Spread-tow fabrics: better than traditional reinforcements in aerospace applications?

As most readers of CompositesWorld know, there’s a lot happening in the area of spread-tow fabrics. If you don’t know, “spread-tow” fabrics are fabricated with carbon fiber tows that have been manipulated and unpacked to form smooth, thin, spread-out bands, which are then converted into unidirectional or woven fabrics, and prepregged in some cases, to form reinforcements with high cosmetic appeal.

Because the fibers are spread out so much, the laminates that result are very thin. The popularity of these thin materials in many sporting goods and automotive applications has soared, and aerospace OEMs have taken note. Nevertheless, aerospace adoption has been slow because of a lack of analytical tools and performance tests, particularly impact tests. A bilateral Swedish/Spanish European program, called DAMTEX, was undertaken in October 2013 to investigate the mechanical properties of thin, spread-tow fabrics, and involves AERNNOVA, Oxeon (maker of the well-known spread-tow TeXtreme fabrics), Swedish research group Swerea SICOMP, and the University of Girona’s AMADE (Analysis and Advanced Materials for Structural Design) testing laboratory. DAMTEX is part of an overarching research program in Europe called AirTN (Air Transport Net) that involves many commercial and transport aviation entities.

The data coming out of the DAMTEX project is promising (here’s a link to a Spanish presentation made at an AirTN meeting by AERNNOVA’s Jose Ramón Sainz de Aja: http://www.airtn.eu/downloads/damtex_nextgen2_workshop_viena_v1_storage.pdf). Oxeon’s Florence Rinn gave a presentation on the project results and Oxeon’s development of a new material form (more on that below) at the recent CFK-Valley Stade Convention in June (watch for more CW coverage on the CFK-Valley event by Ginger Gardiner). Participants worked to develop analytical and finite element (FE) models, using ABAQUS as a starting point, to predict impact damage and damage propagation in thin spread-tow composite panels made with two different weights of TeXtreme fabrics combined with Hexcel’s RTM6 epoxy resin in a resin transfer molding (RTM) process.

After demonstrating manufacturability of 55-ply -panels using 80 gsm and 160 gsm woven TeXtreme materials, extensive material characterization tests were carried out, including in-plane shear, double cantilever beam (DCB), double edge notched under both tensile and compression loading, and residual strength via compression after impact (CAI). After more than 200 coupons were tested, damage tolerance values were similar to standard reinforcements used in aerospace, while in-plane properties were better.

To improve the damage after impact performance of the spread-tow fabric laminates, Oxeon has developed a “novel” TeXtreme material, which incorporates a thermoplastic binder. According to Rinn’s presentation, the novel material shows an improvement of at least 23% in CAI residual strength at low impact energies and at least 16% improvement at high impact energies. DAMTEX partner Swerea SICOMP reported in 2015 that thin ply composites offer reduced or suppressed matrix cracking and higher strains to first ply failure, and discussed analytical models for response and damage initiation during impact, and for prediction of damage growth. Ultimately, the goal is to improve impact damage tolerance in the aerospace industry.

All of this is in line with an article I wrote in January 2013 (here’s the link: http://www.compositesworld.com/articles/bi-angle-fabrics-find-first-commercial-application) about thin, spread-tow biaxial fabrics, in which I included an interview with Dr. Stephen Tsai of Stanford University. In that article, it was clear that Tsai and Bob Skillen, founder and chief engineer at VX Aerospace, believe that “more, thinner plies make a stronger and tougher part than fewer, thicker plies.”

As you can see from the photo, spread tow TeXtreme fabrics have made it onto at least one aircraft, the Blackwing Sweden sport aircraft, and it will be interesting to see if spread tow migrates onto commercial aircraft programs.