TB 26-09 Measured Hammer-Tap Inputs

Hammer tests evaluate structural systems' modal properties by measuring the complete applied force and response. While a half-sine waveform is often used, it is not required. The instrumented hammer test captures the actual force shape, which can include crest ripples and separated contacts. An exact model of the test article will produce identical responses when subjected to the same force and initial conditions.

An approximate model may show different errors if it excites different frequencies, amplitudes, or nonlinear states.

A half-sine waveform is broadband but not flat and contains spectral zeros at specific frequencies. Its appearance alone is insufficient to determine adequate excitation. The spectral content of the measured force history determines the test's adequacy. Crest rippling and timing between separated contacts can introduce additional content and deterministic interference. Therefore, the unnormalized measured spectrum and response signal-to-noise ratio must be inspected for adequacy.

To ensure test adequacy, evaluate the measurements against the intended objective. 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, and not be clipped or saturated. The excitation must cover the required frequency, amplitude, and nonlinear-response ranges. If any deficiencies are identified, the test should be rejected or repeated accordingly.

In conclusion, the appearance of a half-sine waveform does not validate or guarantee the adequacy of a hammer test. Each test should be judged by its purpose and by accurate, complete force and response measurements over the required range.