Feasibility study of performing experimental modal analysis with oblique impact testing using various oblique impact directions

Abstract

Oblique impact excitation has been introduced in Experimental Modal Analysis (EMA), with the great advantage of reducing the conventional EMA’s testing time by a factor of three. One major constraint of this technique is the investigation of the effect of various oblique impact directions towards its accuracy in determining the structural dynamic characteristic. This feasibility study is difficult to be achieved in practice, as it involves a lengthy amount of experimental works using various oblique impact directions. To solve this problem, a mathematical model has been developed to synthesize the FRF due to oblique impact (i.e. oblique FRF) in this study. The synthesized oblique FRFs show great agreement with the measured oblique FRFs in various oblique impact directions, which validate the reliability of the usage of the proposed synthesis method. The performance of the oblique impact testings using various impact angles is investigated. The results show that the oblique impact testing has a high success rate to extract directional modes in many impact directions, however wrong selection of the impact direction will lead to mode estimation failure. Good selection of impact direction based on force and modal strengths are demonstrated to ensure an accurate estimation of the structural dynamic characteristics. Keywords: Dynamic characteristic, Frequency response function synthesis method, Modal testing, Oblique angle, Oblique impact, Tri-axial normal impact testing