One of the limitations of conventional modal testing using a roving impact hammer is that the reference sensor (usually an accelerometer) must remain fixed throughout the test. Since the accelerometer must be connected by a wire to the data acquisition system. a very long wire may be required when testing a large structure. Furthermore, better quality signals are possible if each impact force is applied closer to the response accelerometer. Because it does not require a fixed reference sensor throughout the test, a Rapid Impact Test is faster and easier to use on any size structure.
In this new method, either the impact hammer or the accelerometer can be moved to a different DOF between acquisitions of data. One sensor can be “hopped over” the other in slinky fashion, or both can be moved, provided that a chain of FRFs is calculated from the acquired data. Each FRF has two DOFs associated with it. An FRF chain is formed when each FRF has the same DOF as another FRF.
An FRF chain is a set of multi-reference FRFs. The multi-reference FRFs can be curve fit using single-reference methods, but the resulting modal residues must be further processed to obtain mode shapes. The residue post-processing is based on the relationship between modal residues and mode shapes. Examples using an impact hammer, uni-axial accelerometer, and 2-channel data acquisition, and also using an impact hammer, tri-axial accelerometer, and 4-channel data acquisition are included in this paper.
