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Accelerated Life Testing of De-frost Timers

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Test Procedure : DFT are tested at the rated supply at high temperatures 80°C and 90°C. The De-frost and Compressor timings are noted for each cycle by the LEDs.

Judgment Criteria: Timer Stops and Timings more than the specs is considered as failure.

Software Used in analysis : MINITAB for Data Analysis, Dplot for Graphs

Test Facilities :

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Sample Failed after1 12 23 24 1105 1106 1107 1108 123*

Test Data at 80°C

* indicated censored Sample

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Test Data at 90°C

Sample Failed after1 12 23 784 1065 1216 1217 121 *

* indicated censored Sample

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For Data Analysis, the following are the parameters considered

1. Distribution : Weibull2. Temp. Relation : Arrhenius3. Approximation : Maximum Likelihood Estimation

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80

90

100000010000010000100010010100000000

99

95 90 80 70 60 50 40 30

20

10

5

3 2

1

Failure T

Per

cent

Probability Plot (Fitted Arrhenius) for Failure TWeibull Distribution - ML Estimates - 95.0% CI

Censoring Column in Censor

Shape Scale AD* F/C

0.5906 56.195 6.50 8/1

0.5906 84.216 7.55 6/1

Design Value = 35.00000.5906 6.5756

Probability of failure for 80°C is more than 90°C

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As the probability of failures in 80°C is more than 90°C, which is not suggested, the data has to be modified.

Neglecting the early life failures (1 cycle failures) the data is again analyzed.

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Modified Test Data at 80°C

Sample Failed after1 22 1103 1104 1105 1106 123*

* indicated censored Sample

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Modified Test Data at 90°C

Sample Failed after1 22 783 1064 1215 1216 121*

* indicated censored Sample

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80

90

10000010000100010010100

99

95 90 80 70 60 50 40 30

20

10

5

3

2

1

Failure T

Per

cent

Probability Plot (Fitted Arrhenius) for Failure TWeibull Distribution - ML Estimates - 95.0% CI

Censoring Column in Censor

Shape Scale AD* F/C

1.2015 112.77 8.91 5/1

1.2015 109.82 8.59 5/1

Design Value = 35.00001.2015 129.80

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When doing accelerated life testing, you subject units to levels of an accelerating variable far exceeding normal field conditions to accelerate the failure process. But most likely, the information you ultimately want is, How do the units behave under normal field conditions?

Ask Minitab to extrapolate information gained from the accelerated situation to the design value, or common field condition.

Percentile is How long it takes for 10% of the units to fail, 20% of the units to fail, 20% of the units to fail............

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Percent Temp Percentile10 35 19.9520 35 37.2530 35 55.0440 35 74.2150 35 95.6760 35 120.6970 35 151.4880 35 192.8790 35 259.85

Test Data Extrapolated to use temperature 35°C

So, The No. of Cycles required for

10 % of the samples to fail are 20 Cycles, for

50 % of failure is 95 Cycles and for

90 % of the sample to fail is 259 Cycles.

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Test Data Extrapolated to use temperature 27°C

Percent Temp Percentile10 27 20.5420 27 38.3630 27 56.6740 27 76.4250 27 98.5260 27 124.2970 27 156.0080 27 198.6390 27 267.60

So, The No. of Cycles required for

10 % of the samples to fail are 20 Cycles, for

50 % of failure is 98 Cycles and for

90 % of the sample to fail is 267 Cycles.