Mass–stiffness change method for scaling of operational mode shapes

In Operational Modal Analysis (OMA) the structure is excited by unknown forces and only the response signals are measured. Therefore, the mode shapes cannot be scaled straightforward from the test. The mass change method is a well-known way to estimate the scaling factors of mode shapes by adding mass to the selected points of structure. In the mass change method, the scaling factors are estimated using the frequency shift due to the added masses to the structure. However, in the mass change method the frequency shifts for the first modes of structure are less than those of the higher modes based on the sensitivity analysis. In the conventional scaling methods, the mode shapes are required to change slightly by the mass change for all the modes under investigation. As a result, the first modes do not experience a significant frequency shift and they cannot be scaled precisely. In this paper a method is applied for scaling of the operational mode shapes on a basis of both mass and stiffness change of the test structure. It is shown that the advantage of the applied method is that the first modes of the structure are more affected by the stiffness changes and consequently the corresponding scaled mode shapes are more accurate. The numerical and experimental results show that the accuracy of the scaled mode shapes improves by using the proposed mass–stiffness change method.

► In this study a new application of mass–stiffness change method is proposed.
► It is shown that the first modes of the structure are more affected by the stiffness changes.
► It is shown that the significant frequency shifts can be achieved for the first modes of the structure.
► The method is very useful for scaling the operational mode shapes especially for the first modes.