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Fig. 7.63. Lap Shear Strength Comparison of Different Joining Methods44

Thermoplastic composites do offer some definite advantages compared to ther-moset composites; however, in spite of large investments since the mid-1980s, very few continuous fiber thermoplastic composites have made it into production applications. Compared to thermoset composites, thermoplastic composites offer the potential for short processing times, but their inherent characteristics have prevented them from replacing thermoset composites in the aerospace industry, namely:

• High processing temperatures (500-800° F) increase the cost of both the prepreg and complicates the use of conventional processing equipment.

• The lack of tack and boardiness of the prepreg results in expensive manual handling operations.

• Thermoforming of continuous fiber reinforced thermoplastics has proven to be much more difficult than first anticipated, due to the tendency of the fibers to wrinkle and buckle if not maintained under tension during the forming operation.

• The early claims of superior toughness and damage tolerance have largely been negated by the development of much tougher thermoset resins.

• Solvent and fluid resistance properties remain major barriers to the use of amorphous thermoplastic composites.

In the author's opinion, two criteria must be satisfied to take advantage of continuous fiber thermoplastic composites: (1) the demand for a large quantity of parts, and (2) the process must be automated to remove almost all manual operations. Unfortunately, it is difficult to meet these criteria in the aerospace industry, where lot sizes are small and production rates cannot usually justify the investment in highly sophisticated automated equipment. It is both interesting and insightful that discontinuous GMT have made significant inroads in the automotive industry, where the demand for parts is large and the process has been almost totally automated.

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