0018-9464 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TMAG.2016.2636807, IEEE Transactions on Magnetics C108 0018-9464 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. (Inserted by IEEE.) 1 Advanced NDT Inspection Tools for Titanium Surfaces based on High Performance Magnetoresistive Sensors Fernando Franco 1,2 , Filipe A. Cardoso 1 , Luís S. Rosado 2,3 , Ricardo Ferreira 4 , Susana Cardoso 1,2 , Moisés Piedade 2,3 , and Paulo P. Freitas 1,4 1 INESC - Microsistemas e Nanotecnologias, Lisbon 1000-029, Portugal 2 Instituto Superior Técnico, Universidade de Lisboa, Lisbon 1049-001, Portugal 3 INESC - Investigação e Desenvolvimento, Lisbon 1000-029, Portugal 4 International Iberian Nanotechnology Laboratory, Braga 4715-330, Portugal Eddy current testing (ECT) is a high impact technology to detect surface flaws. However its reliability is reduced for low conductive titanium alloys, which are broadly integrated in the industry. ECT tools based on magnetoresistive (MR) sensors offer advantages over inductive sensors due to an enhanced spatial resolution, high sensitivity and bandwidth. In this work, we demonstrate successful NDT inspection of titanium surfaces, not achieved by other ECT tools. Here, single surface defect (length = 0.6 mm; width = 100 m; depth = 50 m) in a non polished TA6V titanium mock-up was measured using a MR-ECT probe. The detection was made by an array of magnetic tunnel junctions (MTJ) with 50x50 m 2 with optimized field detectivity. A differential measurement employing an heterodyne technique isolated the magnetic field component (fH-fbias = 1 kHz) from the electric biasing component (fbias = 4999 kHz) and electromagnetic coupling (fH = 5 MHz), revealing a bipolar defect signature of 6.01 V0-p amplitude. Index Terms—Eddy Current Testing, Magnetic Tunnel Junction, Magnetoresistive Sensors, Non-destructive Testing, Titanium Alloy. I. INTRODUCTION DEMAND for non-destructive testing (NDT) technologies is rising due to a strong pressure from the industry in order to improve the reliability and product maintenance without compromising profit. NDT gathers methods which address a wide range of industrial sectors where the rupture of a non-evaluated structure can lead to a catastrophic situation involving environmental and public safety. Titanium alloys are broadly integrated in aerospace industry [1] and biomedical applications [2] due to their mechanical properties (high strength to weight ratio and corrosion resistance). In aerospace industry, engine blades are under a constant impact with particles at velocities between 200 to 300 m.s -1 [3], generating initial flaws with an overall depth and length around 250 m. When an external stress is applied, the micrometric defects grow with a higher rate than visible cracks due to additional forces from internal stresses, which can lead to a more serious problem [4]. Thereby, the increasingly higher use of titanium components to develop critical roles in the industry where high criteria of safety must be guaranteed requires an accurate NDT method. Penetrant testing (PT) methods are integrated as an NDT tool to locate surface flaws in non-porous materials from the manufacture up to the maintenance phase [5]. An hybrid PT method based on bacteria cells was successfully implemented as a NDT tool to inspect micro surface defects up to 700 m diameter, in laser welds performed in titanium [6]. However its effectiveness is drastically affected by the surface conditions, requiring a complex procedure to avoid a false negative result [7]. On the other hand, digital radiography methods achieve a detection level of flaws with openings above 100 m width in laser welded titanium specimens [8]. Under a non-complex sample preparation, eddy current testing is an high impact technology to detect hidden (low frequency regime) or surface defects (high frequency regime) in conductive materials upon the application of a time-dependent magnetic field [9]. The presence of a discontinuity acts as a resistive barrier that perturbs the eddy current flow changing the magnetic field generated by it. Furthermore, the resistive losses also promote a thermal distribution along the surface which can be captured by combining thermographic NDT techniques with ECT. In [10] defects with a length of 780 m are the threshold value for eddy current induced thermography employed to fatigue cracks in titanium. From a sensing point of view, inductive coil sensors are a widespread ECT probe [11], however their poor spatial resolution and limited sensitivity at low frequency compromises the detection of deeply embedded flaws and subtle topographic variations. Superconducting quantum interference devices (SQUIDs) have the potential to surpass the inductive coil sensors by detecting deep buried defects [12], however their high field sensitivity compromises the spatial resolution and requires an apparatus which operates at cryogenic temperatures. Therefore, ECT tools based on magnetoresistive sensors offer advantages over inductive coil sensors and SQUIDs due to an enhanced spatial resolution, high sensitivity, large bandwidth and an operating point at room temperature [13], being very promising candidates for an universal integration in NDT tools [14]–[16] to overcome the specifications imposed by the industry and achieve a detection range of micrometric surface A Manuscript received April 1, 2015; revised May 15, 2015 and June 1, 2015; accepted July 1, 2015. Date of publication July 10, 2015; date of current version July 31, 2015. Corresponding author: F. Franco (e-mail: ffranco@inesc-mn.pt). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier (inserted by IEEE).