Material characterization of woven fabrics for thermoforming of composites

Proper material characterization of woven fabric reinforcement is important for accurate modeling of thermoforming. Thermoforming of commingled thermoplastic/glass fabric relies strictly on material deformation at temperatures above the matrix melting temperature. As the temperature increases, the viscosity decreases allowing for the resin to flow. The yarns are freer to rotate, reducing shear loads in the material. The resistance to deformation depends not only on the matrix viscosity, but also on the fabric architecture. The two types of materials used in this study are a plain weave and a 4-harness satin weave structure. An interesting characteristic of these woven materials is that they exhibit shear deformation with concomitant area changes, enabling them to take on shapes of double curvature. This makes them a great candidate for thermoforming of structural parts. To improve the viability of this manufacturing process, simulations and design tools are needed. A standardized test method must be devised to ensure an accurate material characterization for finite element models. Therefore; extensive testing with the use of a trellis shear frame is discussed. Testing shows significant non-repeatability, introducing great concern for modeling purposes. Possible reasons for inconsistency addressed in this paper are fiber tension, alignment and manufacturing. Although all of these may contribute to the inconsistencies seen, yarn misalignment is discussed in greater detail. Correlation between test fixtures of different sizes was established by normalizing the data with respect to area. These results suggest that the number of crossovers, as represented by the area, within the material is critical. Two distinct areas of concern addressed in this paper include: the change in fabric temperature during stamping and the material response at these temperatures.