Tunnel - Eddy Current Array (ECA)

Target of Investigation

The eddy current array (ECA) is primarily used to detect cracks in steel elements. ECA sensors can be placed on a flexible substrate, which allows the sensors to conform to a variety of specimen shapes and sizes. For steel elements in tunnels, ECA can have the following applications: 

  • Detecting surface-breaking cracks. 
  • Inspecting for weld flaws if the weld contour is fairly smooth without deep valleys between weld beads. 
  • Measuring geometric parameters, such as coating thicknesses. 


The ECA is a newer technology that improves upon the principle of conventional eddy current testing (ECT). ECA systems are generally made of conformable ECT equipment that has been specifically designed for model-based inverse methods. Multivariate inverse methods convert the sensor responses into absolute property estimates. Multiple sensor responses for each individual sensing element at each location are converted into estimates of several material properties at each location along the inspected surface of the material. 

Physical Principle 

The ECA is conceptually similar to conventional ECT. For conventional ECT, eddy currents are induced when an energized probe coil is placed near the surface of a conductive material. The ECA, on the other hand, uses an array of multiple coils. The coil must have an alternating current (AC) because a time-dependent magnetic field is required to induce or generate electrical currents. The eddy currents induced at the surface of the test material are time varying and have magnitude and phase. Eddy currents are proportional to the electrical conductivity of the material. Material properties and discontinuities such as cracks disturb the eddy current trajectories, affecting the magnitude and phase of the induced current. The coils sense the magnetic field induced by these currents, producing a complex voltage in the coils. 

Data Acquisition 

Sensor arrays are fabricated on a thin, flexible, printed circuit-board substrate. This allows sensors to conform to the shape of a specimen. Data acquisition equipment includes electronics that drive sensor arrays and measure the response of the individual array coils (figure 1 and figure 2). 

Source: FHWA. 
Figure 1. Photo. Technician performing inspection with an ECA. 
Source: FHWA. 
Figure 2. Photo. ECA instrument and sensor array. 

Sensor arrays can measure multiple qualities, such as coating thickness based on sensor lift-off (distance from the array coil to the specimen), and material properties, such as permeability. Sensor coils can be incorporated with encoders to measure the movement of the coils and correlate measured data with spatial position on a specimen. Equipment is generally designed for laboratory and industrial applications. The ASTM guide covers the use of conformable eddy current sensor arrays for nondestructive examination of electrically conducting materials for discontinuities and material quality.(1) 

Data Processing 

Data processing is done within the instrument hardware and software. System software uses calibrated lookup tables to relate measured signals to desired physical parameters. With the use of position encoders, data can be presented with a spatial reference. Data provide a permanent record and can be used in offline processing. 

Data Interpretation 

The operator interprets processed data while using the ECA. Signal output is observed on the instrument display, and the operator uses his or her judgment to determine the meaning of the signal. Operating equipment and interpreting data requires significant skill. The equipment produces a C-scan image, which uses color contrast to identify areas where the eddy current is interrupted or altered. Figure 3 and figure 4 show data output in the form of C-scan images. 

Source: FHWA. 
1 ft = 0.3 m; 1 mil = 0.025 mm. 
Figure 3. Contour map. C-scan image showing lift-off. 
Source: FHWA. 
1 ft = 0.3 m.
Figure 4. Contour map. C-scan image showing permeability.  

Signal output can be dependent on the movement of the probe over the specimen surface. Irregular surfaces or inconsistent movement of a probe (e.g., lifting the probe off the surface or tilting the probe) can produce signals that could be interpreted as defects. Skilled operators can consistently scan specimens and judge the nature of the output signal. 


Advantages of ECA technology include the following: 

  • Detects near-surface defects through paint. 
  • Provides a C-scan image of a defect. 
  • Measures multiple physical parameters. 
  • Compensates for lift-off and material property variations better than conventional eddy currents. 


Limitations of ECA technology include the following: 

  • Results influenced by magnetic properties of weld materials. 
  • Results affected by probe orientation during scanning. 
  • Operator needs experience and skill. 
  • Costly equipment. 
  • Limited usability in difficult-to-access areas because of probe size. 


  1. ASTM E2884-17. (2017). “Standard Guide for Eddy Current Testing of Electrically Conducting Materials Using Conformable Sensor Arrays.” Book of Standards 03.04, ASTM International, West Conshohocken, PA.