Letter 15001

GM9540P Mass Loss Target Ranges for Coupons 

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I am just curious if anyone has a complete table of target mass loss data for GM9540P besides the end of test ranges outlined in the specification. It seems to me to be a bit confusing as how to calculate backwards the appropriate mass loss for the coupons from just using the end of test mass loss ranges. If anyone has any info or their interpretation, it would be greatly appreciated!

Thanks,

Paul T. Collins
Michigan Testing Institute, Inc. - Sterling Heights, MI

Paul -

The mass loss coupons should be removed every 8 cycles. Take the end of test target number (depending upon the duration of the test and coupon thickness)and calculate the percentage of the test completed. Multiply that amount times the end-of-test number. For example, if you are running method D, 80 cycles with 1/8" coupons, at 40 cycles the mass loss should be 3478 - 3778, which is 50% of the end-of-test numbers.

Cynthia L. Meade
- Sylvania, OH, USA

Cynthia,

First of all, I'd like to thank you for the response, it has been a real egg hunt trying to find an answer to my problem. Actually I have already calculated the mass coupon loss as you suggested, it was the only way that I could see to do it, but I just had some major concerns with this method. One thing is, when you calculated the range instead of a single number, you get a percentage reduction in the spread of the range, as you can see. This seems very incorrect.

I would assume that no matter what the duration is, the overlapping durations should still be approximately the same, e.g. 1 phase is still 1 phase, or 8 cycles is 8 cycles, or 16, and so on. If you calculate out the range all the way up to the 10% completion (1 phase, 8 cycles), using your example, you get a range of 695.7 &endash; 755.7mg with a spread of 60mg. Compare this to Test Duration A with a range of 965 &endash; 1375mg with a spread of 410mg, quite a big difference. And that 410mg seems to be a typo in the specification because it does not follow the patterns of the rest of the coupon thicknesses. It should be at least 400mg anyhow.

A great example of where this problem is so evident, is when we had a customer that wanted to start with a test duration of C - 40 cycles, and wanted to possibly extend the duration to D &endash; 80 cycles depending what his samples looked like at the end of C. Now how do you figure out what the mass coupon loss should be? They pretty much should be the same for the overlapping durations, but I can’t find any evidence supporting this with the way the specification is written. And try to get a hold of someone at GM to clarify anything with any of their specifications, I don’t even know where to start. It’s a major pain!

At least I can say this: if it was a Ford specification, I could call Ford Laboratories and get someone to look up the information. I have had great success with Ford Laboratories in the past, they were always able to straighten out discrepancies in their specifications. GM needs to be a little more accountable in this way.

Well, thanks for the feedback, if you or anyone else has anything further, please, I beg you, let me know!

Sincerely,

Paul T. Collins
- Sterling Heights, MI, USA

Hello Paul,

First of all, I do not work for GM and I have never read GM9540P. But I have a copy of GMW14872 "Cyclic Corrosion Laboratory Test", which is the replacement of GM9540P, first published in November 2006.

I don't think the big difference you mentioned is a typo.

You are probably right that the coupon mass loss from every 8 cycles, no matter they are from the first 8 cycles, or the last 8 cycles, should be approximately the same. And the the deviations of the mass loss from each 8 cycle could be small and close. However, you have to add up the deviations of mass loss from each 8 cycle to the end of test to get the wide range of targeted mass loss. The final targeted mass loss ranges represent the accepted deviations of cpoupon mass loss over the total test cycles. e.g.: your test chamber has a constant overexposure condition that results in 50mg more than the targeted mean mass loss for each and every 8 cycle. At the end of the 80 cycles, your final coupon mass loss could be 50mg x 10 = 500mg more than the targeted mean mass loss.

Sincerely,

Peter Zhang
Major global commercial truck manufacturing company - Portland, OR, USA

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