Downloaded from https://www.everyspec.com MIL-STD-81OD’., 19 July 1983 Endurance testing does not establish fatigue life. This is because: the test item is tested for the amount of stress anticipated for one lifetime but not necessarily to destruction, and (a) .~..? .. ., (b) because. the sample size is too ?mall. ,..,. ,., , ,, Rather endurance testing assures that the required life,can be achieved with reasonable maintenance. The determination of the item’s useful life would require a.combined environments test (method 520) where al1.rel,evant environments were varied realiatioally and ‘asufficient number of .SaMPle.9 were tested to.failure..
A test item which has survived.an endurance test ia not necessarily used up; however, the,risk.of..failure in further Use iS higher:than that of,a new unit. So the test unit should not be used for a true life test, a reliability y demonstration, or a safety-critical application. Other uses are acceptable:if the increased risk of failure is compatible with the use. . 1 - 4 . 4 e effecta. Allowance should be made for mechanical ts whenever the benefits of increased real-ismare worth ‘the time, effort, and cost required for implementation. Equipment structures dynamically influence their own response to an external forcing function.
At structural natural.frequencies where the response stresses are high, the.structure will load the adjacent supporting.structures (i.e., notch the acceleration spectral density at these frequencies). ‘The magnitude of loading effects is related to the relative impedance of the equipment structure and support structure,e. As a rule of thumb, the ,resonagt element exhibits a loading force in proportion to its dynamic weight multiplied by the,corresponding amplification factor. Mechanical impedance effects can be accounted for in establishing vibration test spectra. The depths of notches are determined by measurement or by calculation. Ref 54 gives guidance in this matter.
I-4.5’ for source dwal test- A‘test spectrum ; representative of this type of testing is presented in fig~re 514:3-14. Test” bandwidths.,should be based on measured data if possible. When data are not available,., the bandwidth can be defined as: . ‘Wjd, = Where: BW3dB . the bandwidth at a level 3 db below”the peak PSD level ~ F = the fundamental frequenty, fn or one of the harmonics: .. fl, fz, fs, f~. METHOD 514.3 19 Jllly.1983 514.3-61