Tuesday, August 14, 2007

A cautionary tale on the use of software

An occasional commenter to my blog wrote an entry on "Where is engineering going". One of the subtopics in that was the on use and abuse of computers. In particular the use of computers as a black box. I have a cautionary tale about using computers as a black box.

About ten years a nuclear facility, "the customer," was buying a deployment system from a vendor. The vendor was a well known name in nuclear operations. The deployment system was to insert a high pressure cleaning lance into a waste tank to break up sludge on the bottom the of the tank. Once the sludge was broken up it was to be removed.

The vendor had had a finite element analysis of the entire deployment system performed by an outside consultant. The consultant's analysis showed the deployment system design to be satisfactory.

Because of the seriousness of a failure the customer wanted an independent assessment by an outside third party of the deployment system provided by the vendor. The customer approached my employer to have the assessment performed and the job landed on my desk. I was not to redo the entire analysis but only to analyze the wrist segment of the deployment system which was deemed the most critical. The significance of the wrist was that its failure would result in either the deployment system being unretrievable from the waste tank or dropping the payload which could result in breaching the tank.

I recieved a 3-D CAD file of the wrist, some drawings of the overall mechanism, and a table of loads. The vendor, viewing this whole excersize as a waste of time, was not interested in cooperating further. The cooperation I recieved was only due to arm twisting on the part of the customer.

The mechanism was a complex 3-d mechanism, as opposed two a 2-d planar, mechanism. It took me a few days to decipher the drawings, figure out the static equilibrium equations for the wrist, and set up the finite element analysis model. I ran the model and .... portions wrist near the mechanism were shown to be at yield under static loading conditions with no factor of safety. I was quite dismayed. I automatically assumed I made a mistake.

I went back through my analysis, both the hand calculation to determine the loading on the part, and the finite element analysis - same result. I refined my model of the pin joints to more accurately calculate the stresses at the connection points. It made matters worse - the stresses were now shown to be slightly higher than yield.

I was quite aware of the consequences to the customer/vendor relationship once I reported my results. I had two engineers I worked with review what I had done and they agreed with my conclusions. At this point I was still convinced I had overlooked something. It simply did not seem that an experienced vendor could make this sort of design error. With a heavy heart I called the customer and reported my findings. The customer, of course, was also convinced I had made a mistake but requested that I fedex my calculations and analysis results. The next day the vendor called and after expressing some doubts about my abilities and my employer's reputation also requested a copy of my work.

I heard nothing more for almost a week. By this time I was convinced I was in the dog house when the customer called. They agreed with my results and, reluctantly, so did the vendor.

What had happened?

The vendor's consultant had inadvertently modelled the spherical and pin joints as transferring moment loads where in actuality they can only transfer forces. That error dramatically changed the stresser on the part. The factor of safety showed in the vendor's original analysis was non-existent.

What were the end results of this? The project lead engineer was fired. Although he did not do the analysis, he signed off approving it. The consultant was fired. The customer eventually backed out of the contract. Although this flaw was fixed. This and other technical difficulties led to the customer losing confidence in the vendor.

Ever since then I have been leery of automatically trusting results from computer analyses. This poses a real problem in today's ever shortening design cycles. The assumption is if the computer did it, it's correct. There is no patience for checking the results or for performing alternative manual calculations.

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mechanical design
FEA

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2 Comments:

Blogger X M Carreira said...

Nice reference, JB. In my short professional experience I found that sometimes finite element analysis is required by the client just to boast even with poor soil data. It does not matter if the analysis is very precise and sophisticated if the starting data are inaccurate.

I strongly believe that FEA can be a powerful tool but time to check the process and expert analists are necessary.

Monday, August 20, 2007  
Blogger Justin said...

I appreciate your insight and agree with it. I am just getting into FEA and see just how easily engineers (yes, myself included) are satisfied by colorful plots without questioning how the results were achieved.

Looking forward to reading more of your posts.

Saturday, May 10, 2008  

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