Suspension Bridge Main Cable Inspection – A Challenging Proposition for Advanced Non-destructive Testing

Bridge cables are exposed to a spectrum of load and environmental conditions that create susceptibility to a variety of deterioration mechanisms including corrosion, fatigue, wear and abrasion [1].  Bridge cable damage occurs in the corrosion protection system, cable exterior and interior, and anchoring system.  The primary damage mechanism driving  suspension cable mechanical defects includes corrosion and complex cyclic loading.  The damage mechanisms may be visible on the external diameter of the cables as visible corrosion, noticeable diameter loss, and wire breaks.   This article presents advanced ultrasonic testing and a non-destructive testing technique to screen long sections of cable for  corrosion and wire break driven cable diameter reduction.  A new parameter relating the attenuation in long sections of cables to changes in condition is also introduced.  The guided wave ultrasonic (GWUT) technology can be applied for periodic non-destructive testing as well as long  term structural health monitoring installations.  

Non-destructive Testing of Suspension Bridge Cables

Non-destructive testing of suspension bridges has advanced over the decades by advancing existing NDT and recently structural health monitoring (SHM) solutions.  In parallel to these advances,  human inspector visual inspection has continued to be the primary inspection method.   Drone assisted visual inspection is common practice as well.   Robotic crawlers equipped with vision systems are applied routinely to stayed cable bridge systems [2].  In the example below, high resolution cameras are installed on an MFL measuring head to provide inspection duality.

Figure 1:  Bridge strand cable visual inspection in tandem with MFL robotic inspection showing corrosion, pitting, and diameter reduction.

Acoustic Emission Testing of Bridge Cables

Acoustic Emission (AE) technology is an important non-destructive testing method commonly used to identify fractures and fatigue in metals, providing the advantage of real-time monitoring [3-5].   AE works by capturing high-frequency sound waves that are released when defects form in stressed materials.   These sound waves generate vibrations, which are detected by AE sensors installed on cable sheaths or anchors and converted into electrical signals for further analysis. AE has become widely used for identifying internal issues in bridge cables.  Despite its advantages, AE technology faces several challenges when used on bridge cables. One major issue is the limited durability of sensors, which can fail in extreme or high-stress environments, requiring frequent maintenance. Additionally, interpreting AE signals in real-world conditions demands sophisticated algorithms to eliminate background noise and ensure accurate readings.   While some success with acoustic emission has been achieved on suspension bridge cables, the technology is more universally applied to monitoring for fatigue cracks in steel bridge girder systems as shown below.

Figure 2:  Acoustic emission crack tip location on steel bridge girder web.

Magnetic Flux Leakage Testing of Bridge Cables

Magnetic flux leakage (MFL) is an electromagnetic non-destructive testing technique that uses magnetic-sensitive sensors to detect the magnetic leakage field of ferromagnetic materials such as steel cables or pipelines.  The technology used inductive and Hall sensors to detect cable wire breaks and changes in cross-sectional area.   MFL detects internal defects by generating a magnetic field to magnetize the steel structure. In undamaged areas of the cable, the magnetic flux lines are confined within the steel wires due to their high permeability, while in areas with damage, such as corrosion or cracks, the magnetic field lines are disrupted, causing a “leakage” of the magnetic field. This leakage is detected by sensors, revealing otherwise invisible internal damages [5].  Magnetic flux leakage of suspension cables can be difficult to practically implement due to the periodic presence of suspender ropes and their clamping  mechanisms.  The MFL measuring head must be re-installed between each set of suspender cables and a new test section must be  restarted.

Figure 3:  MFL inspection of bridge cables for wire breaks and cross-sectional area reduction.

Guided Wave Ultrasonic Testing  of Suspension Bridge Cables 

Guided wave ultrasonic testing (GWUT), also named long range ultrasonic testing (LRUT), is a specialized non-destructive testing techniques used to screen long sections of main cable.  The technology can be applied to single strand bridge cable up to about 6” (150mm) diameter.  Larger diameter multi-strand bridge cable can also be tested provided the smaller diameter cables can separated for sensor installation.    The figure below shows and example guided wave bridge sensor installation in the suspension bridge tower at the saddle point and a second example of an installation on a multi-strand bridge cable. 

Figure 4:  Non-destructive testing  of suspension bridge single cables from the bridge tower saddles and installation of guided wave technology on a multi-strand cable. 

Guided-waves refer to mechanical (or elastic) waves in sonic and ultrasonic frequencies that propagate in a bounded medium (such as pipe, plate, and rod) parallel to the plane of its boundary. They exist in many different forms (such as longitudinal, torsional, and flexural waves in cylindrical rod or pipe; shear-horizontal and Lamb waves in plate) and their properties vary with the geometry and size of the medium. For long-range guided-wave inspection and monitoring, a short pulse of guided waves in relatively low frequencies (up to a few hundred kHz) is launched along the structure under testing, and signals reflected from irregularities in the structure such as welds and defects are detected in the pulse-echo method.  From the occurrence time of defect signal and the signal amplitude, the axial location and severity of the defects are determined.  

Guided waves are generated in the bridge cable using the magnetostrictive sensor (MsS).  The MsS consists of an encircling coil in a biasing DC magnetic field that is typically provided using permanent magnetic circuits.  For guided wave generation, a short pulse of electric current is supplied to the transmitting MsS coil.  The time varying magnetic field produced by the transmitting coil in turn expands and contracts the cable material underneath the sensor via the magnetostrictive effect, thus generating the guided waves that travel along the individual wires comprising the steel cable in both directions from the coil. Detection of guided waves is achieved by the reverse process, where the guided waves arriving at the sensor location cause the magnetic induction of the cable material underneath the sensor to change with time. The changing magnetic induction, in turn, induces in the receiving MsS coil an electric voltage (Faraday’s Law), which is detected. 

From a given probe location, the guided-wave method can inspect a long segment of the structure (for example, more than 60 meters) and can quickly detect and locate defective areas in the structure. The primary guided wave mode used for the long-range inspection is the longitudinal (L) wave for rods/cables/ropes in to 10 to 30 kHz range.  

Guided wave ultrasonic testing generates a standard amplitude – distance radio frequency (RF) waveform or A-scan as shown below.  The data is sampled at 1 MHz and converted to distance using the known velocity of the L (0,1) guided wave mode.   The horizontal and vertical axes are distance (m) and amplitude (V).   At 0m, the pulse observed is the main bang, or dead zone, which is crosstalk between the pulsing and receiving electronics.  At approximately 11-12 meters, a strong reflection is observed from the closest clamp to the tower.  Subsequent clamp reflections are observed in equally spaced intervals until the anchor at approximately 82 meters from the sensor position in the north tower.

The RF waveform is converted to a fully rectified video waveform and the phase of each reflector is analyzed to determine if the source is an increase in cross-sectional area (e.g., clamp, paint chip, surface rust, etc.,) or a decrease in cross-section area (e.g., wire break).    Clamps, wire breaks, and indications are marked with CL, D and I, respectively.  Indications are classified as areas that could not be classified as decreases in area via the phased analysis.  

Figure 5: Suspension bridge cable ultrasonic testing data showing conventional A-scan and spectrogram views.   In this instance, the inspection range ws approximately 80 meters from the saddles in the bridge tower.

Summary

This article presents three different advanced non-destructive testing solutions for suspension bridge cables with primary focus on advanced ultrasonic testing of suspension bridge cables with guided wave ultrasonics.  The article demonstrated that the technology can be installed on single strand and multi-strand bridge cable in a conventional non-destructive of structural health monitoring  configuration.  Depending on the cable configuration, clamping mechanisms on the cable, and coating systems the technology can screen up to 60-80 meters from a single sensor position.

1. R. Mayrbaurl, S. Camo, Cracking and fracture of suspension bridge wire, J. Bridg. Eng. 6 (6) (2001) 645–650, https://doi.org/10.1061/(ASCE)1084-0702(2001)6:6 (645).

2. Xu, Fengyu & Wang, Xingsong & Wang, Lei. (2011). Cable Inspection Robot for Cable-Stayed Bridges: Design, Analysis, and Application. J. Field Robotics. 28. 441-459. 10.1002/rob.20390 [2].

3. A. Nair, C. Cai, Acoustic emission monitoring of bridges: review and case studies, Eng. Struct. 32 (6) (2010) 1704–1714, https://doi.org/10.1016/j. engstruct.2010.02.020.

4. G. Drummond, J.F. Watson and P.P. Acarnley, "Acoustic emission from wire ropes during proof load and fatigue testing", NDT International, vol. 40, pp. 94-101, 2007

5. D. Li, J. Ou, C. Lan, H. Li, Monitoring and failure analysis of corroded bridge cables under fatigue loading using acoustic emission sensors

6. Sensors, 12 (2012), pp. 3901-3915, 10.3390/s120403901

7. B. Feng, J. Wu, H. Tu, J. Tang, Y. Kang, A review of magnetic flux leakage nondestructive testing, Materials 15 (20) (2022) 7362, https://doi.org/10.3390/ ma15207362.