DIRECT FIBER MODEL VALIDATION: ORIENTATION EVOLUTION IN SIMPLE SHEAR FLOW

Direct particle models are a promising tool for predicting microstructural properties of fiber reinforced composites. In order to validate our modeling approach for fiber orientation prediction, compression molded reinforced Polypropylene samples were subjected to a simple shear flow in a Sliding Plate Rheometer. Micro computed tomography was used to measure the orientation tensor for deformations up to 60 shear strain units. The fully characterized microstructure at zero shear strain was used to reproduce the initial conditions in the particle simulation. Fibers were placed in a periodic boundary cell and a flow field matching the experiment was applied. Samples created with the proposed compression molding technique showed repeatable and controlled initial orientation. The model showed good agreement with the steady state orientation; however, it showed a faster orientation evolution at the start of the shearing process. Introduction Computational tools to simulate the processing of fiber reinforced composites have become indispensable for the automotive industry. The ability to accurately predict the microstructure of molded components is a key factor not only for design calculations but also for addressing issues such as shrinkage and warpage before mold fabrication. Almost all commercially implemented models (Folgar-Tucker, Anisotropy Rotary Diffusion (ARD) Reduced Strain Closure (RSC)) use experimentally determined fitting parameters. However, these experiments are costly, lengthy and limited in the amount of information they can provide. Particle level simulations have been used in the past to aid in the understanding of these type of systems [1, 2]. In these simulations, each fiber is modeled individually as a chain of rigid elements. Fibers are placed in a predetermined flow field and hydrodynamic forces, as well as fiber-fiber interactions, are computed to predict fiber motion [3, 4]. Obtaining parameters numerically has advantages over obtaining them experimentally. In a numerical setup, all parameters can be accurately controlled, detailed information is always available and the simulations are relatively inexpensive to perform. An additional advantage of these models is the high accuracy that is reached by modeling the actual motion of individual fibers [5]. The objective of this paper is to provide reliable fiber orientation evolution data in a welldefined simple shear flow to aid in the validation and development of a multi-particle model for reinforcing fibers. Simple shear was chosen since it is one of the fundamental flow conditions present in most polymer processes. Selecting it allows us to directly correlate the rate of deformation with the filler’s behavior. Compression molded glass fiber-reinforced polypropylene samples were sheared in a Sliding Plate Rheometer following Cieslinski et al. [6]. As has been shown by the same author, compression molding is not a suitable sample preparation method since it has no control over the planar orientation of the fibers. In this work, we will therefore present a compression molding technique which ensures a controlled and repeatable initial fiber orientation for shear experiments. Results from both simulation and experiment are compared.