Experimental comparison of individual excitation of closely spaced modes using sine sweep and sine burst testing techniques

With the goal of independently controlling the peak responses of 2 closely spaced modes to achieve correct test loads, an experimental comparison of the typical swept sine method and a proposed alternative near-resonance burst sine method was conducted. A test article exhibiting 2 modes, an in-phase mode and an out-of-phase mode, with natural frequencies of 4 Hz separation was built and subjected to swept and burst sinusoidal vibration tests. Mode frequencies and peak responses were verified with finite element analysis techniques. The swept sine input demonstrated a high degree of control over the peak response level of either mode whereas the sine burst input excited only the in-phase mode. The sine burst test method presented an attractive alternative to the swept sine test method if excitation of an in-phase mode was desired while avoiding excitation of an out-of-phase mode. The comparison provides a guideline for strength qualification testing of aerospace structures.