Introduction
Wire rope testing, or bridge cable inspection, is an essential process in fitness-for-serv
ice evaluation of amusement park rides, bridges, cranes, ship loaders and may other load bearing assets. This article briefly introduces the various wire rope testing methods and their respective advantages and disadvantages. The basic wire rope testing methods are visual wire rope testing, magnetic flux leakage (MFL) wire rope testing, long range ultrasonic testing (LRUT) and acoustic emission testing of steel cables.
Visual Inspection of Wire Rope
Visual wire rope is a fast and economical non-destructive testing technique that detects wire breaks on the outside diameter of the wire rope. Wire rope testing is commonly performed using a cloth rag lightly wrapped the rope that catches on protruding wires. Visual wire rope testing is performed routinely across all industries. Visual inspection cannot determine condition under collars, seizing wires, separators, sockets and gatherers since due to accessibility. In addition to wire breaks, this method can detect reductions in diameter, corrosion, birdcage, waviness, kinks and deformations.
Some disadvantages include disadvantages include that it cannot detect corrosion/breaks on the interior strands and under paint. Impossible to size subsurface defects.
Figure 1. Wire rope visual inspection for cross-sectional area changes.
Magnetic Flux Leakage (MFL) Wire Rope Testing
MFL testing of wire rope and steel cables introduces a magnetic field along the primary axis of the wire rope using magnetizing measurement head. Wire breaks cause a disruption in the magnetic field causing it to leak out from the rope. The magnetic flux leakage (MFL) is detected by a Hall sensor in the measuring head. The measuring head is generally equipped with an encoder wheel to accurately track wire break locations.
MFL wire rope testing is most practical and economical on moving ropes since a winching system is not required to pull the measuring head. Additionally, the maximum rope diameter that can be MFL tested is approximately 4”. Lastly, specialized measuring heads, at extra cost, may be required for groups of wire ropes with minimum clearance. Consider that each measuring head has certain size and must small enough to mount in the clearance area between the ropes. Example MFL wire rope testing data is shown in Figure 2. The wire rope testing data are presented as milli-voltage (mV) versus distance correlated to the encoder wheel. The measured mV on the vertical axis is proportional to the magnetic flux leakage caused by wire rope breaks. The upper data (OUT) reports on wire rope testing of the outer strands of the cable. The middle data (INN) reports on wire rope testing of the inner strands of the cable. The bottom (blue) wire rope testing data records loss of metallic area (LMA) during the wire rope inspection.
Figure 2. Magnetic flux leakage (MFL) wire rope testing data.
MFL wire rope testing is performed on ropes in the 0.50 to 4.00” diameter range. Moving rope like those used on amusement park rides, ski-lifts, elevators, and lifting devices are easily tested with and MFL measuring head and encoder wheel. MFL wire rope testing is often performed on standing ropes like those found on ship loaders and bridge stay cables, however, these applications are slightly more complicated, time consuming, and expensive due to a winching requirement.
Acoustic Emission Testing of Bridge Cables
TKS specializes in the monitoring of suspender cables and ropes in cable-stay and suspension bridges, using acoustic emission (AE) for wire breaks. This technology is recommended for larger diameter ropes and cables that may not be suitable for MFL wire rope testing or long-range ultrasound testing (LRUT), also known as guided wave ultrasonic testing (GWUT). Bridge cable inspection using acoustic emission technology has been utilized to provide risk-based inspection and maintenance for fracture critical members in infrastructure such as bridges and storage tanks. The technology provides real-time feedback on structural condition without the need for direct access to the area of interest.
Acoustic emission in bridge cable is produced by wire breaks, strand breaks, corrosion related events that may be correlated to individual strand deterioration, or group of strand deterioration, and other mechanisms. Bridge cable acoustic emission data is analyzed for both intensity and rate at which it is generated. Acoustic emission intensity is analyzed by considering acoustic emission amplitude, acoustic emission energy, and acoustic emission counts. The rate at which acoustic emission is emitted is analyzed to determine if deterioration is progressing. No acoustic emission activity implies that no active AE source was present during the duration of the test. Acoustic emission rates may also be categorized as stable, increasing linearly, and exponentially.
Follow-up visual inspections are recommended based on inspection results. Cable rubbing or fretting at gatherers, collars, separators and sockets generate acoustic emission which may be confused with acoustic emission generated from wire breaks. Broader disadvantages include that multiple sensors are require per cable with significant installation setup time. No defect sizing is possible.
Figure 3. Example acoustic emission date from a bridge main cable.
Long Range Ultrasonic Testing (LRUT) of Wire Rope
Long range ultrasound (LRUT), or guided wave ultrasonic testing (GWUT), is used as a screening tool for oil and natural gas pipelines, bridge pile, and railroad track. The technology provides real-time feedback on structural condition without the need for direct access to the area of interest. Cable inspection typically takes about 20 minutes. The sensor is installed on bottom end of cable with a low frequency ultrasonic wave transmitted from the sensor to top end of cable – typically the main cable. Similarly a guided wave is transmitted downwards to the suspender rope socket. Ultrasound is reflected back from wire breaks and other cable defects and cable defects are identified by location and severity: minor, moderate and advanced. Follow-up visual inspections are recommended based on inspection results.
The technology deploys magnetostrictive sensors (MSS) which may be installed in 15-30 minutes depending on access and cable conditions. The MSS sensor consists of conductors wrapped around the cable and centered between two permanent magnets. Guided waves are generated in the cable through induction. The instrumentation is lap-top based and transports easily to the inspection site and in between inspection locations on-site. Example data is shown below in distance – amplitude response. Reflections from the cable socket and main cable are observable at 0 and 100 feet, respectively. Confirmed corrosion was detected at approximately 85 feet (X1).
Some limitations include 1) Defect sizing is limited to minor, moderate, severe. 2) The technology is not suitable for moving rope inspections.
Figure 4. Example long range ultrasonic testing bridge cable data showing cable socket, intersection with main cable at 110 feet and corrosion at 85 feet.