Development and Validation of Experimental Modal Analysis with Fixture-Free Oblique Impact Testing

Experimental modal analysis (EMA) with oblique excitation (e.g. oblique impact testing) is useful in improving the long testing time problem of conventional EMA with normal excitation (e.g. tri-axial normal impact testing). The long testing time is mainly due to the redundant procedure of performing conventional EMA at 3 principal coordinates, in order to extract all important dynamic characteristics of a 3D complex structure. In this study, a new methodology involving vector projection method is introduced to find the driving point frequency response function (FRF) in the oblique direction, without the need of special fixture with oblique-oriented impedance head. Hence, it presents a low cost and practical solution to scale the mode shape, as compared to the traditional approach. Moreover, the concurrent forces characteristic of the oblique excitation is used in the development of the theoretical relationship between the FRF with oblique excitation and normal excitation. This is important for the validation of the oblique impact testing result, such as the FRF and modal parameter estimations. Experimental results show that the oblique impact testing has reliable result, as compared with the tri-axial normal impact testing in term of the FRF correlation, natural frequency discrepancy, modal damping ratio error and modal assurance criterion (MAC) of the unit modal mass (UMM) mode shape.