A major new breakthrough in composites manufacturing technology is not likely to occur in the foreseeable future. Most likely, there will be a series of improvements to existing manufacturing technologies, and manufacturing concepts already generated will be proven. For composites to become competitive with metals, cost reduction has to occur in three areas: nonrecurring costs, recurring costs, and direct operating costs (DOC) (e.g., durability, maintainability, reliability, and repairability). IPD will continue to infiltrate all the disciplines for improved efficiency in design and manufacturing. It is expected that DOC will become a much bigger issue as many aircraft with composite components enter revenue service. There will be doubts as to whether composites will ever become cost-effective for commercial use; however, these doubts can be assuaged by the facts. The reduction in manufacturing cost realized by improved technology will lose its value if it is offset by an increase in nonrecurring costs and DOC. Thus, life cycle cost analyses should be conducted along with the traditional trade-off studies of weight vs. strength and stiffness vs. cost.
Some of the manufacturing technology developments expected to occur in the foreseeable future are described below.
Small to medium size stitched/RTM parts have been fabricated with some success; however, the fabrication of complete wing skins and box by this method is a long way off (Note: this method is not cost-effective for small to medium size thin parts; to take full advantage of this method, the parts must be thick and large). For this technology to be incorporated into wing design, an appropriate automatic stitching machine has to be developed. This machine must have the capacity to handle various skin thicknesses, ranging from less than 1/4" to more than 1", and with many different shape and thickness stiffeners attached to it. Concurrently, a new cost-effective resin system specifically for RTM application must be developed. Along with stitched/RTM manufacturing technology, other issues (e.g., repair method, certification, and joints) must be addressed and resolved.
This is a mature manufacturing technique which has been in existence for a long time. Improvements in automation, speed, variable thickness, pad-up insertion, consistent quality, flexibility in fiber orientation, control of resin and void content, and shapes other than cylinders will be seen before more versatility appears in application. A combination of robotic and traditional filament winding (with seven to 10-axis) system is already available in crude form. If this system is perfected, it will be able to wind complex non-axisymmetric shapes, such as T and elbow shapes. One of the most critical requirements for a successful implementation of this method is controlling the tension of the deploying filament during the winding processes. This critical problem may be quickly solved with the aid of powerful computers.
This method has the potential for cost reduction, but current technology is limited to constant cross sections and is restricted in fiber orientation. Pultrusion is not as popular as metal extrusions. Metal extrusions are attached to other structural members, such as skins and webs, by hundreds and thousands of fasteners and rivets. This method of assembly is not acceptable for composites, where the strong trend is to eliminate fasteners. Consequently, for pultrusion to become an acceptable and popular composites manufacturing technology, it must be possible to pultrude complex multi-element cross sections, such as J-stiffened panels and constant airfoil sections. It is expected that a new technique for making tapered sections with variable thickness and even variable shapes will be available within this decade; significant progress has already been made toward that end in the last few years. Another new development is curved pultrusion.
Preforming and braided pultrusion are variations of pultrusion for special applications. New developments can be expected in these areas.
This method is already used in production. However, it is limited to making flat constant sandwich panels. Future improvements will increase speed of fabrication and quality. Floor panels, galleys, and partitions are the major uses of flat sandwich panels. Therefore, there is no need for a technology which produces a continuous sandwich panel of complex shapes and variable thickness.
The advantages of 3-D weaving are widely known, but the cost has been prohibitively high. A few automated and semi-automated systems have been created or are under development to reduce cost. Although 3-D weaving is still in its infancy, it has the potential to replace expensive titanium fittings, hinges, engine blades, etc. In addition to reduced costs of weaving, improvements in curing will be seen.
Aircraft components in general and composite parts in particular have been known as hand-made custom products as opposed to automotive and electronic products. Full automation is probably not cost-effective for aircraft applications because of relatively low production rates. However, a semi-automated method using mechatronics may be a viable option for aircraft manufacturing. Currently, mechatronics is not a fully developed manufacturing technology, but its development should be followed with keen interest.
Significant progress has been observed in ATL technology. Both speed and accuracy have increased tremendously when compared to early ATL. Advancements in computer technology (hardware and software) have influenced ATL. Along with improvements in speed and accuracy, the capability in size of layup area has also increased. Although a new breakthrough is not expected to occur in ATL technology, improvements will be incremental but continuous.
This technology has made significant progress in recent years. Three different methods of cutting are used for an APC machine: mechanical, laser, and water. Each has its own advantages and disadvantages. No new breakthroughs are expected in APC technology.
Tow placement is relatively new and has received considerable attention in recent years. It combines the advantages of ATL and filament winding. Tow placement can fabricate complex-shaped structures without limitations on fiber angles. It has the potential to reduce production costs significantly. Under the Air Force MANTECH and NASA ACT programs, this technology has proven its worth; however, its use at high production rates still remains to be seen. Future developments include optimized control systems, head position feedback, and in-process inspection for fast, accurate and high quality parts production.
The advantages of co-curing technology are numerous, but complex tooling, high risk, and the difficulty of adapting it to high production rates inhibit widespread usage. Continuous improvements in prepreg materials, tooling concepts, quick turnaround, and quality consistency may result in the elimination of those hurdles.
These manufacturing methods have great potential for high volume production applications, especially when combined with the use of thermoplastics. Application is limited to small to medium size parts. Sporting goods and industrial products will benefit from this group of technologies.
Repair technology is gaining more attention. Operators of aircraft are discovering that composites are showing a better service record than are metals, mainly due to their better fatigue and corrosion resistance properties. But at the same time composites are more prone to impact damages, which increases the importance of repair. As new generations of aircraft with tremendous amounts of composites enter flight service, both commercial and military operators will demand improved repair technology. Both the cost of repairs and the down-time resulting from the complexity and special facility and equipment requirements are putting severe demands on repair technology. Current repair technology is not satisfactory, and improvements are necessary.
Several years ago, the most popular topic in material technology was the "tough resin" system, followed by "thermoplastics." Today's popular materials are "stitched preforms," "tow placement," and "woven textile." Contrary to the original belief that thermoplastics greatly reduce manufacturing cost and time, the observation is now being made that thermoplastic parts cost more and are difficult to produce. In fact, some of the material suppliers are considering discontinuing thermoplastic production. It is still early to predict whether stitched preforms, tow placement, and textile will replace prepregs by the end of this decade. The next three years will be crucial for these so-called new advanced material systems to become dominant. It all depends upon how well these new materials can be adapted to a production mode where cost, quality and manufacturability play important roles.
Operating temperatures of the High Speed Civil Transport will be 250øF to 450øF, depending upon the location of the structure within the aircraft. Epoxy systems alone cannot handle this temperature range. The race for a new material system has already begun, and it is still too early to predict what will happen in an intensely competitive market. Candidate materials are polyimides, bismaleimides, metal matrix, ceramic matrix, etc.