UBC Theses and Dissertations
Core movement during processing of sandwich panels Pawson, Duncan Joseph
Core movement is a phenomenon that plagues sandwich panel fabrication during autoclave processing. External pressure can result in deformation of the core in its weak, lateral direction. This in turn, drags the facesheet plies inward. Parts displaying notable core movement are unfit for service and must be scrapped. Restraining plies, thereby preventing core movement, has become common practice in industrial processing of sandwich panels. Progress has been made in mitigating the risk of core movement through artificially increasing pressure within the core as well as through development of high friction resin/fiber systems. However, a complete understanding of the physics of the problem is still lacking. The focus of this research is to understand how processing conditions affect core movement and to gain a better understanding of the fundamental nature of core movement. Parameters investigated include, temperature, pressure, and a variety of structural features such as tie downs, core machining stabilization, and core chamfer angles. The problem is investigated using novel techniques. In-situ data collection is performed through use of displacement and in-core pressure sensors. Moreover, the entire event is filmed in real time using an autoclavable camera. This allowed for identification of the exact processing parameters during initiation and progression of core movement. Individual ply movement is determined post-processing. It was shown that failure pressure is dictated by viscosity at lower processing temperatures, but at high temperatures this is no longer the case. This implies a shift in the lubrication regime and alters the mechanics of the problem. A basic mechanical model describing the core movement process is outlined and the conditions necessary for core movement initiation are proposed. It was shown that core movement initiates at the chamfer radius and fluctuates between progressing through the core and down the chamfer edge. The results suggest that friction is the governing mechanism behind core movement initiation, which, if true, has important ramifications for sandwich panel design. This work builds off previous research and lays the foundations for future work.
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