PSI - Issue 52

Miray Yasar et al. / Procedia Structural Integrity 52 (2024) 165–175 Miray Yasar et. al. / Structural Integrity Procedia 00 (2019) 000 – 000

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surface resin was polished on each side using sandpaper to expose the carbon fibre reinforcement and ensure that the terminals of the power supply could be placed in direct contact with the conductive carbon fibre layer. The silicone oil was heated to 100 o C and the composite was immersed in the preheated silicone oil for 30 minutes. A voltage of 10 kV per mm thickness of the PVDF layer was applied for 2 hours to align the PVDF chains. During the poling procedure, the electrodes should not touch each other which is difficult to prevent with carbon fibres. Therefore, it is crucial to use a larger area of PVDF-PPS layer between the carbon fibre plies to prevent shorting during the poling process. 2.2.3. Characterization FTIR spectra for the films was recorded using a Thermoscientific Summit Pro FTIR Spectrometer by accumulating 64 sample scans with a resolution of 4 cm -1 over a range of 700-1600 cm -1 . XRD analysis was performed using a Siemens D500 X-ray Diffractometer between 3-50 o , a step size of 0.02 degrees and 1 sec/step dwell time. The flexural modulus and flexural strength of the composite samples were investigated following 3-point bending tests in accordance with the ISO 14125. The tests were carried out using five samples with a thickness of 2 mm, width of 15 mm and length of 100 mm. The distance between the two supports was adjusted to 80 mm and a cross head displacement rate of 1 mm/min was employed. The interlaminar shear strength (ILSS) properties of the carbon fibre composites were evaluated using ISO 14130. The specimen width was selected as five times the thickness and the overall length as ten times the thickness. The span for the test was five times the thickness and the cross-head displacement rate was 1 mm/min. The mode I fracture toughness of the composites was determined following ISO 15024. Kapton tape was applied on the middle ply stack sequence, simulating a crack as per the standard. The samples were cut from the panel to the final dimensions of 150 mm length and 25 mm width and the load blocks were fixed in place using an adhesive to carry out the tests. Correction fluid was applied on one side to visualize the delamination or crack propagation. The progress of the crack was monitored using a digital camera synchronized with the load and displacement measurements. Five samples were tested in each case at a crosshead displacement rate of 2 mm/min. The mode I fracture toughness ‘G IC ’ of the composites are calculated using Equation 1. = 3 2 ( +| |) (1) where P denotes the load and δ is the corresponding displacement value, b is the sample width and a is the total delamination length which is the sum of initial delamination length a 0 and the measured delamination length increments. F represents the large-displacement correction which is calculated as per the standard. The fractured surface of the delaminated region was observed using a Hitachi TM4000 Tabletop microscope for the SEM analysis at an accelerating voltage of 15 kV, a working distance of 10.9 mm. The piezoelectric behaviour of the samples was investigated by monitoring the voltage output response of the composites when a 10 N square wave load was applied with a frequency of 0.3 Hz. For this test, composites were prepared in a near-net shape by keeping the PVDF-MWCNTs Impregnated PPS Veil larger than the fibres. As noted above, this method prevents the contact of carbon fibre electrodes. The custom-made laboratory setup includes three main parts: a sample holder with a load cell connected to a function generator, a charge amplifier (Kistler Type 5019A), and an oscilloscope. The sample was connected to a charge amplifier to convert very low piezoelectric charge signals into proportional voltage signals. Prior to piezoelectric measurements, as noted above, the surface resin was polished using sandpaper to provide a conductive pathway. Silver conductive ink (Fisher Scientific UK Ltd.) was applied to the upper and lower electrode surfaces, and coaxial wire connections were soldered onto copper tape, which was then adhered to the silver conductive ink. 3. Results and Discussion 3.1. FTIR Analyses The PVDF impregnated PPS veils were analysed by FTIR and the result is shown in Figure 2. The β phase content of the PVDF layer was calculated using Equation 2, where K ⍺ = 6.1x10 4 for ⍺ =766 cm -1 and K β =7.7x10 4 for β=840 cm -1 .