Short Shaped Copper Fibers in an Epoxy Matrix: Their Multifunctional Use — Fracture and Impact Toughening and EMI Shielding

Previous research has shown that short shaped copper fibers can significantly increase the fracture toughness of a thermoset matrix composite. This paper considers the effectiveness of adding these same short shaped copper fibers to improve the composite’s impact toughness. Further, the electromagnetic interference (EMI) shielding effectiveness (SE) of these shaped copper fibers will be analyzed. The matrices used in the impact experiments were two different types of epoxy, a low shrinkage epoxy (LS) and a high shrinkage epoxy (HS). Fiber shapes and lengths were chosen based on previous single fiber pullout experiments. The following shaped fibers were used: straight, flat end-impacted, rippled, and end-oxidized. The two fiber lengths tested were 6mm and 10mm. Results indicate that the largest influence on impact toughness is from the matrix fracture behavior. When the matrix failed around the fibers, the impact toughness was significantly less compared to matrices with the same fibers that had a planar fracture at the notch during impact. Fiber end modification and to a lesser extent the fiber length both also had an effect on the fiber pullout behavior and resulting toughness. Comparing the end-modified fibers to straight fibers, the largest increase in impact toughness was 17%. The matrix used for the EMI shielding experiments was HS epoxy and two fiber diameters were tested: 0.325 mm and 0.162 mm. The fiber shapes used in the experiments were: straight, flat end-impacted, rippled, and acid roughened. An EMI SE of > 40 dB at 1.5 GHz was attained using 15vol% of the 0.162 mm diameter shaped fibers. The composites with 15vol% of the 0.325 mm diameter shaped fibers showed poor EMI SE, < 20 dB, due to large fiber diameter that results in a lower skin depth volume and a lower number of fibers which results in a lower number of conducting paths for attenuation through the composite. Results indicate that besides improving the fracture and impact toughness of a thermoset polymer matrix, short shaped copper fibers can also significantly improve the EMI SE of the composite, resulting in a multifunctional material.