Step-stress testing is an empirical procedure that can be used when resources for elaborate surveys, recording, analysis and technical support are limited. Due to the associated inaccuracies and risks, however, its use must be approved by the government.
The step-stress approach determines the "tolerance limit" or design capability of the hardware for the screen. By knowing this limit, a safe screening level can be determined and changed as required to obtain satisfactory screening results. The overall input level is tailored to the product.
As in Method A, the vibration survey test configuration should replicate the configuration for the proposed screen. The test item must be representative of hardware to be screened. It should be permissible to accumulate vibration time on the test hardware. The fixture, slip-plate, and head expander used for the survey should be the same as for the screen
Step-Stress tests proceed as follows:
1. Use a broadband spectrum of 20 to 2000 Hz and an initial input level of 2 or 3gRMS. Vibrate for 5 minutes for this and each subsequent stress step.
2. Inspect and, if powered, operate the unit during and after each vibration.
3. Analyze all failures occurring during or after the vibration tests to determine whether the problem is design related or a latent flaw.
4. Increase input by 2gRMS for successive vibration tests until tolerance limit is reached. The tolerance limit occurs when the unit does not function properly or at all and flaws are being induced into good hardware.
5. Establish the design capability or overstress region of the hardware (i.e., hardware which has weaknesses removed) based on failure analysis.
6. Use one-half of the design capability (gRMS) as the initial screening level.
Ideally, the final vibration input screening level
should be greater than or equal to one-half the design capability of the
hardware and greater than the operating level or possibly even the design
depicts this concept.