An instrumented hammer test does not need to reproduce an ideal half-sine wave. When the applied force and the structural response are both measured accurately, ripples in the crest and separated contacts are recorded as known inputs to the test rather than invalidating it. The force shape changes which modes get excited, but that does not make the record unusable.

The argument starts from common practice in impact testing for identifying natural frequencies and frequency response functions: hammer strikes whose force shape departs from the ideal half-sine pulse tend to be discarded. The technical note from NASA's Langley Research Center argues that this criterion is too restrictive when the instrumentation captures both the force signal and the system response in full.

The key is measuring both signals accurately. With that in place, imperfections in the strike —ripples on the force crest or contacts that separate and close again— stop being unknowns and become part of the test input. The change in force shape alters the excitation spectrum, but the record remains usable for extracting dynamic information from the structure.

The practical implication is a wider margin in impact tests on aerospace structures: fewer discarded strikes and more flexibility in how the part is excited. The contact listed for the technical note is Joel W. Sills, at NASA's Langley Research Center.