Historically, composites have been used in applications where cost is not a major design consideration. These composites have traditionally been thermosetting matrix composites (TSPCs), but the inherent limitations of TSPCs, most notably the inability to be joined by fusion welding and high manufacturing cost, have led to a surge in the usage of thermoplastic matrix composites (TPMCs) [2]. TPMCs have a number of advantages over TSPCs: higher fracture toughness, better resistance to wet environments, shorter processing cycles which lower manufacturing cost, recyclability [3]. Of consequence for this work is their ability to be melted and re-solidified which makes TPMCs suitable candidates for fusion welding processes. Fusion welding can be broken into two categories: bulk heating, and frictional heating. Bulk heating involves common processes like autoclaving and compression molding, frictional heating includes spin welding, vibrational welding and ultrasonic welding [2]. The technique studied in this work is ultrasonic spot welding (USSW). In this joining process, the energy transferred to a thermoplastic material subjected to ultrasonic vibrations is dissipated through intermolecular friction, leading to heating of the material and localized melting. An ultrasonic welder typically consists of six main components: a base-plate, pneumatic press, control system, transducer, booster, and welding horn [4]. The ultrasonic spot welder used in this work is a 20 kHz Dukane iQ series Ultrasonic pneumatic press 43Q-220 (Rapid City, SD, USA) (Fig. 2.1). When joining materials through USSW, a large range of weld qualities can be obtained by varying weld parameters. Taking this into consideration, a study was performed using varying weld parameters. The parameters used in the ultrasonic welding process were correlated to the quality of the welds created evaluated by lap shear testing in order to select suitable weld parameters. In general, the mechanical behavior of composite materials differs based upon the loading direction. The behavior of the TPMC used in this work was initially evaluated along two perpendicular loading directions. The mechanical response of the material under tensile loading was evaluated with respect to a fiber orientation that was assumed to be caused by the rolling step in the processing of the TPMC sheets. The orientation of the fibers in a material is of consequence when evaluating mechanical properties, particularly for short fiber composites, a common type of TPMC. Changes in fiber orientation are related to a number of factors, such as the geometrical properties of the fibers, viscoelastic behavior of the matrix material, and the change in shape of the material produced by the processing operation [5]. With processes varying between manufacturers, a means of characterizing fiber orientation is necessary. To evaluate the fiber orientation in the TPMC used in this work, the optical methods of x-ray micro computed tomography (μCT) and scanning electron microscopy (SEM) were employed. These methods were used to characterize the fiber orientation of a TPMC before and after ultrasonic spot welding of the material. Fig. 2.1 20 kHz Dukane iQ series ultrasonic pneumatic press 43Q-220 used for this work 12 J.R. Newkirk et al.
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