Ultrasonic evaluation of induction heat treatment applied to thermoplastic matrix CFRP

Abstract Thermoplastic matrix carbon fibre reinforced polymers (CFRP) are extensively utilized for composites structures in the aerospace and aeronautical industries. Diverse techniques were currently applied to joining composite parts, the most promising method is the induction heat treatment. In this paper, experimental tests were performed on thermoplastic matrix CFRP specimens by varying the induction heat treatment parameters: power, frequency and current. An advanced ultrasonic (UT) non-destructive evaluation based on pulse-echo technique was employed for the investigation of the utilized process parameters through the UT evaluation of the process induced damage and its depth along the thickness of the thermoplastic matrix CFRP laminates

Wireless Sensors and Actuators for Structural Health Monitoring of Fiber Composite Materials

This work evaluates and investigates the wireless generation and detection of Lamb-waves on fiber-reinforced materials using surface applied or embedded piezo elements. The general target is to achieve wireless systems or sensor networks for Structural Health Monitoring (SHM), a type of Non-Destructive-Evaluation (NDE). In this sense, a fully wireless measurement system that achieves power transmission implementing inductive coils is reported. This system allows a reduction of total system weight as well as better integration in the structure. A great concern is the characteristics of the material, in which the system is integrated, because the properties can have a direct impact on the strength of the magnetic field. Carbon-Fiber-Reinforced-Polymer (CFRP) is known to behave as an electrical conductor, shielding radio waves with increasing worse effects at higher frequencies. Due to the need of high power and voltage, interest is raised to evaluate the operation of piezo as actuators at the lower frequency ranges. To this end, actuating occurs at the International Scientific and Medical (ISM) band of 125 kHz or low-frequency (LF) range. The feasibility of such system is evaluated extensively in this work. Direct excitation, is done by combining the actuator bonded to the surface or embedded in the material with an inductive LF coil and setting the circuit in resonance. A more controlled possibility, also explored, is the use of electronics to generate a Hanning-windowed-sine to excite the PWAS in a narrow spectrum. In this case, only wireless power is transmitted to the actuator node, and this lastly implements a Piezo-driver to independently excite Lamb-waves. Sensing and data transfer, on the other hand, is done using the high-frequency (HF) 13.56 MHz. The HF range covers the requirements of faster sampling rate and lower energy content. A re-tuning of the antenna coils is performed to obtain better transmission qualities when the system is implemented in CFRP. Several quasi-isotropic (QI) CFRP plates with sensor and actuator nodes were made to measure the quality of transmission and the necessary energy to stimulate the actuator-sensor system. In order to produce baselines, measurements are prepared from a healthy plate under specific temperature and humidity conditions. The signals are evaluated to verify the functionality in the presence of defects. The measurements demonstrate that it is possible to wirelessly generate Lamb-waves while early results show the feasibility to determine the presence of structural failure. For instance, progress has been achieved detecting the presence of a failure in the form of drilled holes introduced to the structure. This work shows a complete set of experimental results of different sensor/-actuator nodes

Directional eddy current probe configuration for in-line detection of out-of-plane wrinkles

Real-time monitoring of carbon fibre composites during Automated Fibre Placement (AFP) manufacturing remains a challenge for non-destructive evaluation (NDE) techniques. An directional eddy-current (EC) probe with asymmetric transmit and differential receive (Tx-dRx) coils is designed, constructed and characterized to evaluate the detectability of out-of-plane wrinkles. Initial studies were conducted to determine suitable excitation frequencies and to analyse the impact of relative orientations of driver and pickup coils on wrinkle detectability. The probe configurations are evaluated experimentally and employ a new finite element modelling approach to better understand the relationship between eddy-current density and defect detection. The findings indicate that a probe configuration with an asymmetric driver coil normal to the material surface and aligned with the fibre directions, and with differential pickup coils 90 degrees to the scanning direction, shows the best capability for out-of-plane wrinkle detection, with SNR >20 for wrinkles over 1.3 mm in amplitude

Development and characterization of sensors fabricated from polymer based magnetoelectric nanocomposites

Tese de Doutoramento em Engenharia Electrónica e de ComputadoresSensors are increasingly used in many applications areas, integrated in structures, industrial machinery, or in the environment, contributing to improve the society level of well-being. It is expected that sensorization will play on of the most relevant roles in the fourth industrial revolution, and allow, together with mechanization and informatization, a full automation. Particularly, magnetic sensors allow measurements, without physical contact, of parameters such as direction, presence, rotation, angle, or current, in addition to magnetic field. In this way, for most applications, such sensors offer a safe, noninvasive and non-destructive measurement, as well as provide a reliable and almost maintenance-free technology. Industry demands for smaller, cheaper and low-powered magnetic sensors, motivating the exploration of new materials and different technologies, such as polymerbased magnetoelectric (ME) composites. These composites are flexible, versatile, lightweight, low cost, easy to model in complicated shapes, and typically involve a lowtemperature fabrication process, being in this way, a solution for innovative magnetic sensor device applications. Therefore, the main objective of this thesis is the development of polymer-based ME sensors to be incorporated into technological devices. Thus, the ME effect is increasingly being considered an attractive alternative for magnetic field and current sensing, being able to sense static and dynamic magnetic fields. In order to obtain a wide-range ME response, a nanocomposite of Tb0.3Dy0.7Fe1.92 (Terfenol-D)/CoFe2O4/poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) was produced and their morphological, piezoelectric, magnetic and magnetoelectric properties investigated. The obtained composites reveals a high piezoelectric response (≈-18 pC∙N- 1) that is independent of the weight ratio between the fillers. In turn, the magnetic properties of the composites are influenced by the composite composition. It was found that the magnetization saturation values decrease with increasing CoFe2O4 content (from 18.5 to 13.3 emu∙g-1) while the magnetization and coercive field values increase (from 3.7 to 5.5 emu∙g-1 and from 355.7 to 1225.2 Oe, respectively) with increasing CoFe2O4 content. Additionally, the films show a wide-range dual-peak ME response at room temperature with the ME coefficient increasing with increasing weight content of Terfenol-D, from 18.6 mV∙cm-1∙Oe-1 to 42.3 mV∙cm-1∙Oe-1. The anisotropic ME effect on a Fe61.6Co16.4Si10.8B11.2 (FCSB)/poly(vinylidene fluoride) (PVDF)/FCSB laminate composite has been used for the development of a magnetic field sensor able to detect both magnitude and direction of ac and dc magnetic fields. The accuracy (99% for both ac and dc sensors), linearity (92% for the dc sensor and 99% for the ac sensor), sensitivity (15 and 1400 mV∙Oe-1 for the dc and ac fields, respectively), and reproducibility (99% for both sensors) indicate the suitability of the sensor for applications. A dc magnetic field sensor based on a PVDF/Metglas composite and the corresponding readout electronic circuits for processing the output ME voltage were developed. The ME sensing composite presents an electromechanical resonance frequency close to 25.4 kHz, a linear response (r2=0.997) in the 0–2 Oe dc magnetic field range, and a maximum output voltage of 112 mV (ME voltage coefficient α33 of ≈30 V∙cm-1∙Oe-1). By incorporating a charge amplifier, an ac–rms converter and a microcontroller with an on chip analog-to-digital converter (ADC), the ME voltage response is not distorted, the linearity is maintained, and the ME output voltage increases to 3.3 V (α33effective=1000 V∙cm-1∙Oe-1). The sensing device, including the readout electronics, has a maximum drift of 0.12 Oe with an average total drift of 0.04 Oe, a sensitivity of 1.5 V∙Oe-1 (15 kV∙T-1), and a 70 nT resolution. Such properties allied to the accurate measurement of the dc magnetic field in the 0–2 Oe range makes this polymerbased device very attractive for applications, such as Earth magnetic field sensing, digital compasses, navigation, and magnetic field anomaly detectors. A dc current sensor device based on a ME PVDF/Metglas composite, a solenoid, and the corresponding electronic instrumentation were developed. The ME sample exhibits a maximum α33 of 34.48V∙cm-1∙Oe-1, a linear response (r2=0.998) and a sensitivity of 6.7 mV∙A-1. With the incorporation of a charge amplifier, a precision ac/dc converter and a microcontroller, the linearity is maintained (r2=0.997), the ME output voltage increases to a maximum of 2320 mV and the sensitivity is increased to 476.5 mV∙A-1. Such features indicate that the fabricated ME sensing device is suitable to be used in non-contact electric current measurement, motor operational status checking, and condition monitoring of rechargeable batteries, among others. In this way, polymer-based ME composites proved to be suitable for magnetic field and current sensor applications.Os sensores estão a ser cada vez mais utilizados em diversas áreas, integrados em estruturas, máquinas industriais ou projetos ambientais, contribuindo para melhorar o nível de bem-estar e eficiência da nossa sociedade. Espera-se que a “sensorização” contribua decisivamente para a quarta revolução industrial, e que permita, em conjunto com a mecanização e a informatização, uma completa automação. Em particular, os sensores magnéticos permitem medir parâmetros como a direção, presença, rotação, ângulo ou corrente, para além do campo magnético, tudo isto sem qualquer contacto físico. Assim, para a maioria das aplicações, estes sensores oferecem uma medição segura, não invasiva e não destrutiva, para além de garantirem uma tecnologia confiável e de escassa manutenção. A indústria procura e exige sensores magnéticos mais pequenos, mais baratos e de baixo consumo, daí a motivação para explorar novos materiais e diferentes tecnologias, tais como os compósitos magnetoelétricos (ME) baseados em polímeros. Estes compósitos são flexíveis, versáteis, leves, de baixo custo, fáceis de se modelar em formas complexas e tipicamente envolvem um processo de fabricação a baixa temperatura, constituindo uma solução fiável e de qualidade para os sensores magnéticos. É da constatação deste potencial que surge este estudo e o objetivo desta tese: o desenvolvimento de sensores ME de base polimérica. O efeito ME é cada vez mais considerado como uma alternativa credível para a medição de campo magnético e da intensidade da corrente elétrica, podendo detetar campos magnéticos estáticos e dinâmicos. De modo a obter uma gama mais alargada de resposta ME, produziram-se nanocompósitos de Tb0.3Dy0.7Fe1.92 (Terfenol-D)/CoFe2O4/poli(fluoreto de vinilideno trifluor-etileno) (P(VDF-TrFE) e as suas propriedades morfológicas, piezoelétricas, magnéticas e magnetoelétricas foram investigadas. Os compósitos obtidos revelam uma elevada resposta piezoelétrica (≈-18 pC∙N-1) que é independente da percentagem de cada material magnetoestrictivo. Por sua vez, as propriedades magnéticas são influenciadas pela composição dos compósitos. Verificou-se que a magnetização de saturação diminuí com o aumento da percentagem de CoFe2O4 (de 18.5 para 13.3 emu∙g-1) enquanto que a magnetização e o campo coercivo aumentam (de 3.7 para 5.5 emu∙g-1 e de 355.7 para 1225.2 Oe, respetivamente) com o aumento da percentagem em massa de CoFe2O4. O efeito ME anisotrópico num compósito Fe61.6Co16.4Si10.8B11.2 (FCSB)/ poli(fluoreto de vinilideno) (PVDF)/FCSB laminado foi utilizado para desenvolver um sensor de campo magnético capaz de detetar tanto a magnitude como a direção de campos magnéticos ac e dc. A exatidão (99% para ambos os sensores ac e dc), linearidade (92% para o sensor dc e 99% para o ac), sensibilidade (15 e 1400 mV∙Oe-1 para o sensor dc e ac, respetivamente) e reprodutibilidade (99% para ambos os sensores) indicam a aptidão destes sensores para aplicações avançadas. Desenvolveu-se ainda um sensor de campo magnético dc baseado num compósito ME de PVDF/Metglas, bem como a correspondente eletrónica de leitura para processar a tensão de saída ME. O compósito ME apresenta uma ressonância eletromecânica de aproximadamente 25.4 kHz, uma resposta linear (r2=0.997) para uma gama de campos magnéticos dc entre 0–2 Oe e uma tensão de saída máxima de 112 mV (coeficiente ME α33≈30 V∙cm-1∙Oe-1). Ao incorporar um amplificador de carga, um conversor ac–rms e um microcontrolador com um conversor analógico-digital (ADC), a tensão ME não é distorcida, a linearidade manteve-se e a tensão ME aumentou para 3.3 V (α33efectivo=1000 V∙cm-1∙Oe-1). O sensor, incluindo a eletrónica de leitura, obteve um desvio máximo de 0.12 Oe com um desvio total médio de 0.04 Oe, uma sensibilidade de 1.5 V∙Oe-1 (15 kV∙T-1) e 70 nT de resolução. Tais propriedades aliadas à medida exata do campo magnético dc entre 0–2 Oe tornam este dispositivo indicado para aplicações como sensores de campo magnético terrestre, compassos digitais, navegação e detetores de anomalia no campo magnético. Foi ainda possível desenvolver e otimizar um sensor de corrente baseado num compósito ME de PVDF/Metglas, num solenoide e na correspondente eletrónica de instrumentação. A amostra ME exibe um α33 máximo de 34.48V∙cm-1∙Oe-1, uma resposta linear (r2=0.998) e uma sensibilidade de 6.7 mV∙A-1. Com a incorporação de um amplificador de carga, um conversor ac/dc de precisão e um microcontrolador, a linearidade manteve-se, a tensão ME aumentou para um máximo de 2320 mV e a sensibilidade subiu para 476.5 mV∙A-1. Estas propriedades tornam este sensor ME apropriado para a medição de corrente elétrica sem contato, para a verificação do estado de funcionamento de motores e para monitorização da condição de baterias recarregáveis, entre outros. Concluindo-se deste modo que os compósitos de ME com base em polímeros provaram ser adequados para aplicações na medição de campos magnéticos e intensidade de corrente elétrica