Seminar Series - 1997-1998

The Effect of Thermoforming on the Tensile Strength and Fatigue Properties of Unidirectional Discontinuous Aligned Fiber Composites

Professor Dr. -Ing. Jens Schuster
Fachhochschule Kaiserslautern
Site Prmasens
Pirmasens  66955 Germany
Sponsored by the Dept. of Engineering Mechanics

Date:  Tuesday, February 24, 1998
Time:  3:30 p.m.
Place:  Nebraska Hall W128


Thermoforming is an economical process for manufacturing complex shaped and stiff parts.  The process is mainly suitable for thermoplastic prepregs and the processing parameters chosen have a great influence on the quality of the parts and the efficiency of the process.  Short production cycles hamper the occurrence of deformation mechanisms (e.g. interply slip, intraply slip) in the laminate resulting in instabilities such as wrinkling and buckling.  In order to both accelerate and ease the processing of such materials, prepregs containing long aligned discontinuous fibers (DAF) have been developed.

This paper described the principal factors dominating deformation load requirements in the processing stage.  It is possible to describe the thermoforming behavior of discontinuous aligned fiber composites by using the Herschel-Bulkley-Model.

The viscosity of the molten PEKK-matrix unidirectionally reinforced with carbon fibers was determined using an expression developed by Coffin and Pipes.  This model enables the determination of internal residual stresses by taking into account thermal stresses introduced to the material during cooling.  Besides fiber orientation in deformation direction, the residual stresses are a major factor to increase the ultimate tensile strength, which can be realistically concluded by a combination of the Hershel-Bulkley-Model and a modified expression of the Jeffrey-Equation.


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