SAE Update - June 2022 - 14

TECH FOCUS:
TESTiNg
Worst-case testing doesn't
have to be at the worst case
A perfect test would only accept
good designs while rejecting all
bad designs. But what is a bad
design? This is often a question
of risk versus resources. The
threshold for accepting a
glovebox that requires a slightly
elevated closing force should be
much lower than that of a fuel
pump that explodes. Perhaps
one in one thousand stiff
gloveboxes is acceptable, but
one in one million exploding fuel
pumps is unacceptable.
Statistically designed tests
balance the risk of accepting a
bad design versus rejecting a
good design, but require
massive sample sizes to achieve
very low risks. How many
random samples would be
required to detect a problem
that occurs at a rate of one per
million? Even if it were possible
to test massive sample sizes,
ABOUT THE AUTHOR
SKF Executive Engineer
for group Quality, Dr.
Bryan Dodson, P.E.,
wrote this article for
Update. The SAE Fellow
(2010) is co-author of an
SAE international book
on accelerated testing.
14
June 2022
Probability of multiple characteristics simultaneously being at the
tolerance edge.
statistically designed tests are far less efficient than
testing the worst-case tolerance stack-up. There is
simply no reason to randomly select samples when it
is possible to choose samples that are likely to trigger
an undesired event. If it is possible to successfully test
the " worst case, " then only a single sample is needed
to verify the design. The difficulty is obtaining the
" worst case. " Whether choosing electrical
components or machining surfaces, it is simply not
possible to obtain all desired characteristics at the
worst case.
Thankfully, the absolute worst case of all
characteristics is not needed. Consider a
manufacturing process with a process capability index
of 1.33; the probability of a single characteristic falling
in the last 25% of the tolerance interval is only 2.3%.
Thus, if five characteristics contribute equally to the
UPDATE
Bryan Dodson

https://www.sae.org/publications/books/content/r-518/ https://www.sae.org/publications/books/content/r-518/

SAE Update - June 2022

Table of Contents for the Digital Edition of SAE Update - June 2022

SAE Update - June 2022 - Cov1
SAE Update - June 2022 - Cov2
SAE Update - June 2022 - Cov3
SAE Update - June 2022 - Cov4
SAE Update - June 2022 - 1
SAE Update - June 2022 - 2
SAE Update - June 2022 - 3
SAE Update - June 2022 - 4
SAE Update - June 2022 - 5
SAE Update - June 2022 - 6
SAE Update - June 2022 - 7
SAE Update - June 2022 - 8
SAE Update - June 2022 - 9
SAE Update - June 2022 - 10
SAE Update - June 2022 - 11
SAE Update - June 2022 - 12
SAE Update - June 2022 - 13
SAE Update - June 2022 - 14
SAE Update - June 2022 - 15
SAE Update - June 2022 - 16
SAE Update - June 2022 - 17
SAE Update - June 2022 - 18
SAE Update - June 2022 - 19
SAE Update - June 2022 - 20
SAE Update - June 2022 - 21
SAE Update - June 2022 - 22
SAE Update - June 2022 - 23
SAE Update - June 2022 - 24
SAE Update - June 2022 - 25
SAE Update - June 2022 - 26
SAE Update - June 2022 - 27
SAE Update - June 2022 - 28
SAE Update - June 2022 - 29
SAE Update - June 2022 - 30
SAE Update - June 2022 - 31
SAE Update - June 2022 - 32
SAE Update - June 2022 - 33
SAE Update - June 2022 - 34
SAE Update - June 2022 - 35
SAE Update - June 2022 - 36
SAE Update - June 2022 - 37
SAE Update - June 2022 - 38
SAE Update - June 2022 - 39
SAE Update - June 2022 - 40
SAE Update - June 2022 - 41
SAE Update - June 2022 - 42
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