Browsing by Author "Chen, Hongyu"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
- Modeling of Microstructures and Stiffness of Injection Molded Long Glass Fiber Reinforced ThermoplasticsChen, Hongyu (Virginia Tech, 2018-11-19)An enhanced demand for lightweight materials in automotive applications has resulted in the growth of the use of injection molded discontinuous fiber-reinforced thermoplastics. During the intensive injection molding process, severe fiber breakage arises in the plasticating stage leading to a broad fiber length distribution. Fiber orientation distribution (FOD) is another highly anisotropic feature of the final injection molded parts induced by the mold filling process. The mechanical and other properties can be highly dependent on the fiber length distribution and fiber orientation distribution. The residual fiber length in the final part is of great significance determining the mechanical performances of injection molded discontinuous fiber reinforced thermoplastic composites. One goal of this research is to develop a fiber length characterization method with reproducible sampling procedure in a timely manner is described. In this work is also proposed an automatic fiber length measurement algorithm supported by Matlab®. The accuracy of this automatic algorithm is evaluated by comparing the measured results using this in-house developed tool with the manual measurement and good agreement between the two methods is observed. Accurate predictions of fiber orientation are also important for the improvement of mold design and processing parameters to optimize mechanical performances of fiber-reinforced thermoplastics. In various fiber orientation models, a strain reduction factor is usually applied to match the slower fiber orientation evolution observed experimentally. In this research, a variable strain reduction factor is determined locally by the corresponding local flow-type and used in fiber orientation simulation. The application of the variable strain reduction factor in fiber orientation simulations for both non-lubricated squeeze flow and injection molded center-gated disk, allows the simulated fiber re-orient rate to be dependent on the local flow-type. This empirical variable strain reduction factor might help to improve the fiber orientation predictions especially in complex flow, because it can reflect the different rates at which fibers orient during different flow conditions. Finally, the stiffness of injection-molded long-fiber thermoplastics is investigated by micro-mechanical methods: the Halpin-Tsai (HT) model and the Mori-Tanaka model based on Eshelby's equivalent inclusion (EMT). We proposed an empirical model to evaluate the effective fibers aspect ratio in the computation for the fiber bundles under high fiber content in the as-formed fiber composites. After the correction, the analytical predictions had good agreement with the experimental stiffness values from tensile tests on the composites. Our analysis shows that it is essential to incorporate the effect of the presence of fiber bundles to accurately predict the composite properties.
- Prediction of Young’s Modulus for Injection Molded Long Fiber Reinforced ThermoplasticsChen, Hongyu; Baird, Donald G. (MDPI, 2018-08-06)In this article, the elastic properties of long-fiber injection-molded thermoplastics (LFTs) are investigated by micro-mechanical approaches including the Halpin-Tsai (HT) model and the Mori-Tanaka model based on Eshelby’s equivalent inclusion (EMT). In the modeling, the elastic properties are calculated by the fiber content, fiber length, and fiber orientation. Several closure approximations for the fourth-order fiber orientation tensor are evaluated by comparing the as-calculated elastic stiffness with that from the original experimental fourth-order tensor. An empirical model was developed to correct the fibers’ aspect ratio in the computation for the actual as-formed LFTs with fiber bundles under high fiber content. After the correction, the analytical predictions had good agreement with the experimental stiffness values from tensile tests on the LFTs. Our analysis shows that it is essential to incorporate the effect of the presence of fiber bundles to accurately predict the composite properties. This work involved the use of experimental values of fiber orientation and serves as the basis for computing part stiffness as a function of mold filling conditions. The work also explains why the modulus tends to level off with fiber concentration.