# Risk-Based Selection of Inspection Method for External Post-Tensioning System of Bridges
## 2 Literature Review
### Defining and Classifying Applicable Inspection Methods
#### 2.2.5 Electromagnetic Methods  
Electromagnetic methods will be divided into four inspection categories: magnetic flux, elasto-magnetic method, electrical capacitance tomography, and magnetostatics.  
Magnetic flux leakage methods include two major categories: active magnetic flux leakage and residual magnetic flux leakage. A portable magnet is used to submit a ferrous material/steel to a high magnetic field in the active magnetic flux leakage technique. This creates flux routes between the two poles of the material. The magnetic field in the material \"leaks\" from its regular path of least resistance to spots where there is a section loss. The leak is detected using a magnetic field detector (made up of Hall-effect sensors) placed between the magnet's poles and is sensitive to the change in the magnetic field. The residual approach involves bringing the steel to full magnetic saturation to erase its unknown magnetic history, then removing the magnet and passing the sensors over the  
Figure 6: Electrochemical impedance spectrometer [9].  
portion to detect the remaining magnetic field [53; 54]. Figure 7[9] shows the schematic of the sensor probe, full head sensor, half head sensor, and the connection of the NDE extension to the computer.  
Elasto-magnetic approach, also known as the magneto-elastic technique, was developed to estimate cable stress and may detect stress changes and section loss caused by corrosion. The approach is based on the idea that magnetic permeability is responsive to stress variations. In laboratory studies, stress measurements utilizing elasto-magnetic sensors were shown to be in good agreement with load cell results. The stress loss due to shrinkage, relaxation, creep, and elastic deformation was clearly observed using elasto-magnetic sensors on the Kamikazue viaduct to quantify stress changes in an external tendon system [55; 56].  
Electrical capacitance tomography gathers capacity data from multi-electrode sensors and creates permittivity pictures of sections over thousands of repetitions. The researchers used a pair of electrodes on a tiny HDPE conduit with one strand and successfully identified both air- and water-filled holes in ducts. When it comes to attaining accurate estimation, however, the electrical capacitance tomography design is challenging; careful design and rigorous verification are necessary [14; 40; 43].  
Magnetic fields cause small changes in the physical dimension of steel, while material strains cause changes in magnetization, according to the technology's idea. As a result, when the magnetic field surrounding the steel element changes, an elastic wave travels in both directions along the wire's length. The stress wave changes the material's magnetic induction, causing the voltage to be produced in the receiving coil. This can be monitored and used for the detection of flaws in magnetostatics.  
Figure 7: Magnetic flux leakage (**a**) sensor probe, (**b**) full head sensor, (**c**) half head sensor, (**d**) the connection of the NDE extension to the computer [9].