Tow steering, Part 3: The birth of tow shearing

In early 2020, NASA issued its final report on the Passive Aeroelastic Tailoring (PAT) project, which evaluated the design, optimization and fabrication of a tow-steered wing skin. The report suggested that the material and hardware surrounding automated fiber placement (AFP) systems would need further maturation to help increase the quality and consistency of tow-steered aerostructures. The report also suggested that ancillary technologies be evaluated more closely, and made specific reference to tow shearing.

Continuous tow shearing (CTS) was invented at the Advanced Composites Collaboration for Innovation & Science at the University of Bristol (Bristol, U.K., currently Bristol Composites Institute) in 2010 under an EPSRC grant by Byung Chul (Eric) Kim (primary inventor), Paul Weaver and Kevin Potter. Evangelos Zympeloudis further developed the technology from 2014 onwards at the university and has been leading the commercialization of the technology. iCOMAT (Bristol) was established to commercialize tow shearing in 2019 by Zympeloudis, Kim and the University of Bristol as a spin-off company, with Evangelos as the CEO of the company and Kim as the chief technology advisor. iCOMAT has worked to mature the overall process and recently rebranded it to RTS (rapid tow shearing) to highlight productivity gains that have resulted from this work.

Zympeloudis says tow shearing was developed as an alternative to tow steering, which uses in-plane bending of tows and is prone to defect generation, including tow buckling, overlaps and gaps. He notes that such defects, even if well managed by the AFP machine, must be accounted for in the design to guard against knockdown, which complicates design, fabrication and testing efforts.

iCOMAT instead uses in-plane shearing to move tows laterally as they are placed and re-orient the fibers along a radius. This is accomplished, says Zympeloudis, through the use of intellectual property (IP) in two places: the material and the placement head. “I can’t go into great detail,” he says, “but we do many things to the material before we place it, and the placement head itself is critical to iCOMAT’s shearing technology.

Zympeloudis says iCOMAT’s technology, delivered through a placement head attached to the end of a 6-axis robot, can shear off-the-shelf dry or prepregged materials to a radius as small as 50 millimeters (2 inches) in tape widths ranging from 6.35-500 millimeters (0.25 inch to 19.7 inches). He says dry fibers are most amenable to shearing, but with proprietary modification, prepregged tapes can be sheared as well. “We can shear any UD prepreg we have tried, and we have tried a lot,” Zympeloudis says. “But we have not tested all UD prepregs on the market.”

One notable feature of tapes placed using iCOMAT’s shearing technology is that as the angle of shear increases, lateral movement of the fibers relative to one another causes the tow/tape width to decrease and thickness to increase. This is a feature, says Zympeloudis, that designers have exploited to locally increase the thickness of a laminate, without increasing the number of plies.

iCOMAT points to several benefits of RTS, including more efficient composites fabrication, reduced waste, better load path management, no gaps or overlaps and no fiber buckling. “It’s a radical way to manufacture with composites that drastically improves structural and production performance,” Zympeloudis says.

The company is currently involved in seven R&D trials with leading OEMs to evaluate RTS and its application. And later in 2021, iCOMAT will place its first system, called RTS 2D, with a customer in a manufacturing environment.

Although RTS appears to be ideally suited for commercial aerospace use — particularly in wing skins — Zympeloudis acknowledges that there is several years worth of design, manufacturing, testing and certification work to be done before shearing technology appears on a commercial aircraft. So, not surprisingly, iCOMAT is targeting defense applications first and hopes to have tow-sheared parts flying by 2023. Zympeloudis also points to automotive as another target. He says RTS could be used to make curved beams for cars and trucks to near-net shape, with minimal waste.

Regarding testing and certification, Zympeloudis notes that there is no established ASTM standard designed specifically for testing steered or sheared fibers. “As soon as you start steering, many of the ASTM standards go out the window, but we think it might be possible to use existing standards, such as ASTM D3039 [standard test method for tensile properties] with RTS. You can go a long way using existing standards, but we are also working to develop dedicated test methods for RTS.”