A multipreferred fiber orientation constitutive model for fiber mat-reinforced thermoplastics with a random orientation applied to the stamp thermohydroforming process

This work focuses on the development of a constitutive relationship for the modeling of a multi-preferred fiber orientation sheet that has several different primary fiber orientations, none of which are necessarily mutually perpendicular prior to, or during, deformation. One of the goals was to develop the constitutive relationship for the deformation behavior of the fiber mat reinforced thermoplastics with a random orientation, a material that is starting to gain in popularity but has not been extensively investigated. Two different types of mat fiber reinforced material were investigated; one a continuous fiber mat and one a chopped fiber mat, both with a polypropylene matrix. Both materials were characterized through a series of squeeze flow and uniaxial tensile tests to determine the preferred fiber orientations as well as the material properties. The constitutive model was implemented through a user-subroutine into the commercial finite element analysis code ABAQUS/Explicit and the numerical results were validated against experimental stamping results. Overall, the multi-preferred fiber orientation constitutive relationship was able to accurately capture the material instabilities that occurred during the stamping process. Since the mat fiber reinforced materials have not been extensively investigated this research creates one of the building blocks that can be used to develop more accurate models in the future. With the addition of a constitutive relationship for the interaction between the layers, this single layer model could be expanded into a constitutive relationship for the full sheet.; In addition to the constitutive modeling aspect of this work there is also an experimental portion that deals with the development, design, build and verification of a new processing method for the shaping and forming of fiber reinforced thermoplastic materials, stamp thermo-hydroforming. Experimentation demonstrated that the process provides a 7–10 percent increase in draw depth when applying small levels of counteracting pressure. In addition, the use of a counteracting hydrostatic pressure during forming led to less delamination of the material. Overall the experimental results demonstrate that the stamp thermohydroforming process is a viable method for shaping thermoplastic materials that warrants additional attention. When the numerical and experimental portions are coupled together a unique design tool is created. The numerical results can be used to eliminate some of the trial and error associated with experimental work and the experimental portion can be used to validate changes to the numerical simulation.