Unforeseen fatigue issue
Unforeseen fatigue issue
(OP)
Hello all.
I have a situation here. Imagine a railcar truck (the steel casting that connects the wheelsets to the locomotive) having been in service for 40 years.
The truck has been subjected to fatigue loading all this time. It's analysis data is unknown, so the stress range is not known, neither are any of the fatigue category parameters.
Then, someone comes along and adds a welded bracket, for which our office is doing the design. This bracket will support a damper which is again a fatigue structure.
What is the common approach in this kind of retrofit? Clearly we are taking care of all the bracket fatigue issues, but what is a practical approach to making sure there are no surprises (cracks in supporting structure)down the line?
Is it common practice to redo the full structural fatigue analysis including the effects of the added bracket?
Thanks in advance.
tg
I have a situation here. Imagine a railcar truck (the steel casting that connects the wheelsets to the locomotive) having been in service for 40 years.
The truck has been subjected to fatigue loading all this time. It's analysis data is unknown, so the stress range is not known, neither are any of the fatigue category parameters.
Then, someone comes along and adds a welded bracket, for which our office is doing the design. This bracket will support a damper which is again a fatigue structure.
What is the common approach in this kind of retrofit? Clearly we are taking care of all the bracket fatigue issues, but what is a practical approach to making sure there are no surprises (cracks in supporting structure)down the line?
Is it common practice to redo the full structural fatigue analysis including the effects of the added bracket?
Thanks in advance.
tg





RE: Unforeseen fatigue issue
RE: Unforeseen fatigue issue
if you haven't got the original stress analysis, and can't estimate a working stress, then i guess you're left conservatively assuming the maximum stress for an infinite life.
consider too the effect of your weld on the fatigue life of the structure.
good luck !
RE: Unforeseen fatigue issue
We have dealt with bolster castings for our new and existing fleet of rail coal cars with regards to potential failures in service. As you are aware failures of bolsters can cause coal train derailments.
Why is the bracket being added? Is there a problem with fatigue crack propagation in service or is the bracket being added to allow for extra hauling capacity?
One of the foundries that manufactures rail car bolster castings and side frames in the US has a large fatigue testing machine that can simulate loading of a full bolster casting in service. We had one of the prototype castings tested for a cost of $10k. You might want to consider similar testing regarding your design.
RE: Unforeseen fatigue issue
RE: Unforeseen fatigue issue
Please forgive my fatigue inexperience, but here goes:
Can you clarify what you meant by assuming the maximum stress for an infinite life? If the existing (pre-bracket) working stress is the maximum stress for infinite life, then I can't add any stress, as I understand it, so I couldn't add the bracket. Is this right?
My concern is that even without adding too much stress, I am degrading the fatigue category near the new welds.
metengr:
The bracket is being added to improve the ride quality for loco. operators by dampening yaw motions in the truck. It's a passenger locomotive trying to be used at higher operating speeds.
tg
RE: Unforeseen fatigue issue
RE: Unforeseen fatigue issue
Here's the key statement :-
"The bracket is being added to improve the ride quality for loco. operators by dampening yaw motions in the truck. It's a passenger locomotive trying to be used at higher operating speeds."
Higher speeds = higher loads
Need I say more ?
RE: Unforeseen fatigue issue
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Unforeseen fatigue issue
RE: Unforeseen fatigue issue
What material is the casting made of?