One of the big challenges of high-performance composites design and manufacturing is to provide isotropic reinforcement with a formable carbon fiber or glass fiber fabric over a curved surface. Fabric buckling caused by curvatures can stress or disorient fibers, thus compromising mechanical performance of the finished structure. One solution is to cut or dart the fabric, but doing so also compromises mechanical performance.
To meet this challenge, Pepin Associates Inc. (Greenville, Maine) has announced the development of DiscoTape, an aligned discontinuous, preimpregnated reinforcement unitape that Pepin says has the ability to stretch up to 30 percent to form complex shapes. Measuring 0.005 to 0.006 inch (0.13 to 0.15 mm) thick, the tape can be made with glass or carbon fiber prepregged with epoxy resins or thermoplastics. Variations of fiber lengths and cut patterns have been investigated to optimize DiscoTape for a variety of applications.
Tests performed by Pepin show that while there is a knockdown of 0° tensile strength in the discontinuous unitape, interlaminar shear and in-plane shear remain relatively consistent with properties of continuous unitape. Tensile modulus is also close to continuous reinforcement values. This would allow use of DiscoTape in structures whose designs are stiffness driven such as bead-stiffened shear panels.
Another possible application of DiscoTape, says Pepin, is radome manufacture. The company has completed a study on the formability and transmissivity of glass DiscoTape in radomes, working with Radant Technologies (Stow, Mass.) to measure the electrical properties of the material. Results show that discontinuity does not always affect transmissivity of the radar signal. Discontinuity may interfere with the signal at X-band frequencies; in the event the material shows signal loss, the length of DiscoTape segments can be altered, allowing the fabricator to “tune” the material as needed. Further, research showed no appreciable effect on material transparency for the Q-band frequency.
John Pepin, president of Pepin Associates Inc., says stiffness is the primary property required for domed structures, and DiscoTape provides that. Strength, he says, can be tuned by extending the length of the discontinuous fibers. “We can adjust the geometry of the discontinuity of the tape,” he says. “We can balance strength and stiffness by modifying the architecture of the fiber structure.” He also reports that thermoplastic candidates for use with the tape include PEEK, PPS and PEI.
Pepin also admits that laying up DiscoTape “is not always easy,” and reports that his company is working on tooling systems that change shapes — morphing from a moderately contoured surface to a more contoured surface. This allows layup to evolve from simple to complex.
Pepin Associates Inc. will present two technical papers at SAMPE 2012 (May 21-24, Baltimore, Md.) on DiscoTape.