Non-linear finite element analysis of apple packaging : a thesis presented in partial fulfilment of the requirements for the Master of Technology in Technology and Engineering at Massey University, Palmerston North, New Zealand
The New Zealand Apple and Pears Marketing Board exports apples to over 50 countries, and is one of the New Zealand's largest export earners, netting approximately 500-600 million dollars per year. Competition on the overseas market is very high, and the importers set strict requirements on the condition of the fruit being exported. Therefore, this fruit must be in optimum condition, wherein packaging methods play a vital role to protect the fruit during handling and transit. The packaging is very important to protect the fruit, but little research has been done on its physical and mechanical behaviour. The apple tray is the core object and an integrated part of apple packaging made out of paper pulp, called "Friday Trays", and here is studied for its mechanical and physical behaviour. The project has two phases, testing the material at different environmental conditions and stress analysis of the tray by using "Finite Element Analysis" technique. In the first phase of research, since the "Friday Trays" are handled at different temperatures and humidity levels, the paper pulp material was tested for its mechanical properties such as Stress, Strain, Young's Modulus and Creep at different moisture content levels. The physical and mechanical properties of Paper Pulp materials are affected by moisture content, which is dependent on the humidity of the surrounding environment. The function of the trays is, firstly to transport the apples from conveyor to apple boxes and remove them from the boxes and secondly to act as a cushioning between apples of different layers to prevent damage. In the second phase of the research, the behaviour of the trays was studied in the above conditions by using finite element analysis. The technique was chosen because it can model very complex shapes. The results are displayed in graphical format and can see with maximum stresses or displacements highlighted. The Z pack 70 count trays are investigated at two different conditions of mechanical properties, at 8% moisture content and 20% moisture content. Creation of FEA models are challenging because of the complexity of the problem and vast size of the computer files. The handling situations are successfully generated and used to investigate the relative advantages of packing fully loaded trays, which are then supported at different positions. The tray performed better while being picked up by the ends of the tray and at 8% moisture content. In this situation, the deflection in trays was less when compared to the other situations and the apples less likely to fall off the tray. It was seen that the apples themselves protected the tray from bending during handling, brought about by the apples making contact with each other. Following from this a complete shell model was generated for holding full apples that has contact between neighbouring apples. This used non-linear controls and slidelines. The results show the apples contacting and supporting each other and produce less impact in terms of load on the tray. Analysis of Friday tray is far from complete, greater computer resources will be needed, and has to be checked for the response of lower material properties. Future work should concentrate on developing a dynamic model of a full carton of apples. From the dynamic model, conclusions can be made about the behaviour of trays within the carton.