Magnetostriction is an inherent magnetic property that changes the volume of a magnetic material when a magnetic field is applied, and vice versa when a strain is applied the magnetisation within the magnetic material rotates and therefore changes the measured magnetic field. Therefore, this property could be used for structural health monitoring (SHM) of aircraft grade carbon fibre structure to study the changes in strain due to damage. Magnetostrictive materials are functional materials that have previously provided high resolution to detect defects or delamination in carbon fibre reinforced polymer (CFRP). Soft magnetic materials such as stainless steel 17/4 ph, Fe3O4 and Ni, are ideal as they have low coercivity, high saturation magnetisation and high anisotropy field. Additive manufacturing of functional materials has been an area of interest in science and engineering to exploit 3D and 4D designs, which saves cost and materials. Therefore, this project studied the design and printing of magnetostrictive material sensors to detect damage on CFRP. Two different types of printing were explored, metal extrusion (desktop metal bound deposition printer) and inkjet printing (JetLab IV) for different soft magnetic materials. Magnetic characterisation of the soft magnetic materials included using a superconducting quantum interference device (SQUID) magnetometer to measure the magnetic hysteresis loop. Structural characterisation included using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and fourier-transform infrared spectroscopy (FT-IR). It was found that printing stainless steel 17/4 ph by a desktop bound metal deposition printer was unsuitable for printing large, thin structures as fractures occurred due to thermal fluctuation and warping during the sintering process.
The Joule magnetostriction and the Villari effect were measured to test the sensor’s performance under magnetic field and under strain respectively. The magnetic response to strain by bending was measured using an inductance coil and a Hall probe. Bending rigs with radius of curvature of 600 mm, 500 mm, 400 mm, 300 mm, 200 mm and 100 mm were used to strain the sensor. The change in magnetic field was measured using a coil inductor with various turns of copper wire. For inkjet printing, five factors were used to assess the print, which were: material choice (ink), print direction, design choice, substrate selection and additive layering. It was found that the magnetite coil design between 10 and 20 layers gave the best response to the change in field as a function of strain.
SHM of CFRP was evaluated by impact testing a defined weight onto a CFRP sheet. The inductance generated from the dropped weight was analysed by testing the inductor coil on a polycarbonate sheet. It was found that printing magnetite directly on CFRP rather than on a substrate, increased the measured inductance and performance during impact. A magnetite line was printed across the sheet to test the SHM before and after impact on CFRP. Multiple layers of magnetite provided a better response to SHM during impact; however, to detect defects near impact, the level of change in inductance would not be suitable for service. Future work will continue to develop new materials for printing and testing on CFRP to replicate SHM technique on aircraft.
