Estimation of rotational frequency responses

Abstract

Rotational frequency response functions are potentially valuable for structural damage detection but are difficult to obtain because of the practical problems of applying a pure moment. In the literature various indirect methods of generating a moment have been presented. In one method a so-called 'T-block' is attached at the point where a moment input is required. Then a force applied to the T-block generates a moment together with a force at the connection point. As well as introducing a moment the T-block may also be used to measure an angular displacement. However, it is the rotational frequency response of the structure without the T-block that is required. The problem may be cast as a special case of excitation by multiple inputs determined from the applied forces and the stiffness and inertia properties of the T-block. The resulting equations are badly conditioned, but may be regularised using other measurements. The paper describes the methodology and the signal processing techniques required to estimate the rotational frequency response function. An experimental example is used to demonstrate the practical application of the method.