Chapter 11 Classification of Low Velocity Impact Using Spiral Sensing Technique Chijioke Agbasi and Sourav Banerjee Abstract In this paper, the non-linear elastodynamics of a flat plate subjected to a low velocity foreign body impact is studied experimentally. The work is based on a central hypothesis that in addition to identifying the impact locations, the material properties of the foreign objects can be classified using acousto-ultrasonic signals. A novel cluster of thin piezoelectric sensors is proposed and a carefully formulated dominant frequency approach is studied to investigate the nonlinearities. Such nonlinearities with their highest resolution are quantified with the proposed Theodorus spiral configuration of the sensors (TSSC). It is found that the frequency and speed of the guided wave generated in the plate can be quantized based on the impactor’s relationship with the plate, i.e. the wave speed and the impactor’s mechanical properties are coupled. In this work, in order to characterize the impact location and mechanical properties of impactors, nonlinear transient phenomenon is empirically studied to decouple the understanding using the dominant frequency band (DFB) and lag coefficients of the acousto-ultrasonic signals through TSSC. Next the understanding was correlated with the elastic modulus of the impactor to predict transmitted force histories. Keywords Impactor • Theodorus spiral • Acousto-ultrasonic • Elastodynamic • Dissipation 11.1 Introduction Over the years, studying impact events and its effect on the host structure have become an important topic, especially in the aerospace industries. Space operating vehicles and aircrafts often receive impacts from the space debris present in the lower and greater earth orbit. However, to estimate the degree of damage in the real time, no comprehensive method exists that can characterize the transmitted force and the elastic modulus of the impactors during the impact, accurately. In this work, we show that this can be achieved through a data driven modified model. A key fact to note is that the coupled nature of the mechanical properties of the impactor and the structure influences the transmitted force in the structure and hence the degree of damage. Transmitted acousto-ultrasonic signals (AUSs) from the impact event can be processed to extract quantitative features that can characterize the physical properties by implementing ad-hoc signal processing algorithms [1, 2]. To localize the foreign impacts, generally, the aero structures are idealized to a flat plate in the laboratories and are considered in this work too. Boundary conditions of the plate, the geometry and the mechanical properties of the impactors and the plate, the impact velocity, etc. influence the nature of transmitted energy in the plate in the form of guided waves. Transient displacements, contact area and transmitted stresses are unknowns and the physical events are simultaneously influenced by the impactor and plate’s response [3]. Most analysis separates the problem into a local contact problem at the impact location and a global problem of the plate’s response, which does not capture the true phenomena. Here, we study the influence of spherical impactors with different mechanical properties on the impact force history and the transient elastodynamics of an isotropic plate undergoing low velocity impact, empirically. To provide sufficient data for the empirical model, the impact events were sensed using a Theodorus spiral configuration of piezoelectric sensors (TSSC) in passive mode. Conventionally, the Hertzian contact law is implemented for the C. Agbasi (*) • S. Banerjee Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA e-mail: agbasi@email.sc.edu; BANERJES@cec.sc.edu N. Sottos et al. (eds.), Experimental and Applied Mechanics, Volume 6: Proceedings of the 2014 Annual Conference on Experimental and Applied Mechanics, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-06989-0_11, #The Society for Experimental Mechanics, Inc. 2015 79
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