CAMX 2015: Highlights from the conference

CW couldn't get to all of the presentations in the conference at CAMX 2015, but we did get to a few. Summaries here include one on manufacturing liability in consumer products, one on high-volume autocomposites, one on DARPA and small parts, and one on aerospace certification.

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This micrograph from Scott Beckwith's presentation on manufacturing practices of some consumer product producers shows the type of porosity he detected in some bicycle forks that failed, causing injury and, in a few cases, death.

It was not possible for CW to attend all of the presentations at the CAMX 2015 conference, but we did get to a few. Below are reports from a few of them. You can find more CAMX reports from other editors in the CW Blog.

 

Design, process control flaws create liability problems for consumer products 

The rigor with which composites designers and fabricators approach manufacture
of parts and structures for aerospace and automotive applications is apparently not being employed in the production of some sporting goods parts, leading to product failure and, in some cases, injury and death. This is according to Dr. Scott Beckwith, principle of BTG Composites (Taylorsville, UT), who offered a review at CAMX of his analysis of evidence in several product liability cases involving carbon fiber bicycle forks and carbon fiber arrows.

Beckwith, who says he has testified in roughly 25-30 civil suits involving failure of advanced carbon fiber composites in a bicycle or arrow, has found in many cases that manufacturers fail to perform funda- mental structural design analysis, design allowables studies, materials allowables studies, process optimization, as-built product testing or rigorous and ongoing quality control. This failure, he notes, spans many manufacturers in many countries and cannot be isolated to one brand or facility or manufacturing process.

“There is, in many cases, a complete lack of good design, analysis and manu- facturing optimization practices,” he says, compounded in many instances by a desire to manufacture parts quickly for large production quantities, which leads to manufacturing shortcuts that cause significant flaws.

He has found in many composite prod- ucts excess moisture in molding processes, loss of pressure, loss of vacuum, tempera- ture deviations, foreign object debris and other problems. The effect on product quality is elevated porosity (some as high as 15 percent), delaminations, wavy fibers, resin-rich regions, resin-starved regions and fiber movement.

The most significant product failures, Beckwith says, involve product flaws that lead to failure of a bike fork at speeds often under 20 mph, which in almost all cases causes head or upper body trauma severe enough to maim or kill the cyclist. The failure of a carbon fiber arrow—as Beckwith graphically showed with one slide—can result in fracture of an arrow shaft, which can then become impaled in the hand or fingers of the arrow shooter. Beckwith says correcting these design and manufacturing problems is not difficult, and he points to easily accessed ASTM D-30 composites standards, which outline good, basic principles of determining composite materials allowables generation, process optimization, as-built product testing and product lot testing.

The question, from the audience, was how bicycle manufacturers might be incentivized to improve their manufacturing operations to help ensure product quality. Beckwith says he hopes that the industry will correct itself using ASTM D-30 standards, but given the liability insurance some manufacturers carry, he fears that injuries from product failure might just be “the cost of doing business.” 

 

Gurit reports success with new snap-cure prepreg for Class A parts out of the mold

“Snap-cure” resins, said William Ricci, Technical Services, Gurit Composite Materials (Isle of Wight, UK) as he introduced “A New Snap Cure System for High-Rate Composite Component Production,” are those that cure in 5 minutes or less. And, he noted, any resin formulator can make a resin cure in one minute. The challenge is how to accomplish a snap-cure yet guarantee that the resin will flow well enough to wet out the fiber, producing a Class A surface and finished rolled edges out of the press (no trimming necessary).

His presentation at CAMX focused on Gurit’s recent trials of a snap-cure resin in a molding process aimed at the auto-industry’s typical 100,000 parts/year production target. Although the trend has been toward the use of resin transfer molding (RTM) or wet compression molding, Gurit determined that kitted prepreg would be the easiest way to ensure a Class A surface, so that’s the direction it took.

Gurit began with a thermoplastic-toughened, multifunctional epoxy base resin, then added a secondary catalyst that remains dormant, preserving the prepreg’s standard dynamic flow, until it is activated at a preset temperature, triggering cure that escalates very quickly after that point. Generally, the cure temperature controls the cure cycle time.

To reach the production goal, parts must exit the mold hot without distortion or fiber print-though. Ricci said the key here is the glass transition temperature (Tg). In very high-temperature molding processes, the part temperature must be allowed to fall below the Tg before demolding, which prolongs the cycle. Gurit’s resin cures at a temperature lower than its Tg.

In its test run, matched-metal tools (female convex and a male concave with a gasketed seal) were prepped with a water-based spray mold release system. Prereg resin content was 35%, and the preformed prepreg was eight plies of uni-tapes. The part was a complex-curved surface with a cutout, cured at 150°C. All edges were finished. Void content of test parts was 1-2%. Ricci says press force need not be high (120 psi) but fast open/close are critical to save cycle time.

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This slide, from the Gurit presentation, outlines the basic steps involved in the company's preform- and prepreg-based fabrication process.

Based on its test parts, Gurit estimates the cost to a molder for a system capable of 20 parts/hr as follows: Matched aluminum tool cost: (2 sets) US$230,000, press, US$900,000, cost per part amortized over one year, US$11.30.

So far, the system is not adaptable to produce cored sandwich constructions, but Ricci said they are working on that. And, he admitted, the preforming process is the weakest part of the system right now (it is in all production molding processes). Currently, it takes 15 minutes to make the preform for a 5-minute molding process, which means a molder would have to feed a press from multiple preforming stations. But Ricci said Gurit is working on a way to quickly form near-net flat laminates of 10-15 layers that will require no debulking. 

 

DARPA: TFF set to make composites more competitive in DoD small-parts programs

Composites compete well in US Department of Defense (DoD) trade studies for large parts. The cost vs. performance trade is justified, because current levels of automation for parts such as aircraft wingskins is easy to justify in comparison to legacy metal systems. “But as you go down in part size, and up in complexity, the automation isn’t as efficient," says Michael Maher, program manager at the Defense Advanced Research Projects Agency (DARPA, Arlington, VA, US), who spoke at CAMX. "There are more cuts and stops.” At around 20-lb part weight, metal wins on cost, despite composites advantages in performance, such as corrosion resistance. “Composites don’t trade as well,” says Maher. This is true of smaller aircraft parts and almost all ground vehicle parts.

That’s the problem. In his session titled, “Aerospace Performance at Automotive Efficiency,” Maher presented DARPA’s already in-process solution for making composites more competitive in small parts: Tailorable Feedstock and Forming (TFF). Conceived on a short three-year timeframe, TFF will address the challenge with a new “part-agnostic, tailorable aerospace-grade” composite material (feedstock) and associated processing technologies (e.g., reconfigurable forming) to reduce manufacturing complexity and enable use of advanced materials for small parts weighing >20 lb at costs competitive with aluminum.

The feedstock is in progress: A carbon fiber smaller in diameter than that currently in the market, because its cross-sectional mass is significantly lower, and would then require a much less expensive carbonization process, reducing its cost. An inexpensive carbon fiber? Maher says, “We believe this is doable.” Most intriguing is DARPA’s intent that the feedstock be discontinuous short fibers. Maher noted research that showed smaller-diameter, short-fiber composites comparing well with continuous fiber composites and performing much better than conventional short-fiber materials. Further, the short-fiber eliminates drapability issues, making the composite material adaptable for for DARPA’s target forming process.

That forming process, still to be determined, is envisioned as some form of stamping or compression molding, but must be capable of complexities that include integral ribs and stiffeners. Further, the process must enable control of short fiber orientation and concentration, a task which Maher admits will the most difficult to accomplish, but is absolutely critical to project success.

The goal is a material and process each of which is flexible and tailorable enough to be used successfully in a wide variety of applications, thus shortening development time and reducing cost. Maher notes that TFF would give development teams, over time, the opportunity to leverage accumulated experience to further accelerate tool and part development cycles.

Maher ended his talk with the observation that, although the focus in on DoD applications, and he believes US Army ground vehicles will greatly benefit, there is no reason why the technology could not transfer to automakers and other commercial processors on Army side will do this, so why wouldn’t it transfer to commercial automakers? And he had a word, too, for proponents of additive manufacturing. DARPA is … skeptical. But willing to listen.

 

To coupon or not to coupon?

There is much discussion in the composites industry about the potential to migrate away from the expensive and voluminous physical testing required certify a composite material for use in aerospace applications. In particular, there is hope that we might be able to dispense with coupon testing, which requires the fabrication and assessment of thousands of plaques for physical testing. 

Thus, it was standing room only for D. Scott Norwood's presentation titled, "Composite Structures Development and Certification for Modern Military Aircraft." Norwood is a senior staff engineer at Lockheed Martin Aeronautics Co. and he talked primarily about efforts at Lockheed to understand the cost and value of composites testing certification programs. Lockheed, he reported, has looked at several years of legacy composites manufacturing programs and developed a database to characterize the depth and breadth of composites testing done. 

Norwood presented data in several slices, looking at composite product form (coupon vs. component, for example), test objectives, material (composites vs. metals) and test type (static vs. fatigue). Of particular interest, Lockheed assessed the cost and time required by the famous "building block" approach typically used for composites certification, starting with coupon testing and evolving through element, sub-component, component and airframe testing. In general, Norwood said, coupon testing (developing allowables) consumes the most time, but at the least cost, while airframe testing is the most expensive, but done relatively quickly.

Also, composites are tested more than non-composite materials, mainly because legacy materials test data is more readily available. And, static testing is by far the most commonly performed test. Norwood was also careful to emphasize the value of physical airframe assessment, including testing for crack spread, interlaminar shear strength and non-normal operating conditions.

The upshot? Norwood said "computational strategies" for certification are currently ill-suited to airframe design, but that it could help "reduce the building block work scope" by way of "more focused testing," primarily in the element/sub-component/component stages. "We can into elaborate testing matrices. We have to become much more focused and efficient."

As for coupon testing, Norwood said, "We still see that as pretty important work that has be done. Coupon data is too valuable to the rest of the test protocols. That's basic homework that has to be done and makes us all feel better."