Structural Health Monitoring of Beam Structures using Shaped Sensors Michael I. Friswell a,1 , Sondipon Adhikari a a School of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP, UK Abstract: This paper is concerned with distributed sensors to measure the response of beam structures. The design of modal sensors for beam structures is well established. Most applica- tions consider vibration control, but this paper is concerned with structural health monitoring, where the sensor output is made sensitive to changes in key stiffness parameters, for example in joints. The procedure is based on finite element models of beams, and thus the distributed transducers may be designed for arbitrary beam structures. A simulated beam excited by a rotating machine is used to demonstrate the approach and a Monte Carlo method is used to highlight the performance of the sensor in identifying damage. Key words: Piezoelectric, Beams, SHM 1. INTRODUCTION The idea of using modal sensors and actuators for beam and plate type structures has been a subject of intense inter- est for many years. Using modal sensors in active control reduces problems of spillover, where high frequency unmod- elled modes affect the stability of the closed loop system. The sensors and actuators may be discrete or distributed, and are usually manufactured using piezoelectric material, such as polyvinylidene fluoride (PVDF) film. For example, a modal sensor for a beam type structure may be obtained by varying the sensor width along the length of the beam. If the sensor covers the whole beam the shape of the sensor may be derived using the mode shape orthogonality property [19, 5]. Modal sensors may be designed that cover only part of the beam [9], or are segmented sensors sensitive to multiple modes. The ef- fect of geometric tolerances during manufacture on the qual- ity of the sensors may be determined [9]. For beam structures the width of the sensor may be parameterized using the finite element method and the underlying shape functions used to approximate the transducer shape [10]. Modal sensors for two-dimensional structures have also been designed. The approach used for beams may be im- plemented by varying the thickness of the PVDF, although this is very difficult to achieve in practice. Sun et al. [24] replaced an actuator layer with variable thickness by many small segments of uniform thickness. Kim et al. [17, 18] de- veloped two design methods for distributed modal transduc- ers for composite plates, the first using multi-layered PVDF Email addresses: m.i.friswell@swansea.ac.uk (Michael I. Friswell), s.adhikari@swansea.ac.uk (Sondipon Adhikari) URL: http://michael.friswell.com (Michael I. Friswell) 1 Corresponding author films with optimized electrode pattern, lamination angle, and poling direction, and the second using PVDF film segments and an interface circuit. Preumont et al. [22] introduced the porous electrode concept, which allows the gains to be introduced by changing the local effectiveness of the elec- trodes. The alternative is to design a distributed modal trans- ducer by optimizing the continuous boundary shape of a con- stant thickness PVDF film by assuming a smooth boundary [15, 16, 25]. This paper uses the design methods for modal sensors for beam structures, but extends this approach to the structural health monitoring application. The identification of the location and severity of cracks, loose bolts and other types of damage in structures using vi- bration data has received considerable attention [7]. Most of the approaches use the modal data of a structure before damage occurs as baseline data, and all subsequent tests are compared to it [4, 8, 12, 13]. Any deviation in the modal properties from this baseline data is used to estimate model parameters related to the damage severity and location. The advantage of using this baseline data is that some allowance is made for modelling errors. However changes in the struc- ture not due to damage, for example due to environmental effects, will be difficult to distinguish from changes due to damage [11, 23]. The approach adopted in this paper is to use shaped transducers to reduce the sensitivity of the sensor out- put to the unmodelled parameter changes and environmental effects. For structural health monitoring this means that the response can be made sensitive to particular regions of inter- est, so that, for example, the sensor may be used to monitor the health of a single joint. The method is an extension of the selective sensitivity technique which was developed to design excitations that produce strong sensitivities to a subset of the Proceedings of ICEAE 2009