A comprehensive collection of news and information about composites.

Posted by: Jeff Sloan

23. July 2014

Although several years behind schedule and many billions of dollars over budget, the composites-intensive F-35 Lightning II fighter jet is expected to be a core of the U.S. and its allies' fighting arsenal for many years to come.

The Soviet Union tested its first nuclear weapon on Aug. 29, 1949, about five to 10 years before the United States expected it would. The test effectively ended the U.S. monopoly on fission-based nuclear weaponry and launched the decades-long arms race that became the hallmark of the Cold War.

The natural tendency for the Americans was to attempt to regain the upper hand, either by building more fission weapons in effort to stay ahead of the Soviets, or build a bigger weapon. The bigger weapon would be a thermonuclear or fusion-based bomb, which until 1949 existed on the drawing board only as a conceptual weapon. A thermonuclear bomb, fueled by hydrogen, would be orders of magnitude more powerful and destructive than the uranium- and plutonium-based bombs dropped on Japan at the end of World War II, but would require a significant technical effort to develop and test. President Harry Truman faced a simple question: Whether or not to try and make a thermonuclear weapon, or just build more weapons based on the “old” fission technology.

Robert Oppenheimer, physicist and “father” of the atomic bomb developed in the Manhattan Project, actually opposed development of a thermonuclear weapon. It was, he argued, not a battlefield weapon, but a tool used only for destruction of entire cities and regions. It would be, in short, too costly to develop and too destructive to apply. However, faced with the prospect that the Soviets would certainly not show similar restraint, Truman had no choice but to authorize a thermonuclear weapon development program. The U.S. successfully tested its first hydrogen bomb in 1954, followed by the first Soviet hydrogen bomb test in 1955.

There are, today, shades of some of the same “race” logic at work with the F-35 Lightning II Joint Strike Fighter, being built by Lockheed Martin for sale to the U.S. and several partner countries. The composites-intensive F-35, as has been well documented, is about seven years behind schedule and billions of dollars over budget. This has prompted calls for the program’s end, which seems unlikely given that the F-35 and its variants are in testing and early production. The argument has also been made that many of the current jet fighters in the U.S. arsenal can do what the F-35 can do.

Indeed, the U.S. and its allies have relied on fourth-generation fighters like the Boeing F/A-18E/F Super Hornet, the Eurofighter Typhoon, the Dassault Rafale and the Lockheed Martin F-22 Raptor for about 20 years. These planes have served well and continue to perform — and might be adequate for the foreseeable future. The problem is that, like it or not, other countries that may or may not be antagonistic to the U.S. and its allies are developing fifth-generation fighters on par with the F-35. These include the Russian Sukhoi PAK FA and the Chinese Chengdu J-20, so keeping up with the competition seems to demand a fighter like the F-35.

But is fighter jet development done at any cost? Had the U.S. Congress known up front what the true cost of the F-35 would be, it likely would not have authorized it. And it would be nice to think that the U.S. and its allies had gotten so good at airplane development that cost and time overruns were the exception, not the rule. But the F-35 is a complex system, and complex systems invite unpredictability.

There is hope, however, in the fighter jet development world for creativity, speed and efficiency. Witness in particular the recently debuted Scorpion, an all-composite, tandem-seat, twin tail tactical jet fighter, self-funded and developed by Textron Airland LLC (Wichita, Kan.), a joint venture between Textron and Airland Enterprises. The plane went from concept to first flight in just two years, was designed and produced in secret and demonstrates clearly the potential for efficient and on-time fighter development. Scorpion is not an F-35 replacement, but it does prove what’s possible and might be more a harbinger of what’s to come.

The Textron Airland LLC Scorpion fighter, which was introduced in July 2014, was self-funded and evolved from concept to first flight in just two years.

18. July 2014

SMARTFIBER concept of SHM system using Fiber Bragg Grating sensors embedded into a composite tidal turbine blade. SOURCE: SMARTFIBER project.

Structural health monitoring (SHM) is an emerging widespread discipline in a variety of structures, including steel and concrete bridges and buildings. In composites, SHM has been pursued for decades with several goals:

  • Prevent failures from overload, manufacturing defect or damage;

  • Reduce the frequency and cost of inspection during service;

  • Provide real-time understanding of the fracture mechanics in composites and validation data for existing finite element models with a view also toward increasingly complex designs.

SMARTFIBER is a 42-month project (Sept 2010 to April 2014) funded by the European Union Seventh Framework Programme with the goal of embedding a miniaturized complete sensor system — including both sensor array and autonomous readout unit, or interrogator — into a composite structure, in this case, a tidal turbine. The seven partners claim they have demonstrated the world’s first miniaturized fiber-optical sensor system than can be fully embedded in a composite material. They assert that this achievement  paves the way toward smart composites that enable continued and automatic monitoring of the structural health of composite material in structures such as tidal blades, wind turbine blades, airplane fuselage and wing components or marine structures (masts, antennas, hulls and propellers).

The SMARTFIBER partners include:

As can be seen in Fig. 1 above from the project’s final white paper, the SMARTFIBER approach consists of embedding a system of Fiber Bragg Grating (FBG) sensors and low-cost, miniaturized interrogator into the composite structure, enabling wireless power and two-way communication with an external read-out unit (see Figure 2 below). Automated (vs. manual) placement of the sensors and interrogator unit during structure manufacturing was also demonstrated.

The key to the SMARTFIBER SHM system is a low-cost, miniaturized interrogator for the FBG sensors, embedded into the composite structure and enabling wireless power as well as two-way communication with an external read-out unit. SOURCE: SMARTFIBER

FBG sensors were used due to their ability to provide continuous monitoring coupled with their compactness, lightweight, immunity to electromagnetic interferences (EMI), high resistance to corrosion, high temperature capacity and multiplexing capabilities. The team believed them superior over other strain monitoring techniques (e.g. classical electrical strain gauges) especially when pursuing automated embedding during manufacture.

Silicon photonics technology enabled the high miniaturization and low cost sought for the FBG interrogator. The core of the interrogator is a photonic integrated circuit (PIC) that transmits a signal which, after being processed, is received by the external read-out unit. The latter also supplies wireless power to the interrogator. Another key component of the interrogator is a tailored Array Waveguide Grating (AWG) filter. The SMARTFIBER paper asserts that all of these devices can be manufactured using well-established industrial silicon-based microfabrication techniques and infrastructure.

The sensor system was assembled by Optocap on an electronic board designed by Xenics. The optical subsystem consisted of a silicon photonics integrated circuit developed by IMEC and photodiodes and read-out integrated circuits (ICs) provided by Xenics.

Fraunhofer Institute for Integrated Circuits IIS was responsible for the wireless interface which provides power to the embedded system and at the same time reads out the acquired data at high speed.

After being connected to an optical fiber sensor chain manufactured by FBGS International, the sensor system was cast into an epoxy shape specifically designed by Ghent University to minimize the impact on the composite material. Together with the attached fiber sensor chain, the sensor system was then embedded into the blade of a tidal turbine by Airborne International, which also conducted studies into automation of this process during blade fabrication.

The complete system was integrated within the tidal turbine blade, a sandwich construction with a foam core and non-crimp fabric faceskins. Light resin transfer molding (LRTM) was used to mold the blade. Extra layers of glass fabric were placed to minimize resin build-up around the embedded system and to reduce the abrupt change in stiffness from laminate to embedded elements. The optical fiber was placed along the longitudinal length of the blade in loops from tip to root and held in position by small dots of glue. A strategy for automated placement was also developed.

The embedded system was tested and compared to a commercial interrogator. Its performance was on target with the project goals. “We are investigating steps to be taken towards commercialization of the device,” says project leader Dries Van Thourhout, “Input from potential partners is welcome.”

Posted by: Ginger Gardiner

16. July 2014

New vs. traditional approach to design allowables.
SOURCE: Dr. Stephen W. Tsai and Jose Daniel D. Melo.


HPC reported on this new approach in the July issue: Overnight design allowables? An invariant-based method for accelerating aerospace certification testing. For more details and the full series of charts and graphs, see the white paper from Dr. Tsai and Daniel Melo posted on CW’s White Papers page.

Dr. Tsai comments, “The recognition of invariants that can significantly simplify design and testing of composites structures was discovered by Daniel and me by accident.  The number of tests for design allowable generation can be reduced from one thousand to a few dozen if not fewer.  More importantly, key tests can be made a matter of hours instead of days, weeks or months.” He adds that the only stiffness data needed is the value of trace, a sum of the diagonal components of the stiffness matrix.  “For strength, one tensile or compressive strength of open hole coupons at room temperature can be sufficient to generate allowables, and the accelerated testing of fatigue strength can also be done based on the shift factor of viscoelastic behavior of composites.”

To put it more in layman’s terms and get a critical review, I asked Dr. Rik Heslehurst to read and comment on the white paper. Heslehurst is a former aeronautical engineering officer in the Royal Australian Air Force (RAAF), in charge of material and process engineering for almost two decades. He also instructs advanced composite engineering courses for Abaris Training (Reno, Nevada) and has consulted for a wide variety of companies including Boeing, the U.S. Air Force, Lockheed Martin and Bombardier. Heslehurst’s response:

"The basic premise is that all composite materials (plies and laminates) have a stiffness relationship that is essentially invariant (statistically low variation).  This invariant term is called TRACE.  With knowledge of the axial longitudinal modulus value for any composite material the TRACE factor can be determined and the resulting stiffness matrix of a ply and thus a laminate can be determined.  This approach is a significant and positive step in reducing the cost of determining materials allowables.

I would caution, however, that the assumptions made, such as the major Poisson’s ratio as 0.3 and the derivation of the shear modulus term, require a sufficient knowledge of composite materials properties computation as well as design allowables generation — for example, the differences between that for unidirectional tape and cloth.

I believe that the development of the shift factors for open hole, environmental and fatigue is also a very positive step, and fortunately, there is now extensive materials data available on these effects to allow close comparison with simulated data.

I am encouraged by the approach and already see several applications in my own work that will allow early and rapid development of composite designs, but I would reiterate that a novice not use this approach without due understanding and caution."

The foundation of invariants and implications on composites design allowables testing will be covered extensively by several speakers in the upcoming Composites Durability Workshop 19 (July 27-29)  hosted by Stanford University’s Department of Aeronautics & Astronautics (Stanford, Calif., USA).

Proposed advances that can simplify design allowables generation will be presented by representatives of Stanford, Wichita and other universities, as well as end users and the FAA. Potential issues in expanding the adoption of this approach will also be discussed and help in using tools and templates for one master ply will be available during and after workshop.

A formal technical paper, “An invariant-based theory of composites,” has just been published online in the journal Composites Science and Technology.

Posted by: Jeff Sloan

10. July 2014

The big news in composites this week came out of carbon fiber and resin supplier Cytec Industries, which announced on July 8 that is establishing a business relationship with German acrylic producer Dralon GmbH to evaluate the possibility of manufacturing large-tow, industrial-grade carbon fiber. Cytec CEO Shane Fleming cited the automotive industry's lightweighting efforts as the principle driver of the decision.

Cytec has a long history with aerospace-grade carbon fiber, and through Umeco (ACG), which Cytec acquired two years ago, it has a long history with automotive composites. Now, with the decision to work with Dralon to develop acrylic for what is assumed to be a low-cost precursor for large-tow carbon fiber, Cytec joins a growing list of carbon fiber manufacturers who, apparently, see real potential in automotive composites.

Indeed, SGL Group, as has been well documented, created in 2010 a joint venture with BMW Group for the supply of large-tow carbon fiber used in the all-electric BMW i3. And in late 2013 the world's largest supplier of carbon fiber, Toray, bought Zoltek, which for many years was the world's largest supplier of large-tow carbon fiber and occasionally derided for the niche it occupied in the composites market.

In fact, it seems that almost every major supplier of carbon fiber is either seeking or working to develop a lower-cost precursor for the manufacture of large-tow carbon fiber for automotive and industrial applications. The poster child for this effort is U.S.-based Oak Ridge National Labs, which has its own carbon fiber pilot line and is evaluating a host of alternate precursors. 

Despite all of this activity, it's hard to say definitively that carbon fiber and the automotive industry have, finally, married for good. Despite the efforts of BMW and Volkswagen to integrate carbon fiber into their vehicles, there remains at other automotive OEMs much skepticism about the utility and sustainabilty of carbon fiber. Ultimately, however, it will be the consumer who decides. If car-buyers demonstrate a preference for the benefits carbon fiber offers, then we might yet see the long-awaited marriage of composites with cars.

10. July 2014

The VX-1 KittyHawk 1/4-scale model completes first flight test. SOURCE: VX Aerospace

The VX-1 KittyHawk is a blended wing body aircraft made by VX Aerospace (Morganton, N.C., USA) using C-ply advanced carbon fabrics from Chomarat North America (Anderson, S.C.). The aircraft offers structural efficiency, lightweight and unusually large payload space as an unmanned aerial vehicle (UAV) or cargo space as a sport aircraft. This combination of design and large volume allow the KittyHawk to incorporate compressed natural gas (CNG) as a fuel with no aerodynamic compromise, and thus, the potential to provide new levels of fuel efficiency and emissions reductions.

A ¼ scale model recently completed the aircraft’s first flight test and performed well. “The test flight showed that the plane is dynamically stable and that the control systems work well,” said VX Aerospace Chief Engineer Bob Skillen. “It confirms the aircraft’s potential and we are processing the data to plan our next flight test in late summer.”

Skillen describes the aircraft as “dynamically stable”  but in the video below, the airplane model looks very “tippy”. Skillen explains, “"Every motion you see the airplane make in the video is in response to a pilot input.  It's the first time this shape has ever been flown.  It takes a few moments for the pilot and aircraft to become acquainted.  Also, a lot of the maneuvers were intentional for control system/response data."

Lars Soltmann, a Ph.D. student at North Carolina State University (NCSU, Raleigh, N.C., USA) and UAV researcher who performed the computational fluid dynamics (CFD) analysis on the KittyHawk, explained that lifting body designs (blended wing body aircraft) can be a bit “twitchy” because of the lack of damping from a tail. “Also, flying wings generally have a lower pitching moment of inertia because their mass is less spread out vs. traditional aircraft designs,” he adds. “Stability can be increased by shifting the center of gravity forward.” So a bit of modification will take place before the next test flight.

Skillen points out that the team can actually play with a variety of design changes to the aircraft and then test them virtually in the computer. “This is possible because the flight data collected matches perfectly with the CFD and wind tunnel projections generated by Lars.” Soltmann expounds, “Bob gave me a shape and other details, and I put that through CFD computations. Then we 3-D printed a model of the aircraft, with a 1-ft wingspan, and tested that in the wind tunnel. The data matched almost on top of each other.”  This confidence in their performance predictions and a now well-established baseline is what will enable the team to refine the VX-1 KittyHawk with virtual testing until they are ready to flight test again.

The test pilot, R.J. Gritter, is also from N.C. State and is a very experienced full-scale aircraft pilot as well as an expert remote control/unmanned aerial vehicle pilot, winning several  championships and representing Team USA in the next international radio control aerobatics competition.

The dives mentioned above are called pitch doublets, where RJ pulls back on the stick, holds, pushes forward, holds and then returns back to neutral. “These excite longitudinal aircraft modes that we then compare back to the computer simulations,” explains Soltmann. Again, the test data looks good and the team has a clear idea of what they need to do next.

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