TB 26-09 Measured Hammer-Tap Inputs

A NASA instrumented hammer test demonstrates that measured force and response data must be evaluated as a whole, rather than relying on the appearance of a half-sine waveform. While a half-sine is broadband, it contains spectral zeros at certain frequencies, which may not provide adequate excitation for all structural responses. Crest rippling and timing between separated contacts can add content and create deterministic interference, potentially filling half-sine notches.

To ensure test adequacy, the complete applied force should be measured with adequate bandwidth, calibration, dynamic range, and time synchronization. The response measurements must also have adequate bandwidth and signal-to-noise ratio, without being clipped or saturated. The excitation must cover the frequency, amplitude, and nonlinear-response ranges required by the intended test objective.

Ultimately, a hammer test should be judged by its purpose and by accurate, complete force and response measurements over the required range. Validity and predictability depend on the measured force history, not just the waveform shape. By following these guidelines, researchers can ensure that hammer tests provide reliable and meaningful data for modal analysis and pyroshock testing.