A program now in progress is dedicated to making improvements in automated fiber placement (AFP) for the manufacture of advanced composite-material (matrix/fiber) structural components — especially skin and shell components for aircraft. [In AFP, a composite-material part is formed by laying down fiber tow or tape pre-impregnated with a matrix resin (“prepreg”) onto a mandrel that defines the shape of the part.] Improvements are sought for the following reasons:
- The AFP machinery and processes in question were originally designed for handling epoxy matrix materials.
- It is required to adapt AFP to bismaleimide (BMI) matrix materials, which can endure higher temperatures and have been selected for the next generation of composite aircraft components.
- In their original forms, the AFP equipment and processes in question are not optimum for manufacture of BMI-matrix materials, which, in comparison to epoxies, present greater difficulty in processing.
- As a consequence of the greater difficulty, AFP deposition rates for BMI-matrix composites have been lower than those for epoxy-matrix composites. The main goal in seeking improvements is to enable deposition at rates comparable to those of epoxy-matrix composites.
The approach taken in making improvements has involved (1) examination of newly available BMI materials to determine whether they offer advantages over an older baseline BMI material now used in the production of F22 and F35 fighter airplanes, in conjunction with (2) modifications of process conditions and equipment. The examination of materials is conducted with a view toward selecting the best among them on the basis of a number of criteria, including mechanical properties of finished composite parts, physical and chemical processing properties, and costs. The main material/processing issues that must be addressed in order to effect the desired improvements are (1) insufficient tackiness of prepregs and (2) buildup of resin and fiber fuzz (see figure) with consequent clogging of processing equipment.
The tackiness issue has been partially addressed through modifications involving humidity and temperature. Tackiness increases with humidity, making it desirable to process at the highest possible humidity that can be used without adversely affecting other aspects of processing. A relative humidity of 40 percent has been selected as an interim level of humidity that affords sufficient tackiness, pending further study to determine an optimum processing humidity. The tackiness issue has also been addressed partially by modifying the AFP equipment to enable heating to a higher temperature at the laydown/compaction point. Work continues to develop means of preventing or reducing the accumulation of fuzz, typically by collecting the fuzz at various locations before the component materials are fed into the processing equipment.
Among the newer BMI materials examined, two have been selected as candidates for use in manufacturing high-performance aircraft by AFP. Both of the selected materials have enabled attainment of increased production rates and reduced down time for removing fuzz from AFP equipment in fabrication of aircraft parts, relative to those attainable using the baseline BMI material. The mechanical-strength parameters of the selected BMI materials were found to be slightly smaller than those of the baseline material, the majority of corresponding parameters of all three materials being within 5 percent of each other.
This work was done by Jonahira Arnold of the Air Force Research Laboratory and Vernon M. Benson of ATK Space Systems, Inc.