Instrumented Hammer Tests Explained
Instrumented hammer tests do not depend on a simple half-sine waveform to be effective. By accurately measuring both the force applied and the corresponding response, it is possible to understand phenomena such as crest ripples and separated contacts. The shape of the force applied influences the excitation, but it does not necessarily compromise the integrity of the recorded data. The adequacy of the test is contingent upon several factors, including measurement precision, bandwidth, signal-to-noise ratio, and how well the test aligns with its objectives.
Understanding Hammer Test Measurements
Hammer tests play a crucial role in modal analysis and may serve as a precursor to pyroshock testing. The differences between a half-sine waveform, maintained-contact rippling, and separated contacts are significant. Notably, even during intervals of zero force, the structure remains in a state of vibration, and any re-contact will affect this ongoing vibration.
When a precise model and test article experience identical forces and initial conditions, the result is a unique response for each measured history. Conversely, an approximate model may exhibit varied errors as different inputs stimulate various frequencies, amplitudes, or nonlinear responses.
Half-Sine Waveform Limitations
While a finite half-sine waveform is considered broadband, it possesses spectral zeros at specific intervals. The frequency ratio is defined as f/fo = 3, 5, 7, etc., with fo representing the reciprocal of twice the pulse duration. The mere visual appearance of this waveform does not guarantee effective excitation.
Significance of Measured Force History
The spectral content is influenced by the history of the measured force. Each spectrum can be normalized to its DC value, and crest rippling can enhance the spectral content, helping to mitigate the notches within the half-sine waveform. The timing between separated contacts can result in deterministic interference. To evaluate adequacy, it is essential to inspect the unnormalized measured spectrum and the signal-to-noise ratio of the response.
Guidelines for Test Adequacy
To ensure the effectiveness of the hammer test, the following practices are recommended:
- Accurately measure the applied force while ensuring adequate bandwidth, calibration, dynamic range, and synchronization over time.
- Confirm that response measurements possess sufficient bandwidth and signal-to-noise ratio, and that they are neither clipped nor saturated.
- Validate that the excitation is adequate across the necessary frequency, amplitude, and nonlinear response ranges relevant to the test objective.
- Only consider rejecting or repeating a test if there is a specific deficiency identified in measurement, excitation, or coverage.
Assessing Hammer Test Validity
The appearance of a half-sine waveform does not serve as a definitive measure of validity or adequacy. Each hammer test should be evaluated based on its specific objectives and the accuracy of the complete force and response measurements throughout the required range.
Source: NASA
