the expert to
build confidence in their failure hypothesis before
completion of their expert report. For example, the
growth of micron-size silver dendrites across an insulator within a failed electronic part showed a failure
mechanism that requires the presence of water
vapour/moisture to occur. Direct access to the laboratory enabled the part to be quickly analysed by Karl
Fischer titration to confirm the presence of moisture
without risking its loss whilst transporting the sample
to another laboratory.
EXPERT WITNESS JOURNAL
Conversation between the expert and an experienced
analyst being able to examine the failed equipment
first-hand can be invaluable in selecting analytical techniques capable of fully exploring and proving or disproving a working failure hypothesis. In a recent
investigation conducted by RINA the analyst was able
to suggest the use of electro-backscatter diffraction
(EBSD) to show residual stress within an overhead line
copper conductor where arcing had caused localised
heating and a reduction in the wire’s tensile strength.
The expert was not aware of this technique’s capability until the conversation over the failed conductor
whilst agreeing the best locations from which sample
cross sections should be prepared.
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