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  #1  
Old December 10th, 2015, 03:09 AM
viswachenna viswachenna is offline
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Default Query in Reliability prediction

Hi,

In temperature Vs Failure rate graph shows that if temperature increases failure rate also increase. In case of negative temperature, failure rate increase or decrease?

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  #2  
Old December 10th, 2015, 05:33 PM
David David is offline
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Default Re: Query in Reliability prediction

I assume you referring to prediction standards such as the ones available in Lambda Predict. It depends on the standard since some will not handle negative temperature values, depending on the scale.
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Old December 10th, 2015, 11:25 PM
viswachenna viswachenna is offline
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Default Re: Query in Reliability prediction

Thanks for reply David.


If I draw a failure rate vs temperature graph from -40C to +70C, the failure rate is increasing with increasing of temperature to +70 but if i look towards -40C the failure rate is decreasing. Is it correct graph in MIL-HDBK 217?

Any standard/ideal graph is there to refer.
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Old December 11th, 2015, 04:10 PM
David David is offline
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Default Re: Query in Reliability prediction

This might be a concern if the pi factor is less than one for temperatures below the reference temperature of the component. The reference temperatures are between 0C and 25C for most components in MIL-217. The failure rate tables in MIL-217 usually start at the reference temperature and go up to about 150-200C. I would certainly try to stay in this range since this seems to be the intent of the authors. However, they do also provide an Arrhenius-type equation to go with the tables which allows extrapolation both below and above the values in the table so using this equation it is entirely possible to get pi factors less than one and failure rates that decrease below the reference temperature.
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Old February 4th, 2016, 07:09 AM
Richard_UK Richard_UK is offline
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Default Re: Query in Reliability prediction

I always recommend reading the instructions for use at the front of Mil Handbook 217. In this case Paragraph 3.4.1 states "Extrapolation of any of the base failure rate models beyond the tabulated values such as high or sub-zero temperatures (etc) ...is completely invalid."

This of course does not necessarily imply that reliability will not continue to improve as the temperature decreases below zero degrees C, merely that extrapolation of the prediction methodology does not claim to represent it.

As David points out, the Arrhenius based equations might be used to extrapolate below zero degrees C. There is not much experience or information about this in the industry. So any extrapolation must be treated with caution. Never assume that just because the Arrhenius model shows hypothetical improvement right down to absolute zero this is representative of practical experience in the real world. There will be a point where low temperature failure modes start to dominate.

Also of course be careful to take account of the lowest specified operating temperature of the devices.
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Last edited by Richard_UK; February 4th, 2016 at 07:11 AM. Reason: correcting typos
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Old April 7th, 2016, 07:22 AM
bpankaj358 bpankaj358 is offline
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Default Re: Query in Reliability prediction

Calculation as per Part stress Method (MIL-HDBK-217) shows that with negative temperatures FR will tend to reduce. However, point to be noted that at -Ve temperatures Rate of change of 'Failure Rate' is very minimal.

For +ve temperatures, change in Failure Rate is more per 'Temperature Interval'.

I have checked this in several part prediction analysis.

Thanks.
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