Dry carbon-fiber fabrics are sewn together and draped over a mold.

Dry carbon-fiber fabrics are sewn together and draped over a mold.

Manufacturers typically pick a sewing yarn based on the seam's function. Polyester yarns, for example, are sufficient for assembling preforms without the need for structural improvement. These seams just prevent the relative movement of the preform layers or subpreforms.

However, the seams must not disturb the textile structure or degrade the material in-plane properties. Typically, designers can expect about a 2% drop in static compression strength. Stitch type, needle and yarn thickness, yarn tension, and fiber undulation or curvature, are the most critical parameters needing tight control.

Threads for structural seams must meet more-stringent criteria. Their main function is to carry and distribute out-of-plane and/or shear stress. Obviously, therefore, they must be thicker, but many other properties play key roles as well. The most important is the yarn modulus in combination with the resin interface. Carbon sewing threads are the optimum choice. They provide the highest combination of stiffness and ultimate strength compared to glass and aramide yarns.

Compression after impact (CAI) tests at EADS-CRC show that the delamination area drops significantly as stitch density rises. The compression strength after impact rises 80% from 172 to 312 MPa.

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