FMEA and test Plans
Back in the USA and teaching at one of the Japanese automaker’s design facilities in the Detroit Area today. The topic came up as to how the Test plan can be affected by the Design FMEA activity.
My response to the question was as follows:
One of the Design FMEA’s jobs is to determine that the testing and prevention methods are adequate to either prevent a failure mode and cause mechanism from happening, (Prevention affects Occurrence) and to determine if a test or groups of tests will excite a failure mode either directly or through a cause which was deemed probable during the analysis (This is the Detection number) A detection ranking is assigned to each test and the lowest number is placed in the Detection column. Here is where the process goes wrong.
The purpose of the the detection ranking is to give an honest assessment of the tests to determine if they are harsh enough and have the causes which have high Occurrence represented within it. If the test does not consider a cause that has a high Occurrence then the ranking should climb. A ranking greater than 3 (my opinion not to standard) then an action should be considered to improve the test to make it more representative of how the team invisions a failure. The Detection action (with no regard to RPN) is captured and given a timing and responsibility for follow-up. The RPN has nothing to do with determining if a test improvement is necessary.
The RPN is assigned to the action (not the other way around) and the resulting RPN after the action is taken is compared to the original for overall relative risk improvement. In fact the RPN is the only way to tell (numerically) if actions were successful in general. The RPN has each element where actions can be taken so it represents a good overall risk reduction comparison. The key word is comparison. OOOPs getting off topic here. Back to testing.
The test improvements are intended to excite the failure mode with the high occurrence causes present in order to fail the part or system, etc… On the other side the design engineer, knowing a risk exists takes an action to prevent a failure so the two opposing forces are equaled. What do I mean by this? The test guys are trying to fail the part with excitation (within specification limits of course) and the product design guys are trying to prevent the failure by strengthening the area (cause) that was determined to have risk. So try as they might, the test guys will experience a pass not a failure.
Remember we should not learn anything through this test plan as we are trying to verify that the design is acceptable. The design verification should be confirmed or backed up by the test after having first gained confidence in the design through prevention means (standards, FEA, Best practices and Product DNA)