the x-axis of the sample, indicating the mechanical properties of the ESP 105 CC were roughly orthotropic. The orthotropic nature of the material under axial loading was observed in the study performed during mechanical characterization of the polypropylene composite and correlates well to these μCT findings (Fig. 2.4). From these scans, fiber diameter was approximately 13–15 microns and fiber length was approximately 270 microns. After establishing a baseline of the fiber orientation for the as-received TPMC, the effect of the USSW process on the orientation of the fibers was investigated. This study was performed on 25.4 mm 25.4 mm 1.52 mm thick TPMC samples welded using the parameters chosen during the weld parameter study (Fig. 2.2). Figure 2.6a shows a schematic of the cross-section of the joint of two sheets. The area under investigation in theμCT image of Fig. 2.6b is highlighted with the box in Fig. 2.6a. This area encompasses the region just to the right of the void left where the weld horn was inserted. Here, the orientation of the fibers of each sheet was roughly perpendicular to one another, with the top sheet in Fig. 2.6b having fiber orientation along the x-axis and the bottom sheet having fiber orientation along the y-axis. In the left region of Fig. 2.6b, which was directly in contact with the weld horn, fiber folding in the direction of travel of the horn is present. In addition, the flow of the weld has a distinct, cone-shaped region converging towards the right of the sample at the interface of the two sheets. In this region, there are two distinguishable fiber orientations relative to the flow. At the edge of the flow, the fibers are aligned with the weld flow. In the core of the weld flow, the fibers are oriented perpendicular to the direction of the weld flow. This behavior draws parallels to behavior observed in the study of injection molding of TPMCs [16]. When injection molding short fiber TPMCs, orientation of fibers is caused by differences in velocity between the two ends of a fiber. When the melt flow is homogenous, no change in the orientation of fibers will take place. When the flow is not homogenous, the variation in velocity can be perpendicular or parallel to the flow direction depending on whether the flow is convergent or divergent [16]. This behavior may lead to the differing alignment of fibers at the edge of the weld flow region. It was also apparent that the fiber orientations seen in the two pieces of the welded sample were oriented roughly perpendicular to one another, and this should be taken into consideration when analyzing the changes in fiber orientation throughout the weld affected region of the sample. From this analysis, it can be seen qualitatively that the fiber orientation is more randomly oriented in the region affected by the weld than it is in the rest of the sample and this will behavior will need to be further quantified. The fiber orientation of the ESP 105 CC composite material was further characterized using scanning electron microscopy (SEM). As-received samples were polished along the short axis of the sheet in order to view the apparent fiber orientation throughout the cross section of the sheet. A SEM image of the cross section of as-received ESP 105 CC is shown in Fig. 2.7b. In this cross section, three distinct regions exist in which the fiber orientation is grouped. In Fig. 2.7c, the fibers generally have an orientation in line with the length of the sheet, which is in agreement with the orientation observed throughμCT. There is a region in the middle of the cross section, Fig. 2.7a, which exhibits a much more random fiber orientation than observed in the outer layers, Fig. 2.7c. It is thought that these zones are a product of the rolling manufacturing method. In terms of evaluating the mechanical response of this material under loading, the interaction between these three zones will need to be characterized for the constitutive behavior of the polypropylene to be modeled effectively throughout the entire cross section. Fig. 2.6 Schematic of USSW cross section (a) with μCT field of view outlined in red, and USSW ESP 105 CCμCT image (b) 2 Characterization of Thermoplastic Matrix Composite Joints for the Development of a Computational Framework 17
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