1O.1109ULTSYM.2014.0091 Ultrasonic assembly of short ibre reinforced composites Marc-S. Scholz Bruce W. Drinkwater Richard S. Trask ACCIS, Dept. of Aerospace Engineering University of Bristol, Queen's Building University Walk, Bristol, BS8 1TR United Kingdom Dept. of Mechanical Engineering University of Bristol, Queen's Building University Walk, Bristol, BS8 1TR United Kingdom ACCIS, Dept. of Aerospace Engineering University of Bristol, Queen's Building University Walk, Bristol, BS8 1TR United Kingdom Email: M.Scholz@bristol.ac.uk Email: B.Drinkwater@bristol.ac.uk Email: R.S.Trask@bristol.ac.uk Abstract-Taking a counter-propagating wave approach, a new type of device was developed to fabricate thin layers of anisotropic material. To investigate the effects of various design parameters, enhance device performance, and improve the composite fabrication process, inite element (FE) analysis was employed. Speciically, the COMSOL Multiphysics package was used to couple together the equations of structural mechanics, piezo-electric devices, and pressure acoustics in a single model. Special attention was paid to the shape and quality of the acoustic standing wave ield, the magnitude of the resulting radiation forces, and the response of ibrous particles to ultrasonic pressure gradients. Further, the formation of structurally interesting ibre architectures was explored by studying the possible standing wave patterns in the device's particle manipulation cavity. I. INTRODUCTION Ultrasonic assembly may be deined as the process of dis tributing one or more small objects according to predetermined patterns through the external application of acoustic radiation force ields. With applications most cnonly found in the biological and life sciences, ultrasonic devices are generally tailored to and optimised for the aquaeous environment. Only recently, interests have also focused on the manipulation of particles in polymeric compounds (e.g. acrylics, agar, epoxy, polyester, polysiloxane) [1]-[7] , and the manufature of meta materials via the ultrasonic assembly process [8], [9]. In this study, we analyse a new type of ultrasonic device that allows for the rapid and repeatable fabrication of thin layers of anisotropic short ibre reinforced composite mate rials, both experimentally (section II) and using FE analysis (section III). Initially, the device concept is introduced, the acoustic ield is visualised experimentally, and the ultrason ically assisted fabrication of discontinuous ibre composites is outlined. An extension of the device design is then dis cussed together with the generation of more intricate ibre architectures. A linear acoustic FE model is used to idntify key design parameters allowing for further enhancements in device performance and to improve composite manufacturability. II. EXPERIMENT A. Ultasonic device The ultrasonic device used for the manufacture of short glass ibre reinforced composite lamina [7], and further exper imental and FE investigations, here, is shown in Figure 1. Two 978-1-4799-7049-0114/$31.00 ©2014 IEEE 369 Fig. 1: Ultrasonic device. A standing wave acoustic ield is formed inside a central particle manipulation cavity from two counter-propagating waves. Two opposing PZT transducers are placed inside the two adjacent cavities on either side of the device, and held in place by small compressions springs. For cooling purposes,the active elements may be submerged in water. 0.975 mm X 15 mm x 2 mm soft-doped lead zirconate titanate (PZT) (Noliac group, NCE51) elements were placed on op posite ends of the device, each separarted from the cen tral cavity by a 5 mm sacriical poly(methyl methacrylate) (PMMA) boundary. On either side of the device, a further water illed chamber served the purpose of a necessary heat sink at high driving voltages. The PMMA frame was mounted on a glass substrate using adhesive tape (tesa 64621-00007-01). For both acoustic drivers to be more easily recycled, a small compression spring secured each of them against the adjoining PMMA boundary; this resulted in a signiicantly increased composite production eficiency. The central cavity measured 30 mm x 15 mm x 2 mm, a size large enough to manufacture test specimens for mechanical characterisation. To operate the device, a voltage of 80 V pp was applied across the transducers, thus generating a standing acoustic ield inside the central cavity by interference of two counter-prop agating waves. While the counter-propagating wave method is not the only approach capable of generating patterns of acoustic radiation forces, it is considered to be relatively less sensitive to changes in the resonsant requencies of the device cavity in the presence of large numbers of particles, and thus most suited to the present application [10], [11]. Furthermore, the position of nodes is independent of geometry, and only their separation is ixed at half-wavelength intervals [11], [12]. B. Schlieren imaging Schlieren imaging [13] describes a technique, that enables the experimental observation of acoustic pressure variations inside an optically transparent luid volume. Given the optical 2014 IEEE Intenational Ultrasonics Symposium Proceedings