Nondestrutive Testing of Wire Rope and Bridge Cable Sockets

Introduction

Wire ropes serve as critical lifelines in countless engineering structures—from towering suspension bridges to powerful cranes and expansive cable-stayed constructions. At the heart of their reliability are wire rope sockets, essential mechanical connectors that anchor and transfer immense loads safely and securely. Often overshadowed by the wire rope itself, the sockets are equally important from a load bearing, fracture critical and fitness-for service perspective.

Figure 1: Wire rope socket showing key areas for non-destructive testing.

Wire rope sockets, including bridge sockets, spelter sockets, and swage sockets, securely terminate wire ropes, providing robust load-bearing connections to structural elements. Bridge sockets are proven anchors in suspension and cable-stay bridges, where precision, strength, and durability are required.    Proper visual and non-destructive inspection of these sockets not only prevents catastrophic failures but may also significantly extends the service life of critical infrastructure.

In this article, the non-destructive testing options for wire rope and bridge cable sockets are reviewed. 

Description of Wire Rope Socket

In this article, a will be considered and non-destructive testing options will be presented. The closed spelter socket for wire rope consists of several distinct geometric areas—each critical for strength, load transfer, and proper bonding of the rope with the socketing material (typically molten zinc or resin). These areas are often considered during design, inspection, and testing and include:

Socket Body (Housing): The socket body is typically conical or tapered cylindrical.  It contains the socketing medium, often zinc or resin, that encapsulates the splayed wire rope.  The socket body is often tested nondestructively during fabrication using magnetic particle testing and ultrasonic testing.

Throat Area (Neck or Eye of the Socket):  The throat area is circular or oval eye opening at the closed end and is used as a connection point to shackles, clevises, pins, etc.  Stress

Basket Area (Tapered Cavity): The basket area is a tapered cone inside the socket body where the where the fanned-out strands of wire rope are embedded into the socketing compound. 

Wire Rope Entry Zone: The wire rope entry zone is cylindrical and aligned with the axis of the socket and serves as the entrance for the wire rope into the socket before the strands flare into the basket.

Socket Shoulder (or Flange): The socket shoulder is a radial flange or shoulder outside the socket opening that acts as a stop and provides structural support.

Bonded Area (Internal Contact Surface): The area is most critical to load transfer between the wire rope and socket.  The bonded area is a conical surface in direct contact with the resin or zinc and the splayed wire strands.

Figure 2: Non-destructive testing of wire rope / bridge cable sockets requires visual inspection, conventional non-destructive testing, and advanced non-destructive testing approaches.

Visual Inspection of Wire Rope and Bridge Cable Sockets

Typical practice includes [1-5] Closely examining transition for cracks  or Fractures. Areas of stress concentration areas, typically at corners, transitions, and attachment points are critical areas.  The socket should also be inspected for deformation, wear and abrasion, and corrosion or rust.  These steps include checking for distortions or elongation indicating overload or misuse, excessive wear on contact surfaces and general corrosion or rust and pitting.

Ultrasonic Testing of Wire Rope and Bridge Sockets

The forging and castings from which sockets are  made may be nondestructively examined using ultrasonic testing for inherent forging and casting defects [6-7].  Ultrasonic testing is conducted utilizing both conventional straight beam and shear wave angle beam methods.  Ultrasonic testing of wire rope and bridge sockets are typically performed on sockets 1-5/8" and larger at fabrication.

Magnetic Particle Testing of Wire Rope and Bridge Sockets

Dry magnetic particle testing and wet fluorescent magnetic particle testing are both used at fabrication and in-service to detect surface breaking cracks and other discontinuities.   Tears can occur at  the transition from the socket body to throat area.  Similarly longitudinal tears can occur in the tapered socket body housing if the hoop stress is high enough

Figure 3: Wire rope socket nondestructive testing using magnetic particle testing – socket body area and throat area.

Strain Gaging or Wire Rope and Bridge Cable Sockets

Hoop and longitudinal strain are measured using 90-degree strain rosettes installed at various axial locations on the socket [8]. The strain is measured for both static and dynamic loading with the  hoop strain always exceeding the longitudinal strain.  The strain gages may be used to acquire baseline strain data to compare to future data or to calculate the actual stress levels in the socket. 

Figure 4:  Strain gage installation on wire rope socket.   Hoop strain and longitudinal strain are measured using a 90 degree strain rosette.

Acoustic Emission Testing of Wire Rope and Bridge Sockets

Acoustic emission testing of larger bridge cable strand for wire breaks has been demonstrated at the research and development level and implemented in the field using standard acoustic emission sensors and fiber optic acoustic emission sensors.   The  technology has been applied to bridge sockets on smaller scale.  By correlating increases of hoop and longitudinal strain to acoustic emission activity relevant versus non-relevant acoustic emission can be isolated.  Through acoustic emission and strain data fusion, the crack initiation and existing crack propagation may be detected.

Summary

The article discusses various aspects of a wire rope and bridge socket geometric features and non-destructive testing methods for sockets including visual inspection, ultrasonic testing, magnetic particle testing, strain gaging and acoustic emission testing. 

References

1.         ASME B30.26 American Society of Mechanical Engineers. (2022). ASME B30.26: Rigging Hardware. New York, NY: ASME.

2.         OSHA 29 CFR 1926.1413 Occupational Safety and Health Administration. (2023). 29 CFR 1926.1413: Wire Rope—Inspection. Washington, DC: U.S. Department of Labor.

3.         ISO 17558 International Organization for Standardization. (2006). ISO 17558:2006: Steel wire ropes—Socketing procedures—Molten metal and resin socketing. Geneva, Switzerland: ISO.

4.         EN 13411-4 European Committee for Standardization (CEN). (2021). EN 13411-4:2021: Terminations for steel wire ropes—Safety—Part 4: Metal and resin socketing. Brussels, Belgium: CEN.

5.         Federal Specification RR-S-550 General Services Administration. (2014). Federal Specification RR-S-550F: Sockets, Wire Rope. Washington, DC: GSA.

6.         ASTM A609/A609M-12(2018) Standard Practice for Castings, Carbon, Low-Alloy, and Martensitic Stainless Steel, Ultrasonic Examination Thereof

7.         ASTM A609/A609M-12(2018) Standard Practice for Castings, Carbon, Low-Alloy, and Martensitic Stainless Steel, Ultrasonic Examination Thereof

8.         Evaluation of performance deterioration in compact strand wire ropes Prepared by the Health and Safety Laboratory for the Health and Safety Executive 2006