Comparing Steady State and Impulse Test Methods to Measure the Damping of Composites Applied to Homogeneous Substrates

Long bar testing provides useful damping performance measurements of materials used for vibration control. The testing is often conducted with either shaker-based steady-state excitation or hammer-driven impulse excitation depending on the lab performing the study and the specific goals for the testing. This study was done to help identify and quantify the differences in the data acquired through both techniques while gaining a clear understanding of the benefits and limitations of each approach. Both methods were used on identical substrates in repeatable conditions to help isolate the differences. Specific variables of each method were then explored individually in various conditions to characterize their effect on the respective method. While the data provided by the hammer testing was simpler to acquire and more coherent, it was not reliable for higher frequency ranges on some of the tested substrates. The main limitations in the shaker method result from added mass to the system through mounted transducers and wiring, poorer coherence due to the length, stiffness, and damping properties of the chosen stinger, and reduced modes excited by transducer and excitation location choice. Recognizing the merits of both methods, hammer testing was chosen for on-going quality control of an aircraft skin damping composite.