Quality Progress - November 2014 - 52

STATISTICS ROUNDTABLE BY NECIP DOGANAKSOY, GERALD J. HAHN AND WILLIAM Q. MEEKER Timely Reliability Assessment Using destructive degradation tests OFTEN, WE MUST demonstrate long- In contrast, in this column we will time and is designated as failed when its term product or component reliability consider the frequently encountered case strength falls below 0.5 newtons. It was de- within a relatively short time window. For in which the degradation measurement sired to demonstrate with 95% confidence example, it is not uncommon to need to is destructive and, consequently, only a that 10-year seal reliability in the use envi- assess 10-year reliability from a six-month single degradation measurement can be ronment of 100°C exceeds 99% (that is, less duration test program. In such situations, obtained on each test unit. In such cases, than 1% of the seals have a 10-year strength basing life data analyses on appropri- the reliability assessment may be based below 0.5 newtons or, equivalently, the ately selected degradation data (that is, on an accelerated destructive degradation probability of seal failure after 10 years is monitoring change of one or more relevant test (ADDT). We explain ADDTs using an less than 0.01). This demonstration, more- quality or performance characteristics) example dealing with a seal to be used in a over, needed to be achieved after only 25 when coupled with test acceleration can new product. weeks of testing. Seal strength application 100°C for 25 weeks will not provide the umn1 and an earlier one,2 we illustrated A severely weakened seal adversely affects desired reliability demonstration because the benefits of using degradation data in product performance and could lead to the anticipated result of no failures after reliability analyses. These case studies safety concerns. The seal's strength is mea- 25 weeks tells us nothing about what will dealt with battery and gallium arsenide la- sured as the force required to bend under happen after 10 years. Therefore, the team ser life, respectively. In both cases, it was a predefined loading system and typically turned to an ADDT for the desired reliabil- possible to obtain degradation measure- is initially between 20 to 30 newtons. The ity assessment. ments on the same unit over time. seal is expected to degrade slowly over Conducting time-to-failure testing at be highly useful. In our last Statistics Roundtable col- Developing an ADDT Seal strength lognormal probability plots at various temperatures, test times and maximum likelihood fits / FIGURE 1 0.1 0.2 0.95 0.90 0.85 0.75 0.65 0.55 0.45 0.35 0.25 0.15 0.10 0.05 0.5 1 2 3 4 5 10 20 40 Strength (in newtons) 0.1 0.2 0.1 0.2 0.5 1 2 3 4 5 10 20 40 Strength (in newtons) 0.5 1 2 3 4 5 10 20 40 Strength (in newtons) 52 QP * www.qualityprogress.com celeration variable, as well as a degradation characteristic to be measured. We also seek a physically meaningful engineering model ing variable to the measured degradation. Next, a test plan is developed to obtain data to fit the assumed model and to assess goodness of fit of the model to the data. After validation, the fitted model is used to 0.1 0.2 0.5 1 2 3 4 5 10 20 40 Strength (in newtons) Unexposed baseline 0.95 0.90 0.85 0.75 0.65 0.55 0.45 0.35 0.25 0.15 0.10 0.05 physically and empirically appropriate ac- to relate test duration and the accelerat- 0.95 0.90 0.85 0.75 0.65 0.55 0.45 0.35 0.25 0.15 0.10 0.05 350°C Probability Probability 300°C 0.95 0.90 0.85 0.75 0.65 0.55 0.45 0.35 0.25 0.15 0.10 0.05 250°C Probability 0.95 0.90 0.85 0.75 0.65 0.55 0.45 0.35 0.25 0.15 0.10 0.05 200°C Probability Probability Unexposed baseline In developing an ADDT, we first identify a 5 weeks conduct the desired reliability evaluations. All of this must be accomplished within the designated elapsed time limitations. 15 weeks Accelerating variable and degradation measurement 20 weeks Fortunately, the development team had a 25 weeks great deal of experience with ADDTs on 10 weeks seals and similar materials, including the 0.1 0.2 0.5 1 2 3 4 5 10 20 40 Strength (in newtons) construction of underlying models. In ad- http://www.qualityprogress.com

Table of Contents for the Digital Edition of Quality Progress - November 2014

Up Front
LogOn
Expert Answers
Keeping Current
Mr. Pareto Head
Finding Harmony
Multipronged Approach
Bank on It
The System Is the Solution
Innovation Imperative
Quality in the First Person
Career Corner
Statistics Roundtable
Standards Outlook
QP Toolbox
QP Reviews
One Good Idea

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