Research Group Eddy Current Testing (ET) Technique Professor Pedro Vilaça * * Contacts: Address: Puumiehenkuja 3 (room 202), 02150 Espoo, Finland pedro.vilaca@aalto.fi October 2017 Contents Historical scope of NDT by Eddy current Testing (ET) technique Electromagnetic fundaments of NDT by ET Formulation of depth of penetration and evaluation of the effect of frequency, conductivity, permeability and lift-off Classification and characterization of conventional and advanced probes for ET Characterization of signal from probes in the impedance plan Applications of conventional and advanced probes Advantages and limitations 2 1
Learning Outcomes At the end of the seminar the student should be able to: To establish the fundaments of NDT by ET To apply the formulation of depth of penetration To know the effect of frequency, conductivity, permeability and lift-off on depth of penetration To classify and characterize conventional and advanced probes for ET To characterize the signal from probes in the impedance plan To establish solution based on ET for industrial applications To identify advantages and limitations of the technique 3 Historical Scope Eddy current (ET) are almost 200 year old discovery. Their application in NDT has evolved significantly since then What would become ET testing had its origin in Michael Faraday s discovery of electromagnetic induction in 1831 Later, in 1879, David E. Hughes studied the changes in the properties of a coil when placed in contact with materials of different electrical conductivity and magnetic permeability However, it was only during world war 2 that this knowhow was applied in NDT 4 2
Historical Scope ET is based on Eddy (or Foucault, for the French's, in memory of Michel Foucault 1926-1984) Current one of the NDT techniques based on the electromagnetic induction phenomenon Defects are detected based on the change in electrical impedance of a coil of conductive wire, excited with an alternating electrical current, when interacting with a conductive material 5 Fundaments Method based on variation of a magnetic field in a coil, in reaction to magnetic or electric conductive materials in its vicinity Coil of conductive wire excited with AC produces a (primary) magnetic field around itself. When the coil approaches a conductive material, this (primary) magnetic field induces electrical currents (eddy currents) in the conductive material In practice it is like an electric transformer, where the coil acts as the primary winding and the conductive material acts as the secondary winding The presence of defects in the test object ( secondary winding ) causes a change in eddy current and a corresponding change in phase and amplitude that can be detected by measuring the impedance changes in the coil ( primary winding ). Such changes reveal the presence of defects The current and voltage measured, represents the impedance of the coil and test material and other somewhat complex electromagnetic phenomena 6 3
Fundaments H P eddy currents H S An alternate electrical current flow through a cylindrical helicoidally spiral (coil) probe generates a primary magnetic field (H P ) This H P induces electrical (eddy) currents on a conductive material (test piece) in its vicinity I turn, the eddy currents generate a secondary magnetic field (H S ) that opposes the primary field H P and induces a current in the probe s coil The presence of a defect the limits the flow of the eddy currents. As a consequence, the secondary magnetic field will be less intense. This change in the intensity of the H S can be measured, enabling the detection of the defects (i.e. variations in the H S are measured by measuring the variations in the electrical impedance in the probe/coil) 7 Fundaments Numerical simulation showing the intensity of the eddy currents on a sound part (left) and a part with a defect (right) Magnetic ferrite core of Eddy current probe Qualitative representation of location and intensity of the eddy currents produced by a cylindrical helicoidally coil with 4 windings Note: Magnetic ferrite cores are dense, homogenous ceramic structures made by sintering (at 1000-1500 C) mixing Fe oxide with oxides or carbonates of one or more metals, such as, Zn, Mn, Ni or Mg The eddy currents NDT method can be applied to both ferromagnetic and nonferromagnetic materials, as long as they are electrical conductors Due to the skin effect (the currents are concentrated at the surface), only surface or sub-surface defects can be detected 8 4
Fundaments As the probe travels over the defects the probe s signal in the impedance plane varies, indicating the presence of the defects Probe Signals from defects Defects Electrical impedance plane 9 Depth of Penetration The eddy currents in a conductive material are not uniformly distributed through the thickness of that material. They are more concentrated at the surface and become gradually less intense until they are nullified at a certain depth: I x I 0 e x( f ) 1 2 I [A m -2 ] current density f [s -1 ] frequency µ [H m -1 ] magnetic permeability (µ = µ 0 µ r ) σ [%IACS] electrical conductivity To have a sense of what is the thickness of a part that can be inspected the depth of penetration x d [m] is defined, corresponding to the depth at which the current density has decreased to e -1 ( 37%) of its value at the surface: Depth of Penetration: d ( f,, ) 1 f I x I 0 1 e 1 2 37 % x( f ) 1 For a given material with known µ [H.m-1] and σ [%IACS] this equation enables to calculate: The frequency of a probe for a wanted depth 10 5
Depth of Penetration d ( f,, ) 1 f 11 Depth of Penetration d ( f,, ) 1 f 12 6
Depth of Penetration Modeling the effect of frequency on the depth of penetration of ET: 50 khz 250 khz 1000 khz Modeling the effect of lift-off on the depth of penetration of ET: 0.1 mm 0.5 mm 1.0 mm 13 Depth of Penetration 14 7
Depth of Penetration Depth of penetration for aluminium alloys with slightly different conductivity: 15 Depth of Penetration Depth of penetration for different materials: 16 8
Depth of Penetration Electrical conductivity units: % IACS International Annealed Copper Standard: Electrical conductivity as a the percentage of the conductivity of standard pure copper annealed at 20 ºC 100 % IACS = 5.8x10 7 Siemens/m 172.41 / resistivity = % IACS 17 Depth of Penetration Source: Eddy Current Technology Incorporated 18 9
Depth of Penetration k x General law of exponential decay I I 0 e X-rays (XR): Radiation absorption phenomenon I x I 0 e ( x) µ - Linear absorption coeficient Ultra-sounds (US): Acustic attenuation phenomenon P t [s] Eddy currents (ET): Depth of penetration phenomenon P P e 0 d α Acustic attenuation coeficient I x I 0 e x( f ) 1 2 19 Eddy Current Behaviour Eddy currents behave like a compressible fluid. For example near the edges of a conductive material (edge effect) Defects that are aligned with the eddy currents are less of a disturbance. Inversely, defects that are perpendicular to the ET flow cause a larger disturbance 20 10
Eddy Current Behaviour Eddy currents essentially flow in planes that are parallel to the surface (in the case of cylindrical probe positioned perpendicularly to the surface) Defects that are perpendicular to the surface are easier to detect Defects that are parallel to the surface are more difficult to detect Probe with a different configuration, adapted to the detection of defects parallel to the surface Horseshoe probe 21 Eddy Current Probes - Classification Nowadays there are many different Eddy probes configurations, adjusted to different inspection objectives. A possible classification criteria is: Operating mode Absolute Differential Eddy Currents Probes Receiving mode Geometry Double function Emission-Reception Cylindrical coil Planar spiral Others: Measurement of conductivity Measurement of thickness (amplitude/phase sensitive) Type Internal External 22 11
Eddy Current Probes - Classification Absolute probes Absolute probes are base on a single coil that acts both as an excitation element and a receiving element In this configuration the total impendence variation of the probe is analyzed Differential probes In differential probes the impedance variation of two coils is compared, where one coil is positioned over a reference material, or next to the other coil but with the winding in the opposite direction In this case it is the difference impedance between the two coils that indicates the presence of the defect Schematic representation of the absolute and differential operating mode 23 Eddy Current Probes - Classification Double function probes Double function probes are characterized by having a single coil that simultaneously induces the eddy currents and detects the changes in the flow of the eddy currents. Emission-reception probes Emission-Reception probes are characterized by having two coils: one to induce the eddy currents in the test piece and another to detect the changes in the flow of the eddy currents. 24 12
Eddy Current Probes - Modelling of Architecture a) Single loop b) Cylindrical coil c) Planar spiral Magnetic field: Eddy current intensity: 25 Eddy Current Probes - Classification Internal probes For the inside of tubes or holes External probes For the outside of tubes or shafts 26 13
Eddy Current Probes Special Solutions Recent developments feature research on planar spiral probes printed on PCB substrates These probes exhibit some attractive properties such as: Smaller lift-off due to the closer proximity between the coil and the test piece Easy to manufacture The signal s response, that is, the electrical impedance, will vary with the orientation of the discontinuity relative to the probes axis of symmetry, which can be taken advantage of to characterize the defects 27 Eddy Current Probes Special Solutions Recent developments feature research on planar spiral probes printed on PCB substrates: Ability to inspect non-planar geometries, if the substrate is flexible (e.g. Kapton). 28 14
Eddy Current Probes Special Solutions Meandering Winding Magnetometer (MWM) probe is based on: A primary excitation winding and two secondary receiving (sensitive/pick-up) windings, immediately adjacent to the primary, arranged in parallel or coincident planes The filament s path can have several configurations, printed in Arrays and Grids Examples of MWM a) MWM probe printed on PCB substrate. b) Schematic representation c) permanent application to a hole under fatigue stress 29 Eddy Current Probes Special Solutions Eddy Currents Array Array probes Alternated excitation of the probes 30 15
Eddy Current Probes Special Solutions 4 planar sensitive coils in the quadrants defined by 4 crossing orthogonal excitation filaments, enabling the electronic rotation of the magnetic field of excitation Cross Convergent / Divergent Convergent / Convergent magnetics fields of excitation 31 Impedance Plan Purely resistive circuit Purely inductive circuit Purely capacitive circuit 32 16
Impedance Plan Electrical Impedance (Ohm): Measure of the opposition of a circuit to the flow of an alternating electrical current U Z I Z Z X L X arctg R 2 2 R X Z R 2 X 2 L XC C X L L Z: electrical impedance [Ohm]; R: electrical resistance [Ohm]; X: electrical reactance [Ohm]; ω: angular frequency [rad/s]; L: electrical inductance [Henry]; C: electrical capacitance [Farad]. 1 X C C 33 Impedance Plan Defects are identified based on the response in the ET Impedance plane 34 17
Characterization of Signal from Probes To characterize ET probes it is necessary to know, in impedance plane, its curves for: Frequency Conductivity Lift-off (angular and planar) Reference blocks for conductivity and planar lift-off Reference blocks developed for the characterization of ET probes Accessories for characterization of the angular lift-off of helical cylindrical probes 35 Characterization of Signal from Probes Impedance plane with Frequency, Conductivity and planar Lift-off curves 36 18
Characterization of Signal from Probes Impedance plane with Frequency, Conductivity and planar and angular Lift-off curves 37 Characterization of Signal from Probes Impedance plane with Frequency curves. Comparison of two probes AA2024-T351 (30 %IACS) Planar Lift-off: 10 μm Planar spiral probe with 20 windings Conventional helicoidally coil 38 19
Characterization of Signal from Probes Frequency sweep in Re(Z) for different Lift-off values Planar spiral probe with 20 windings 39 Characterization of Signal from Probes Influence of frequency and lift-off in the impedance plane for probes with different number of windings and different geometry Note: the red lines correspond to constant frequency for different lift-off 20 circular windings 10 circular windings 10 square windings 40 20
Characterization of Signal from Probes Influence of frequency and lift-off in the impedance plane for probes with different number of windings and different geometry Note: the red lines correspond to constant frequency for different lift-off 20 circular windings 10 circular windings 10 square windings 41 ET NDT + Automated Scanning Systems b) a) f) e) c) d) g) h) Overview of the system s architecture a) ET probe b) XY table c) φ and A excitation and pick-up driver d) Signal generator e) Data acquisition and XY table control f) Sine-fitting measurement module g) Stepper motor power suply h) Dedicated Software 42 21
Application Matrix Sweep of FSW @ Root Side Example of automated inspection where probe executes a matrix sweep (X,Y) over a component. The movement of probe is controlled by a computer. The signal is acquired at multiple points on the surface Signal from defects Probe Movement in Y Movement in X Movement in X Movement in Y 43 Application Sensitivity to LOP Defect in FSW Transverse sweep of 3 FSW beads There is no distinctive feature in signal that enables differentiation of the 3 different defects There is a variation in impedance field just because the material has been processed 44 22
Application Limitations Case Study: Matrix sweep over a hole with Ø = 3 mm in AA2024 plate Planar spiral probe with 20 windings High sensitivity to several kinds of external interference Loss of resolution 1 Gx 2 1 0 0 0 Sobel operator 1 1 2 Abs( Z); G y 0 1 1 2 0 2 1 0 Abs( Z) 1 45 Other applications Assessment of Conductivity Case Study: Scanning of conductivity in cross-section of FSW of AA7075 plate Planar spiral probe with 20 windings 46 23
Other applications Assessment of Conductivity Case Study: Scanning of conductivity in cross-section of FSW of AA7075 plate (cont.) Planar spiral probe with 20 windings Retreating side Stirred zone Advancing side 47 Other applications Assessment of Conductivity 48 24
Advantages/Limitations and Applications Input Alternating electrical current Output Electrical impedance variation Parameters: Frequency, Probe (type, geometry, electric architecture, ), Coupling probe/material Advantages: Good ability to detect small defects Adequate for near surface located defects Limitations: High sensitivity to coupling Not easy to provide the dimensions of defects Applicability domain: Conductive materials Surface and sub-surface defects Locally planar surfaces 49 25