Production of STF involves spreading a high count carbon fibre tow (eg 12k – 12,000 filaments) into thin-and-wide Spread Tow Tapes (STT) and then weaving them into a lightweight fabric using a novel tape-weaving technique. STF offers relatively low crimp and higher coverage and can thereby improve mechanical performance, enable a 20-30% weight savings and enhanced drapeability.
Through this novel STF route, it becomes possible to replace conventional low areal weight woven carbon fabrics, produced using 1k–6k tows, using 12k or higher carbon fibre tows while obtaining an even lower areal-weight reinforcement. The carbon fibre producers thus have the opportunity now to deliver essentially a couple of tow counts, e.g. 12k and 24k, instead of producing a variety of carbon fibres in the 1k–6k range.
The concept of STF is simple and direct. By arranging the fibres in the woven structure in the straightest orientation possible, both in-plane and also out-of-plane, the fibre properties can be exploited most efficiently in tensile loading. A sheet of STF performs mechanically similarly to a cross-ply (0°-90°) construction made using UD tapes, with the advantage in that STF offers improved draping ability and delamination resistance compared to cross plied UD.
Through a controlled operation, carbon fibre tows, usually 12k and heavier count, are gently spread to an STT of desired width and stabilized to prevent the tape folding back into the original tow/yarn form. The spreading action renders the obtained tapes extremely thin. These STT are then used as warps and wefts in the tape-weaving process for producing STF. Compared to conventional fabrics, the use of spread fibre tapes in producing STF directly results in a woven material with high fibre volume fraction and fewer interlacing points, thereby correspondingly lower crimp frequency and crimp angle.
Fig. 1 compares fabric cover factors of traditional 1k fabric, flat tow fabric produced using 12k tow and an STF also produced using 12k tows. As can be observed, the traditional and flat tow fabrics can have openings at every interlacing point whereas the STF is solid.
The lower mechanical performance of traditional woven reinforcement fabrics can be attributed to crimp angle and crimp frequency due to use of tows which are comparatively narrower and thicker than the STT. These factors, which adversely affect the performance of a woven material for composites applications, are significantly reduced by weaving STT as warps and wefts into an STF.
Alongside other reinforcement materials such as rovings, unidirectional tapes (UD), non crimp fabrics (NCF) and conventional yarn/tow woven structures, STF is now an accepted member of the textile reinforcement family and it is being increasingly used in a number of advanced composite applications.
Published courtesy of Dr. Nandan Khokar & Fredrik Ohlsson
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