NASA issues guidance for evaluating measured hammer tap inputs

NASA said in an announcement published Monday 02 November 2026 that instrumented hammer tests should be judged by their intended purpose and by the accuracy and completeness of force and response measurements. The guidance applies to hammer tests supporting modal analysis and tests preceding pyroshock testing. The announcement lists Joel W. Sills at Langley Research Center as the contact.

NASA said an instrumented hammer test does not require an idealized single half-sine waveform. When the complete force and response are measured accurately, crest ripples and separated contacts remain known inputs. Changes in force shape alter excitation, but they do not inherently invalidate the recorded response.

During a zero-force interval, the structure continues to vibrate. Re-contact acts on that vibration, the agency said. NASA said an exact model and a test article receiving the same force and initial conditions produce identical responses for each bounded measured history. An approximate model can show different errors as the inputs excite different frequencies, amplitudes or nonlinear states.

A finite half-sine pulse is broadband but contains spectral zeros, so its appearance alone cannot establish adequate excitation, NASA said. Crest rippling can add spectral content and fill notches in a half-sine input. Timing between separated contacts creates deterministic interference. NASA recommended inspecting the unnormalized measured spectrum and the response signal-to-noise ratio.

NASA recommended measuring the complete applied force with adequate bandwidth, calibration, dynamic range and time synchronization. Response measurements should have adequate bandwidth and signal-to-noise ratio and should not be clipped or saturated, the agency said. The excitation should cover the frequency, amplitude and nonlinear-response ranges required by the intended test objective.

The agency said a hammer test should be rejected or repeated only for an identified measurement, excitation or coverage deficiency. NASA concluded that a half-sine appearance neither establishes validity nor guarantees adequacy. Each hammer test should be evaluated against its purpose and accurate, complete force and response measurements over the required range.