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Solution found through
understanding the mechanics of failure |
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 DESCRIPTION
OF THE FAILURE
A railway vehicle antennae
raft is cantilevered the bogie transom
by 4 M16 bolts. The bolts had repeatedly failed in
service and was considered a severe hazard as a detached raft could in
a worst case scenario derail a train and result in loss of life.
MoreVision were invited
to investigate the failure. The bolt samples were sent to a
metallurgical laboratory and examination of the bolt fracture surfaces
showed all the characteristics of a fatigue failure. The bolts were
renewed every month as a risk mitigation action until the
investigation reported its conclusions.
MoreVision initially performed bolt fatigue calculations using the loading assumed in
the design case. The calculations did not predict a bolt failure so it
was suspected that the actual service loads were greater than that
assumed for design purposes. Another possibility was that some element
in the bolted joint was relaxing giving rise to loss of bolt
pretension and a premature fatigue failure.
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DESIGN AND CALIBRATION OF STRAIN GAUGE BOLTS
After disappointing performance with commercially available bolt
sensors MoreVision devised and produced special test bolts by introducing flats on the shank of the
bolts and attaching standard linear gauges (photo on the left). The
bolts were calibrated in a laboratory load cell shown in the photo on
the right.  |
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BOLT RELAXATION TEST
 The
test bolts were installed on a stationary vehicle in a depot. During
installation the load in the bolt was measured as the bolts were
torqued up. The bolt force was measured for a number of hours to see
if any element in the joint would relax resulting in a significant
loss of bolt pretension and premature fatigue failure. The test
concluded that relaxation of the joint was within normal acceptable
levels and the threat of bolt pretension loss was ruled out. |
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DYNAMIC 'HAMMER' TEST
 Whilst
the the vehicle was stationary in the depot the antennae raft was
struck with a rubber hammer allowing the raft to vibrate. The response
was also seen by the bolt sensors. A spectrum analysis of the bolt
test signals illuminated the frequencies associated with the natural
modes of vibration. Closer examination of the phasing of the time
signals showed the fundamental 'springboard' mode directly (see plot
on left). |
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ON-TRACK TESTING
 The data
acquisition equipment and computer were installed in the drivers cab.
A test route of over 200miles was considered representative of normal
service. It was even possible for the train to remain in service as
the data was acquired which helped the client maintain his train
availability and service levels. |
POST-PROCESSING OF TEST SIGNALS
1) FILTERING NOISE
When all the train systems were started up in the depot it was
noticed that some noise was picked up by the strain gauges. The
characteristic of this noise was very high frequency (greater than
500Hz). The hammer test has shown that the predominant structural
frequency was less than 50Hz so frequencies of over 200Hz were taken
out using software filters.
2) DAMAGE CALCULATION
The filtered signals were the assessed using rainflow counting
techniques to reduce the signals to stress histograms. The fatigue
life could be calculated from the stress histograms. |
CONCLUSION
Local vibrations due to the resonant response of the raft resulted in
higher levels of load than was assumed in the design loadcase. This
gave rise to a premature bolt fatigue failure. |
SOLUTION
MoreVision working together with the depot mechanical engineering team
devised an end support bracket for the antennae. MoreVision prepared a
finite element analysis justification for the new end support bracket
together with revised bolt calculations and the solution was also
proven by further on track testing. |
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