# Non-Destructive Testing Applications for Steel Bridges
## 3 NDT Methods with Potential Application to Steel Bridges
### Test Methods Applicable to Internal/Subsurface Defects
#### 3.2.1 Ultrasonic Testing (UT)  
The application of Ultrasonic Testing (UT) to damage detection in steel bridges is widespread. It can be used to detect subsurface volumetric flaws, including slag inclusions and cluster porosity, surface-breaking flaws (e.g., cracks), and material thickness, to measure corrosion and construction errors. It is the most practical NDT method for detecting planar defects, including cracks, a lack of fusion, and a lack of penetration. It has also been implemented for wire break detection at anchorage zones for stay cables and tendons [10]. The UT method uses tools that include a pulser-receiver, electronic signal controls, display screen, transducers, lead wires, reference gages, and a couplant. Sometimes, surface preparation is required, which requires wire brushes, grinding disks, scrapers, and hammers. In this method, the structural component is tested using ultrasound waves at frequencies more than the audible ranges [26; 27]. The reflection of waves on the UT monitor indicates the exact distance of any subsurface/internal flaws from the surface [28] (Figure 9). Figure 8: Example of the eddy current method for detection of flaws on a steel pipe [22]. The signals from the eddy current method can be affected and distorted by factors such as lift-off (distance between the test coil and the specimen), electrical conductivity, magnetic permeability, inhomogeneity of the material, or the thickness of the test piece [23]. Lift-off is one of the most common problems for EC tests, slowing down and limiting the growth of its applications. Further factors that can affect the received signals are the noise and low-level signals [24; 25]. To provide for the desired parameters and accurate readings, signal processing, signal analysis, and feature extraction and classification of the model must be performed. Figure 9: UT Methodology [28; 29]. Specific amplitudes and frequencies of the electronic pulses are generated through the lead wires that are connected to piezoelectric crystal probes. For steel elements, the frequency is typically between 2 and 5 Hz. Two types of transducers exist depending on the kind of waves that are generated: straight beam and angle beam transducers. Straight beam transducers generate compression waves consisting of alternating layers of steel atoms that expand and compress due to the of the elastic motion. Angle beam transducers generate shear waves that oscillate at right angles to the direction of the wave motion [10]. The presence of air gaps between the contact of UT transducers and the surface of a tested piece may cause wave scattering. Accordingly, a gel couplant is applied to the surface to strengthen the in-situ bonding and to prevent wave scattering [30]. Figure 10 shows the application of UT for the steel bridges. UT is considered a fast NDT method, and its cost is moderate. Its accuracy in detecting defects, portability, and high safety are other advantages of this technique. However, this method is less effective for inspecting very thin components, fragile materials, and elements with a complex geometry [31]. In order to enhance the capabilities of the ultrasonic tests, several methods have been developed; the following are more details of these techniques. Phased-array ultrasonic technology (PAUT)-- Phased-array ultrasonic technology (PAUT) is one of the advanced UT methods developed to detect cracks located at different depths, with random orientation probes at fixed conditions (Figure 11). The main property of PAUT is its computer-controlled excitation of individual elements with a multi-element probe, which can generate an ultrasound focused beam with the capabilities of modifying the beam parameters [32] (Figure 12). These parameters include the beam angle, focal distance, and focal spot size, allowing this technique to detect cracks in various orientations located away from the beam axis [33]. Figure 10: Applications of UT for steel bridges [10]. PAUT is known for its simplicity in implementing multiple transducers.