Caracterização de defeitos em placas de aluminio através da análise da impedância de cerâmicas piezoelétricas e propagação do ultrassom

Abstract

Aluminum alloy is widely used in aerospace, automotive, prosthetics, orthotics, and other applications due to its lightness and mechanical strength. However, it is subject to failures due to fatigue, corrosion, and cracking, which can compromise its performance. Monitoring and detecting these damages are essential for ensuring the quality of devices and equipment. This work aimed to propose a methodology for nondestructive testing (NDT) and material characterization using the modulus and phase angle of the electrical impedance of piezoelectric ceramics coupled to aluminum plates, as well as parameters related to ultrasonic wave propagation. Six samples with dimensions of 500 x 500 x 5 mm³ were used, one of which was kept intact (reference), and in the others, three holes of the same diameter were drilled aligned in the center of the plate, separated by 125 mm, simulating flaws with 2 mm, 4 mm, 6 mm, 8 mm, and 10 mm in diameter. Four piezoelectric ceramic discs of the APC 855 type (12.7 mm in diameter and 2.0 mm thick) were fixed to each of the six plates using silver-based conductive glue. The electrical impedance was measured using an Agilent 4294A analyzer in three frequency bands: 40 kHz to 1.3 MHz (encompassing all vibration modes), 150 kHz to 200 kHz (radial mode), and 950 kHz to 1.3 MHz (axial or thickness mode). The RMSD (Root Mean Square Deviation) index compared the signals between the reference and defective samples. To evaluate the parameters of the ultrasonic waves propagating in the samples, a Tektronix AFG3021 function generator was used to excite the ceramics with sinusoidal pulses with an amplitude of 20 Vpp and containing one, five, or nine cycles, at frequencies of 150 kHz (radial mode), 400 kHz (transverse mode), and 1050 kHz (thickness mode). The signals from the receiving ceramics were acquired using a Tektronix TDS3034B digital oscilloscope and then transferred to a computer for processing in MATLAB. The RMSD values obtained for the ceramics located in one of the four positions of the reference plate and the defective samples ranged from 0.17 to 0.63 for all vibration modes, from 0.83 to 2.29 for the radial mode, and from 0.17 to 0.53 for the thickness mode, indicating that all defective plates presented a difference in impedance modulus when compared to the reference plate, with the radial mode of vibration presenting the most significant difference. Similar results were obtained for the other ceramics in the plates. Analyzing the results for ultrasonic wave propagation, it was found that all ceramics coupled to the defective samples produced different signals compared to the reference sample. Thus, it was concluded that it is possible to use electrical impedance analysis of the ceramics or ultrasound signal analysis to compare the defective samples and the reference sample to identify and characterize the materials using all the vibration modes of the ceramics or the radial and axial modes alone. Since the radial mode operates at a lower frequency range, more straightforward and cost-effective electronic circuits can be designed to function within this frequency range.