Abstract
Components designed for nondestructive evaluation or structural health monitoring applications often leverage smart materials, either mounted on the surface or embedded within, as a sensing element. The principle of magnetostriction represents a viable relationship to exploit in health assessment, especially with carbon fiber composites. A commercial carbon fiber filament for additive manufacturing, Onyx, consists of chopped fibers suspended in a polymeric host to enable heating and deposition on a build plate. Physical vapor deposition (PVD) sputtering provides an inert environment for safely depositing thin films of Terfenol-D, a highly pyrophoric magnetostrictive material when utilized in micro-sized particulate form, onto the surface of substrates. The sputtering process deposited magnetostrictive thin films onto a 6-millimeter diameter indentation surface in the gage area of 3D printed Onyx specimens. Heat treatment pre- and post-sputtering altered the crystal structure of the Onyx substrate, evident by the results of quasistatic uniaxial tensile experiments. Examination of the stress and strain data from the tensile tests found the elastic modulus and yield stress of the heat-treated, sputtered specimens to increase by nearly 77% and 62%, respectively, compared to the baseline specimens. A magnetometer aligned with the sputtered Terfenol-D regions provided magnetic flux density readings throughout the quasistatic tests. The alignment of the magnetometer probe relative to the sputtered surface influenced the magnitude of the magnetic flux density reading. Elongation of the Onyx material displaced the sputtered region from the initial alignment with the probe during testing resulting in variations in the magnitude of the magnetic flux density measurements. The ability to capture a magnetic response from a surface-deposited thin film of the highly magnetostrictive material Terfenol-D, with an estimated volume of 0.035 cubic millimeters, makes sputtering an excellent technique for developing a small displacement sensing element to detect damage in CFRP laminates and 3D printed composite components.
