Qiming Lou

Degree Name

Master of Science (MS)

Semester of Degree Completion

1995

Thesis Director

Ping Liu

Abstract

This thesis studied the nonlinear micromechanic behavior of thermoplastic composite of recycled high density polyethylene (HDPE) reinforced with short glass fibers using finite element method. The composite material was modeled using a micromechanic unit cell to simulate the stress distribution between the plastic matrix and the fiber in the composite. Nonlinear behavior of recycled HDPE and imperfect bonding between the fiber and matrix were investigated. Load-bearing capability of the fiber was evaluated using stress partition ratio (SPR) in the composite models. The effect of fiber aspect ratio on the stress distribution of the composite was studied to optimize the material performance. The strength of the composites with perfect and imperfect bonding were predicted, respectively.

The following conclusions were drawn according to the nonlinear finite element analysis:

1. With the increase of external stress, the average stresses both in matrix and in fiber increased, in the cases of both perfect and imperfect bonding. The stress supported by fibers is much higher than that by recycled HDPE matrix.

2. When a composite with imperfect bonding was applied with external stress, the stress in fiber was much lower, and the stress in matrix was much higher than that in the composite with perfect bonding, at the same level of external stress.

3. Stress partition ratio can be used effectively as a means of evaluating the strength of the composite. As the fiber aspect ratio increased, the stress partition ratio increased consistently for the composite with perfect bonding.

4. In the case of imperfect bonding, the stress partition ratio increased up to a fiber aspect ratio of 150, and leveled off as the fiber aspect ratio increased.

5. Under the perfect bonding condition, the predicted strength of the composite increased consistently with an increasing fiber aspect ratio from 50 to 298.

6. Under the condition of 20% debonding, the predicted strength of the composite increased as the fiber aspect ratio increased up to 150. However, further increasing fiber aspect ratio may not raise the composite strength. Therefore, the aspect ratio of 150 for glass fiber may offer the optimum strengthening for the composite.

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