As mentioned above you should determine what level of failure you (or your customer) can accept. The capability index Cpk is one way. You may want to look into what the part is used for, if there are other considerations that are equally important other than just minimum breakage force. Check up on MTBF and DPMO on this site. But it sounds like you just need a simple breakage strength test so don’t overengineer it.
But first you need to perform a measurement system analysis (MSA) in order to assess the influence of your measurement system. Google or isixsigma is your friend here. And Minitab :)
For your test plan the trick is to make sure you include all plausible sources of variations, so things like mould batches or production machines will not give reason for higher variation in the future.
You want to determine a good sample size for testing, because you can’t sell tested parts. Here you can check the power of your analysis, again Minitab is your friend. Power gives you the strength of your prediction. Lots of good links to articles here on that too.
During production you should test the strength regularly. Here SPC is a good tool, it can help detect variation that is not caused by common causes.
A note: Breakage force often follow the Weibull distribution. That is for your calculation of variation.
BR, Peder
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