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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum ( )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Posted by: chandu ( )
Organization:Nagarjuna univeristy
Date posted: Tue Mar 4 6:33:32 US/Eastern 2003
I have some laboratory data (accelerated test)for Grade 1 item and Grade 2 item, i have estimated parameters of weibull distribution. For the Grade 1 item i have field data and i have estimated the parameters of weibull distribution. Here i have a problem in correlating the field data with laboratory data for grade 1 item. How can i correlate this Grade 1 data? By using this same same logic How can one predict the field data parameters for Grade 2 ? Please help me in this case....


Subject: Test Extrapolation
Reply Posted by: bwd ( )
Organization: RAC
Date Posted: Tue Mar 4 16:15:06 US/Eastern 2003
The problem that you try to describe is difficult to understand. It seems that you have performed stress testing and are trying to correlate this with actual operating data. The correlation is showing that the stress data is not in agreement. This indicated to me that you have overstressed the accelerated test parameters resulting in failures that are not observed at normal conditions. Therefore, if grade one and two are similar items similar test conditions will result in overstressing this sample. Since we do not know anything about the grades and items being tested it is impossible to project field parameters based on the failure testing.


Subject: Weibull accelerated and field reliability
Reply Posted by: Larry George ( )
Organization: Problem Solving Tools
Date Posted: Sun Mar 9 16:28:59 US/Eastern 2003
The Weibull shape parameters estimated from accelerated and field reliability estimates should approximate each other, UNLESS: 1. acceleration changes failure modes, 2. Weibull is a lousy fit, or 3. other interesting reasons. The Weibull shape parameter in P[Life > t] = exp[-(t/a)^b] is b. The reason is that the failure mode(s) should be the same, and b contains information about infant mortality or wearout, which are presumably related to failure modes. I have actually seen nearly equal Weibull shape parameters for accelerated and field data! First, use a likelihood ratio test to test whether the grade 1 Weibull shape parameters differ. If shape parameters accelerated and field don't differ statistically significantly, then estimate the grade 2 Weibull parameters from the accelerated data. Assume the grade 2 Weibull parameter is the same whether accelerated or not. Assume the grade 2 scale parameters have the same ratio as the grade 1 scale parameters. Estimate the grade 2 scale parameter as a(2; field) = a(2; acc)*[a(1; field)/a(1; acc)]. Send your data, and I will check whether Weibull fits tolerably and, if so, do the analysis.


Subject: Correlating field life and lab data life
Reply Posted by: Hari ( )
Date Posted: Tue Apr 1 6:28:40 US/Eastern 2003
Thank you very much for responding to my query. I will give my data for grade 1: lab data: on 7 samples: 80 42 43 72 49 44 87 And field failure data are as follows: Mileage Total Producion number:1395 16346.3 28435.0 10428.1 3009.4 15973.1 14001.9 10241.3 11098.8 11098.8 8839.4 16953.1 20065.6 20017.5 For grade 2: lab data is on two samples: 538 537 I will be very grateful to you, if you provide the answer.

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