NASA Technical Bulletin TB 26-09, “Measured Hammer-Tap Inputs,” outlines how to assess the adequacy of instrumented hammer tests. The bulletin states that crest ripples and separated contacts do not automatically invalidate a test when the complete force and structural response are measured accurately.
The document explains that hammer tests are used in modal analysis and can also precede pyroshock testing. During a zero-force interval between contacts, a structure can continue vibrating, while a subsequent contact acts on that existing motion. The resulting response therefore depends on the measured force history and the conditions of the test.
According to the bulletin, an exact model and test article subjected to the same bounded force history and initial conditions will produce identical responses. In approximate models, however, different force shapes can lead to different errors because they may excite different frequencies, amplitudes or nonlinear states.
TB 26-09 also notes that a finite half-sine pulse is broadband but not spectrally uniform. It contains spectral zeros at odd multiples of the ratio f/f₀, including 3, 5 and 7, where f₀ is defined as 1/(2T) for a pulse lasting T. The visual appearance of a pulse alone therefore cannot demonstrate that the excitation is adequate.
The bulletin says that rippling near the crest can add spectral content and fill some half-sine notches, while the timing of separated contacts can create deterministic interference. Evaluations should use the unnormalized measured spectrum and consider the signal-to-noise ratio of the response.
NASA recommends measuring the complete applied force with suitable bandwidth, calibration, dynamic range and time synchronization. Response measurements should likewise have sufficient bandwidth and signal-to-noise ratio and should not be clipped or saturated. Test adequacy should be judged against the required frequency, amplitude and nonlinear-response ranges for the intended objective.
The bulletin recommends rejecting or repeating a test only when a specific measurement, excitation or coverage deficiency has been identified. For more information, it lists Joel W. Sills of NASA’s Langley Research Center as a contact.
Sources: NASA Breaking News.