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Machado, MA, Rosado LFSG, Mendes NAM, Miranda RMM, dos Santos TJG.  2021.  New directions for inline inspection of automobile laser welds using non-destructive testing, sep. The International Journal of Advanced Manufacturing Technology. AbstractWebsite

An innovative pilot installation and eddy current testing (ECT) inspection system for laser-brazed joints is presented. The proposed system detects both surface and sub-surface welding defects operating autonomously and integrated with a robotized arm. Customized eddy current probes were designed and experimentally validated detecting pore defects with 0.13 mm diameter and sub-surface defects buried 1 mm deep. The integration of the system and the manufacturing process towards an Industry 4.0 quality control paradigm is also discussed.

Machado, MA, Silva MI, Martins AP, Carvalho MS, Santos TG.  2021.  Double active transient thermography, nov. NDT & E International. :102566. AbstractWebsite


Machado, MA, Antin K-N, Rosado LS, Vilaça P, Santos TG.  2021.  High-speed inspection of delamination defects in unidirectional CFRP by non-contact eddy current testing, nov. Composites Part B: Engineering. 224:109167. AbstractWebsite


Machado, MA, Rosado LS, Mendes NM, Miranda RM, Santos TG.  2021.  Multisensor Inspection of Laser-Brazed Joints in the Automotive Industry, nov. Sensors. 21:7335., Number 21 AbstractWebsite

Automobile laser brazing remains a complex process whose results are affected by several process variables that may result in nonacceptable welds. A multisensory customized inspection system is proposed, with two distinct non-destructive techniques: the potential drop method and eddy current testing. New probes were designed, simulated, produced, and experimentally validated in automobile's laser-brazed weld beads with artificially introduced defects. The numerical simulations allowed the development of a new four-point probe configuration in a non-conventional orthogonal shape demonstrating a superior performance in both simulation and experimental validation. The dedicated inspection system allowed the detection of porosities, cracks, and lack of bonding defects, demonstrating the redundancy and complementarity these two techniques provide.

Duarte, VR, Rodrigues TA, Machado MA.  2021.  Benchmarking of Nondestructive Testing for Additive Manufacturing. 3D Printing and Additive Manufacturing. AbstractWebsite

Defect detection in additive manufacturing (AM) is of paramount importance to improve the reliability of products. Nondestructive testing is not yet widely used for defect detection. The main challenges are a lack of standards and methods, the types and location of defects, and the complex geometry of many parts. During selective laser melting (SLM), several types of defects can occur such as porosity, cracking, and lack of fusion. In this study, several nondestructive tests were conducted in a highly complex shaped part in AISI 316L stainless steel with real defects manufactured by SLM. Two additional artificial defects (one horizontal and one flat bottom hole) were produced and the defect detectability was evaluated. The techniques used were as follows: dye penetrant, infrared thermography, immersion ultrasonic, eddy current, and X-ray microcomputed tomography to assess different types of defects in the as-built part. We conclude that no single technique can detect every type of defect, although multiple techniques provide complementary and redundant information to critically evaluate the integrity of the parts. This approach is fundamental for improving the reliability of defect detection, which will help expand the potential for using AM to produce parts for critical structural applications.

Costa, FB, Machado MA, Bonfait GJ, Vieira P, Santos TG.  2021.  Continuous wave terahertz imaging for NDT: Fundamentals and experimental validation. Measurement. 172(108904) AbstractWebsite

Continuous wave terahertz (CW THz) imaging, is a variant of terahertz imaging that has been gaining scientific
and technological relevance in multiple areas. In this paper the fundamental phenomena of CW THz were
studied and a mathematical model was developed that successfully describes the Fabry–Perot interference for
such a system, opening the possibility for measurement of thicknesses and surface curvatures. The capabilities
of the system were tested using different types of defects, such as voids, water infiltrations and thin metallic
wires. The interactions between different materials, features and the radiation beam were numerically studied
using finite element method and the results agreed with the experiments. By comparing the results with other
Non-Destructive Testing methods, it was found that CW THz imaging is particularly interesting to image water
infiltrations and composite materials that incorporate conductive wires.

Santos, TG, Oliveira JP, Machado MA.  2020.  Reliability and NDT Methods. Advanced Structured Materials. :265-295. Abstract

Composites are finding increased use in structural high demanding and high added value applications in advanced industries. A wide diversity exists in terms of matrix type, which can be either polymeric or metallic and type of reinforcements (ceramic, polymeric or metallic). Several technologies have been used to produce these composites; among them, additive manufacturing (AM) is currently being applied. In structural applications, the presence of defects due to fabrication is of major concern, since it affects the performance of a component with negative impact, which can affect, ultimately, human lives. Thus, the detection of defects is highly important, not only surface defects but also barely visible defects. This chapter describes the main types of defects expected in composites produced by AM. The fundamentals of different non-destructive testing (NDT) techniques are briefly discussed, as well as the state of the art of numerical simulation for several NDT techniques. A multiparametric and customized inspection system was developed based on the combination of innovative techniques in modelling and testing. Experimental validation with eddy currents, ultrasounds, X-ray and thermography is presented and analysed, as well as integration of distinctive techniques and 3D scanning characterization.

Sorger, GL, Oliveira JP, Inácio PL, Enzinger N, Vilaça P, Miranda RM, Santos TG.  2019.  Non-destructive microstructural analysis by electrical conductivity: Comparison with hardness measurements in different materials, mar. Journal of Materials Science & Technology. 35:360–368., Number 3 AbstractWebsite


Braz Fernandes, FM, Camacho E, Rodrigues PF, Inácio P, Santos TG, Schell N.  2019.  In Situ Structural Characterization of Functionally Graded Ni–Ti Shape Memory Alloy During Tensile Loading, dec. Shape Memory and Superelasticity. 5:457–467., Number 4 AbstractWebsite


Machado, MA, Antin K-N, Rosado LS, Vilaça P, Santos TG.  2019.  Contactless high-speed eddy current inspection of unidirectional carbon fiber reinforced polymer. Composites Part B: Engineering. 168:226-235. AbstractWebsite

This paper presents the development and the results of a customized eddy current (EC) non-destructive testing (NDT) system for highly demanding online inspection conditions. Several planar eddy current array probes were designed, numerically simulated and experimentally compared for the inspection of low conductivity unidirectional carbon fibre reinforced polymer (CFRP) ropes. The inspections were performed using a dedicated scanner device at 4 m/s with 3 mm probe lift-off where defects under 1 mm were detected with an excellent SNR. Different defect morphologies and sizes, such as broken fibres and lateral cuts, were successful detected and compared to conventional probes.

Antin, K-N, Machado MA, Santos TG, Vilaça P.  2019.  Evaluation of Different Non-destructive Testing Methods to Detect Imperfections in Unidirectional Carbon Fiber Composite Ropes. Journal of Nondestructive Evaluation. 38(23) AbstractWebsite

Online monitoring of carbon fiber reinforced plastic (CFRP) ropes requires non-destructive testing (NDT) methods capable of detecting multiple damage types at high inspection speeds. Three NDT methods are evaluated on artificial and realistic imperfections in order to assess their suitability for online monitoring of CFRP ropes. To support testing, the microstructure and electrical conductivity of a carbon fiber rope is characterized. The compared methods are thermography via thermoelastic stress analysis, ultrasonic testing with commercial phased array transducers, and eddy current testing, supported by tailor-made probes. While thermoelastic stress analysis and ultrasonics proved to be accurate methods for detecting damage size and the shape of defects, they were found to be unsuitable for high-speed inspection of a CFRP rope. Instead, contactless inspection using eddy currents is a promising solution for real-time online monitoring of CFRP ropes at high inspection speeds.

Machado, MA, Antin KN, Rosado LS, Vilaça P, Santos TG.  2019.  High speed inspection of UD CFRP composites. 58th Annual Conference of the British Institute of Non-Destructive Testing, NDT 2019. , Telford, UK Abstract

Online monitoring of carbon fibre reinforced polymer (CFRP) components requires a Non-Destructive Testing (NDT) method capable of contactless sensing of damage, while enabling high inspection speeds needed for monitoring large components. Eddy current testing (ECT) of CFRP components has great potential for two reasons. First, ECT probes are capable of operating without contact, although minimizing the lift-off is preferred. Second, impedance analysers with high sample rates make high-speed inspection possible. This research assesses the damage detection capabilities of eddy current probes on CFRP samples with artificial and realistic damage. To support the aptitude of the ECT method for these needs, the CFRP material is characterized and numerical simulations are performed in order to develop optimized and tailored ECT probes for the detection of defects with different morphologies, namely fibre breakage and delaminations, and to take into consideration the highly anisotropic electrical bulk resistivity of the CFRP material. Different ECT probes were designed, produced and experimentally validated. The experiments were performed at a high inspection speed (4 m/s) and the high sensitivity of the probes was demonstrated.

Machado, MA, Inácio PL, Santos RA, Gomes AF, Martins AP, Carvalho MS, Santos TG.  2019.  Inspection of composite parts produced by additive manufacturing: Air-coupled ultrasound and thermography. 58th Annual Conference of the British Institute of Non-Destructive Testing, NDT 2019. , Telford, UK Abstract

Polymeric parts produced by Fused Deposition Modelling (FDM) Additive Manufacturing (AM) has no special safety requirements, and therefore, NDT is not required. However, the use of AM to produce Fibre Reinforcement Thermoplastics (FRTP) parts means that structural applications with safety requirements are envisaged, demanding reliable NDT methods. This paper presents experimental results and numerical simulation by Finite Element Method (FEM) of the NDT inspection of different parts of polymeric and RFTP composite materials. The parts were produced by FDM Additive Manufacturing and different delamination defects were introduced at different positions and with different dimensions and morphologies. Two different NDT techniques were used, exploiting different inspection parameters: air-coupled ultrasound, using frequencies between 50 and 400 kHz and active transient thermography, in both reflection and transition modes. The influence of the curvature of the parts was analysed, from the experimental point of view, and the results were compared with different numerical simulation strategies. It was shown that, both NDT techniques can detect the defects, with good spatial resolution, being the thermography reflection mode the fastest and expedite for curvature parts. The numerical simulation corroborates the experimental results allowing a deeper insight on the physical phenomena involved.

Sorger, GL, Oliveira JP, Inácio PL, Enzinger N, Vilaça P, Miranda RM, Santos TG.  2019.  Non-destructive microstructural analysis by electrical conductivity: Comparison with hardness measurements in different materials. Journal of Materials Science and Technology. 35:360-368. AbstractWebsite

The use of non-destructive evaluation (NDE) techniques for assessing microstructural changes in processed materials is of particular importance as it can be used to assess, qualitatively, the integrity of any material/structure. Among the several NDE techniques available, electrical conductivity measurements using eddy currents attract great attention owing to its simplicity and reliability. In this work, the electrical conductivity profiles of friction stir processed Ti6Al4V, Cu, Pb, S355 steel and gas tungsten arc welded AISI 304 stainless steel were determined through eddy currents and four-point probe. In parallel, hardness measurements were also performed. The profiles matched well with the optical macrographs of the materials: while entering in the processed region a variation in both profiles was always observed. One particular advantage of electrical conductivity profiles over hardness was evident: it provides a better resolution of the microstructural alterations in the processed materials. Moreover, when thermomechanical processing induces microstructural changes that modify the magnetic properties of a material, eddy currents testing can be used to qualitatively determine the phase fraction in a given region of the material. A qualitative relation between electrical conductivity measurements and hardness is observed.

Lopez, A, Bacelar R, Pires I, G.Santos T, PedroSousa J, Quintino L.  2018.  Non-destructive testing application of radiography and ultrasound for wire and arc additive manufacturing. Additive Manufacturing. 21:298-306. AbstractWebsite

The present work addressed the challenges of identifying applicable Non-Destructive Testing (NDT) techniques suitable for inspection and materials characterization techniques for Wire and Arc Additive Manufacturing (WAAM) parts. With the view of transferring WAAM to the industry and qualifying the manufacturing process for applications such as structural components, the quality of the produced parts needs to be assured. Thus, the main objective of this paper is to review the main NDT techniques and assess the capability of detecting WAAM defects, for inspection either in a monitoring, in-process or post-process scenario. Radiography and ultrasonic testing were experimentally tested on reference specimens in order to compare the techniques capabilities. Metallographic, hardness and electrical conductivity analysis were also applied to the same specimens for material characterization. Experimental outcomes prove that typical WAAM defects can be detected by the referred techniques. The electrical conductivity measurement may complement or substitute some destructive methods used in AM processing.

Machado, MA, Rosado L, Pedrosa N, Miranda RM, Piedade M, Santos TG.  2017.  Customized Eddy Current Probes for Pipe Inspection. Studies in Applied Electromagnetics and Mechanics. :283-290.: IOS Press Ebooks Abstract

A novel Eddy Current (EC) probe configurations were developed to detect millimeter defects with any orientation on inner or outer pipe surfaces. The probes were designed and experimentally validated in different materials where the defects tested were identified with a high sensitivity and good signal-to-noise ratio.

Machado, MA, Rosado L, Pedrosa N, Vostner A, Miranda RM, Piedade M, Santos TG.  2017.  Novel eddy current probes for pipes: Application in austenitic round-in-square profiles of ITER. NDT&E International. 87:111-118. AbstractWebsite

Novel eddy current probes were developed to detect sub-millimetre defects with any orientation on the inner surface of pipes. Five different probes were designed, produced and experimentally validated. These probes include arrays of planar trapezoidal coils in a flexible substrate used alone or together with different winded drive coils. Numerical simulations with Finite Element Method were used to predict the probe response to defects with any orientation. Experimental results in austenitic steel jackets used in ITER revealed that the new probes have an improved reliability compared to conventional toroidal bobbin probes, allowing a higher sensitivity to circumferential defects.

Santos, TG, Miranda RM, Vieira TM, Farinha RA, Ferreira TJ, Quintino L, Vilaça P, de Carvalho CCCR.  2016.  Developments in micro- and nano-defects detection using bacterial cells. NDT & E International. 78:20-28. AbstractWebsite

This paper describes improvements to the Nondestructive Testing (NDT) technique recently proposed, based on the use of bacterial cell suspensions to identify micro- and nano-surface defects. New bacterial strains were used with magnetic fields to improve bacteria mobility. Different materials and defect morphologies were tested, including nanoindentation defects, micro-powder injection moulding components and micro-laser welding. Nanoindentations with 0.6 µm depth and 5.3 µm side length were successfully detected. Bacterial cells allow identifying different topographic attributes of the surfaces, such as roughness. Cracks of about 0.5 µm wide and 10 µm depth in a reference test block Type 1 were successfully detected.

Rosado, LS, Santos TG, Ramos PM, Vilaça P, Piedade M.  2015.  A new dual driver planar eddy current probe with dynamically controlled induction pattern, 2015. NDT and E International. 70:29-37.: Elsevier Ltd AbstractWebsite
Gomes, JF, Miranda RM, Santos TJ, Carvalho PA.  2014.  Emission of nanoparticles during friction stir welding (FSW) of aluminium alloys, 2014. Journal of Toxicology and Environmental Health - Part A: Current Issues. 77(14-16):924-930.: Taylor and Francis Inc. AbstractWebsite
Santos, TG, Miranda RM, Vilaça P.  2014.  Friction Stir Welding assisted by electrical Joule effect, 2014. Journal of Materials Processing Technology. 214(10):2127-2133.: Elsevier Ltd AbstractWebsite
Santos, TG, Miranda RM, Vilaça P.  2014.  Friction stir welding assisted by electrical joule effect to overcome lack of penetration in aluminium alloys, 2014. 17th Conference of the European Scientific Association on Material Forming, ESAFORM 2014. 611-612:763-772., Espoo: Trans Tech Publications Ltd AbstractWebsite
Vilaça, P, Santos TG, Rosado L, Miranda RM.  2014.  Innovative concept and application of EC probe for inspection of friction stir welds, 2014. International Journal of Microstructure and Materials Properties. 9(3-5):314-326.: Inderscience Enterprises Ltd. AbstractWebsite


Santos, TG, Miranda RM, De Carvalho CCCR.  2014.  A new NDT technique based on bacterial cells to detect micro surface defects, 2014. NDT and E International. 63:43-49. AbstractWebsite
Santos, TG, Sorger G, Vilaça P, Miranda RM.  2014.  A non-conventional technique for evaluating welded joints based on the electrical conductivity, 2014. 17th Conference of the European Scientific Association on Material Forming, ESAFORM 2014. 611-612:671-676., Espoo: Trans Tech Publications Ltd AbstractWebsite