Press Forming and Formability of Polypropylene, Dry Fabrics and Fabric-Reinforced Polypropylene

With recent technological advancements, the need for reduced manufacturing costs and improved damage tolerance of composite structures, thermoplastic matrix composites are being increasingly considered for a variety of automotive and aerospace applications. Not only do they have low density, high strength, excellent fatigue durability and design flexibility, but also lower processing time, better damage tolerance and higher recyclability compared to thermoset matrix composites. Press forming is the leading manufacturing process for making fabric-reinforced thermoplastic composite parts since it is a fast, repeatable and cost-effective process. The objective of the current research is to develop process simulation models to study press forming and predict press formability of polypropylene, glass fabric, and glass fabric-reinforced polypropylene. Polypropylene is selected as the matrix since it is a widely used thermoplastic polymer for automotive applications. Plain glass fabrics are selected as the reinforcement based on their low cost, high strength and good drapeability on mold surfaces. Press forming of polypropylene involves a warm deep drawing process which is different from thermoforming processes commonly used for thermoplastics. The forming temperature, which is much lower than the usual thermoforming temperature, and forming speed are selected to make the production rate high. A modified temperature and strain-rate dependent material model was developed to conduct the finite element simulations. It is shown that the deep drawability of polypropylene cups is influenced by the blank holder force, initial blank temperature, die corner radius and forming speed. The failure modes that limited the formability of polypropylene are flange wrinkling and tearing of the cup wall either at the top corner radius or near the bottom corner radius. To simulate fabric deformation behavior for preforming fabric shapes, a superimposed finite element model of membrane and shell elements was used with the in-plane shear behavior governed by the membrane elements and the out-of-plane bending behavior governed by the shell elements. Using this approach, a study on the effect of the shear-tension coupling on the deformation of an E-glass fabric shows that with increasing applied tensile loads, the fabric exhibits a stiffer shear response. The limits of the press forming operation are determined by the ability to form the shape and retain it without defects such as fiber distortion, wrinkling, and tearing of the fabric. A safe window in which the forming limit of the fabrics is determined as a function of blank holder force and forming depth is presented. The punch corner radius, which controls the shape of the fabric during draping, plays an important role on the forming limit of the fabric. A comparison of the experimental and simulated results of the bias-extension tests and press forming shows that the superimposed approach can reasonably accurately predict the outer shape profile and the force required for fabric deformation. A numerical method to simulate the press forming of woven fabric-reinforced polypropylene is presented. It uses the concept of a superimposed approach with the fabric layers superimposed on the matrix layers. The failure occurs first in the polypropylene layer at the location of the maximum shear deformation in the fabric layer, which is along the diagonal at the die entry radius. Fabric layer exhibits buckling tendency after failure is initiated in the polypropylene layer. Draw depths of the fabric-reinforced polypropylene do not vary much on either the forming temperature or the punch corner radius.PhDCollege of Engineering & Computer ScienceUniversity of Michigan-Dearbornhttp://deepblue.lib.umich.edu/bitstream/2027.42/175048/1/Chandra Kishore Reddy Emani Final Dissertation.pdfDescription of Chandra Kishore Reddy Emani Final Dissertation.pdf : Dissertatio