| This thesis discusses the orientation characteristics of Poly (ethylene terephthalate), PET, and deals with the development of a strain energy function. The strain energy function thus developed can be utilized in carrying out finite element analysis of the blowing process, as encountered during orientation and stretch blow molding of the PET preform into the container. An alternative stress-strain model has also been given which fully characterizes the stress-strain behavior of PET, especially at higher extensions where the stress levels show a steep increase with increase in the amount of extension.; A systematic way of showing the sequence of preform blow molding has been discussed. This approach can be used to Obtain an estimate of the free blow or the natural draw ratio for any particular set of temperature and extension rate-from the corresponding stress-strain curve. Also this methodology, together with the stress-strain model, can be extended to simulate the preform blowing process, and thereby calculate the material distribution and the wall thickness of the blown container.; A thorough investigation of the mathematical models available such as Rivlin's formulation and Ogden's Model, was carried out and based on the analysis of those models, as applied to the PET data, a simple strain energy function was proposed. It was found that the constants of the functional form proposed could be easily obtained by linear regression as compared to other complicated forms available at present. The problem with this type of strain energy function is to find a relation between the constants and the orientation conditions. Also the stress-strain behavior in the sequential biaxial mode makes the situation very complicated to handle, and therefore a unique stress-strain model was developed keeping in mind the above encountered difficulties. This stress-strain model takes into account the effects of different modes of extension, including the sequential biaxial mode, as well as the effects of temperature, extension rate, moisture content, and amount of extension. The model was developed in terms of two important characteristics of the stress-strain curve, namely, the yield stress and the strain hardening point. In total four parameters were used in this stress-strain model, including yield stress and strain hardening point. The model thus also characterizes the strain hardening region of the stress-strain behavior of PET, which is of the utmost importance to a design engineer. (Abstract shortened by UMI.)... |
