Tunnel - Ultrasonic Testing (UT)

Target of Investigation

Ultrasonic testing (UT) technology can be applied to steel elements in a tunnel, such as girders, truss members, and other elements with a plate-like geometry. UT can also be applied to structural elements, such as pins, rods, and trunnion shafts. UT technology can be used to determine the thickness or length of an element, and to detect flaws such as: 

  • Cracks. 
  • Weld flaws such as cracks, slag inclusions, weld porosity, and lack of fusion during fabrication or in existing welded members.(1–3) 
  • Fractured anchor bolts and hanger rods. 
  • Section loss from corrosion. 


UT is a nondestructive method of flaw detection in which high-frequency acoustic waves are introduced into a material by a transducer placed on the surface. The acoustic waves propagate through the material and are reflected by discontinuities in the material. The reflected acoustic waves are detected by the transducer, and the resulting waveform is analyzed to identify discontinuities in the material. The technology can be used to detect weld flaws, as well as to detect cracks in steel members and components.(1) UT is implemented using one of two configurations: straight beam (figure 1) or angled beam (figure 2). A straight-beam configuration is usually used to measure plate thickness, and an angled-beam configuration is usually used to detect weld flaws. 

Source: FHWA. 
T1 = thickness 1; T2 = thickness 2. 
Figure 1. Illustration. Straight-beam UT for thickness measurement. 
Source: FHWA. 
Figure 2. Illustration. Angled-beam UT of a transition butt-weld plate. 

Physical Principle 

The physical principle of UT is that acoustic waves (sound, elastic, or mechanical waves) passing through a material will be fully or partially reflected by discontinuities in the material or a boundary of the material. Acoustic waves are introduced into the material using an ultrasonic transducer placed on the surface. The transducer launches a pulse of an acoustic wave that passes through the material. After the wave pulse has passed through the material, an ultrasonic transducer is used to detect and display the resulting waveform. The waveform is analyzed to assess reflections from the back wall or those caused by discontinuities in the material and the boundaries of the material.(1) For a thickness measurement, the thickness of the material is determined from the equation in figure 3. 

Figure 3. Equation. Thickness of material.(1) 


d = thickness of the material (inches). 

TOF = time of flight of the acoustic wave (seconds). 

C = wave velocity in the material (inches/second). 

Thickness measurements are made with an ultrasonic thickness gauge, which is a handheld, battery-operated instrument that displays the measurement on a digital display.(1) 

Data Acquisition 

Data are acquired by using a handheld, battery-operated ultrasonic flaw detector. The flaw detector is composed of an ultrasonic pulser–receiver that generates and receives the acoustic waves, a screen on which waveforms are displayed, and a control panel where adjustments to instrument parameters can be made. Onboard data storage for waveforms and other test data is typically provided. UT is implemented using a straight- or angled-beam configuration. 

The straight-beam configuration is used to detect defects in pins, trunnion shafts, anchor bolts, and other steel components in a tunnel. A longitudinal wave is launched normal or nearly normal to the surface of the material being tested. Figure 4 illustrates the application of straight-beam UT for the detection of a crack in an anchor bolt. Small angles (up to 10 degrees) may be used to improve the alignment of the beam path with the defect or to improve coverage when access to the surface of the material is limited. 

Source: FHWA. 
Figure 4. Photo. Straight-beam ultrasonic transducer being used to inspect an anchor bolt. 

The angled-beam configuration is used for detecting flaws in welds (figure 5). This configuration is commonly used for fabrication inspection and may also be applied in the field. Angled-beam UT involves launching a shear wave into the testing material at an angle of 33 degrees or greater; standard angles are 45, 60, and 70 degrees. The angled beam penetrates the weld area after reflecting from the back side of the plate. 

Source: FHWA. 
Figure 5. Photo. Angled-beam transducer on the surface of a test plate. 

Data Processing 

Data processing for thickness measurements typically consists of a simply determining the time it took the acoustic wave to travel from the transducer through the material, reflect off the opposite surface, and return to the transducer. TOF is then used to estimate the length of the travel path (i.e., the thickness of a plate) based on the velocity of an acoustic wave in the material. These data are shown on a digital or graphical display. 

Data processing for flaw detection consists of amplifying the received acoustic waveforms to obtain a relative signal magnitude that allows the reflected wave amplitude to be compared to a threshold value. The tester finds and records the amplification (decibel gain) required to match the magnitude of the reflected amplitude with that of the predetermined threshold. 

Data Interpretation 

The thickness measurement from UT is commonly used for assessing the extent of section loss from corrosion. The thickness measurement for the test section is compared to the thickness measurement on an intact section. 

Data interpretation for flaw detection consists of comparing the amplitude of a detected waveform with a threshold value. Reflected waveforms that exceed a certain amplitude threshold are interpreted as flaws. Reflected waveforms that do not exceed the threshold are interpreted as noise or nonrelevant indications.(2) 


Advantages of UT include the following: 

  • Rapid, low-cost, widely available technology. 
  • Effective in difficult-to-access areas because access to only one side of the plate is required. 
  • Surface and subsurface flaws detectable. 


Limitations of UT include the following: 

  • Surface preparation required to achieve highly accurate results. 
  • Time consuming. 
  • Well-trained operators required. 


  1. American Society of Nondestructive Testing. (2005). Nondestructive Testing Handbook, Third Edition, Volume 7, ASNT, Columbus, OH. 
  2. American Welding Society. (2010). Bridge Welding Code AWS D1.5, AWS, Miami, FL. 
  3. Moore, M., Phares, B., and Washer, G. (2004). Guidelines for Ultrasonic Inspection of Hanger Pins, Report No. FHWA-HRT-04-042, Federal Highway Administration, Washington, DC.