all the samples were rinsed with ethanol and dried at 60 C in a vacuum oven. The uptake of nanotubes was recorded by weighing specimens before and after treatment. Percent uptake was approximately 10 % by mass for yarns, and up to 1.6 % for fabrics depending on the treatment conditions used. 7.2.2 Yarn Scale: Quasi-Static and High Rate Tensile Testing Rate effects can influence the mechanical properties of polymeric materials, which is an important consideration since ballistic impact is a high rate event. Therefore, tensile tests of the ballistic yarns at both quasi-static and high-rates are required to determine how the constitutive properties change with strain rate. These properties feed into the finite element method (FEM) models and can help predict how changes in strength and modulus translate to behavior at larger length scales and in more complex systems. Yarn-level results for completed quasi-static tensile tests are discussed below while tensile testing of Kevlar yarns using a split-Hopkinson pressure bar (SHPB) is currently in progress. Treated and untreated K129 1420 denier yarns were tested using a bollard type fixture with pneumatic grips on an Instron 5942 load frame as shown in Fig. 7.2a. ASTM D7269 Standard Method for Tensile Testing of Aramid Yarns was used as a guide. Load was applied by moving the crosshead at a rate of 127 mm/min while measuring the force with a 500 N load cell. The gauge length of the yarns, from grip to grip, was 254 mm. Tensile tests enable the measurement of the strength and modulus of the augmented Kevlar on the yarn scale. By examining the stress strain curves such as those in Fig. 7.2b, changes in the intrinsic material properties important for ballistic performance can be identified. Currently, no statistically significant improvements to strength or modulus have been measured as a result of MWNT treatment. However, modifications to the synthesis process are being pursued that are expected to increase the intrinsic strength and modulus. Fig. 7.1 Roadmap of multi-scale testing, and modeling. Nanoscale reinforcements augment Kevlar yarns and fabrics. Experiments are conducted on the yarn, inter-yarn, weave and fabric scale to measure relevant properties. Properties are fed into finite element models, which range in scale from molecular to continuum, and enable synthesis optimization 7 Multi-scale Testing Techniques for Carbon Nanotube Augmented Kevlar 61
RkJQdWJsaXNoZXIy MTMzNzEzMQ==