Short Fiber Reinforced Thermoplastics: Prediction of Stiffness in Injection Molded PS-PPO Blends

The prediction of stiffness in short fiber reinforced thermoplastics is stud ied as a function of fiber length using injection molded blends of PS and PPO. The theoret ical models for predicting composite stiffness are reviewed. The results are first compared with the theoretical models advanced for uniaxially aligned composites. These models predict higher than experimental values. However, agreement between the predictions and experimental values improves when the effect of fiber orientation distribution in the injec tion molded samples is taken into account and as the ductility (or the PPO content) of the matrix increases. Cox's model when used with the "laminate analogy" gives the closest prediction to the experimental stiffness. Reinforcement efficiency factor for stiffness is a strong function of retained fiber lengths. The dependence of composite stiffness on the matrix ductility and the effects of compatibility on the mechanical properties of PS-PPO blend system are also discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68613/2/10.1177_089270579100400205.pd

Short Fiber Reinforced Thermoplastics.

The PS-PPO blend system was studied systematically as a "model" matrix system to provide some underst and ing of special problems associated with SFRTP's. Mechanical properties and failure behavior of short fiber composites are complicated by the non-uniformity of the fiber length, distribution of the fiber orientation and fiber end configuration. One of the objectives of this study was to explore the simplest predictive theories for composite stiffness and strength in injection molded SFRTP materials which would be easy to use and would be accurate over a wide range of materials. The intention of this study was also to try to underst and the predicted dependence of stiffness and strength on fiber length for SFRTP's. The effects of matrix ductility on the mechanical properties and failure mechanisms of SFRTP's were investigated by controling the matrix ductility via composition in PS-PPO system. In addition to matrix ductility, consideration was given to the following parameters: fiber length distribution (FLD), fiber orientation distribution (FOD), fiber volume fraction, aspect ratio, and fiber end configuration to underst and the role they play on the mechanical properties, efficiency of reinforcement, and failure mechanisms of SFRTP's. Sub-surface analysis by transmission optical microscopy under polarized light was utilized along with fracture surface analysis and found to be a useful technique in determining the detailed microdeformation mechanisms of both matrix and short fiber reinforced systems. Fiber reinforcement efficiency in terms of both stiffness and strength was found to be strongly dependent on the fiber length and fiber volume fraction. Proposed failure mechanisms of SFRTP's were classified into three different modes with respect to the matrix ductility: (i) ductile matrix, (ii) brittle matrix, and (iii) transition between ductile and brittle matrix. Final failure mechanisms of SFRTP's were found to be very much dependent not only on the matrix ductility but also on the fiber length, fiber volume fraction and fiber end configuration. The macroscopic failure occurred due to the inability of the material to carry the load in a critical zone. The detailed microdeformation mechanisms and the size of this zone varied as the parameters involved in a specific composite changed. (Abstract shortened with permission of author.)Ph.D.Materials scienceUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/161732/1/8801430.pd