A Comparison of Multiple Impact Testing Methods

Abstract

Impact testing has proven to be a valuable tool for performing modal tests, especially when the frequency response functions from many single-input impact tests are combined together to help extract all observable modes from a structure. This type of testing is historically known as multiple reference impact testing (MRIT) when multiple response measurements are acquired for each test. In each of these single-input tests, the impact hammer is used to excite the structure once per frame of data and all response signals are allowed to decay to zero before the frame ends.

Impact testing may also involve situations where there is more than one impact per frame, whether the multiple impacts are due to a single impact hammer impacting the test article multiple times, multiple impact hammers impacting the structure once each per frame, or multiple impact hammers impacting the structure multiple times per frame. The potential reason for performing multiple impacts per frame of data is to introduce more energy into the test article. The potential reasons for using multiple impact hammers are, as with multiple shaker testing, to decrease testing time and to distribute energy throughout the structure.

This paper investigates and compares testing results from each of the testing methods described above, and then discusses the signal processing issues involved with each method. Single impact per acquisition frame methods provide the highest coherence levels at the peak frequencies. However, implementing different signal processing methods can significantly improve coherence at resonance in the case of the multiple impact per frame tests. Some clear benefits in frequency response function quality can be observed when comparing the various impact techniques.