Learning from Failure: The Case of the Corroded Cable

INTRODUCTION

Forensic engineering often requires at least some of the skills of Sherlock Holmes; however, many failures teach lessons beyond just the details of a failure. One such failure that had both very specialized and global insights to teach, involved the failure of a stainless-steel wire cable. A cable was supporting a vertical spar on an object on a trailer. It was slack when the trailer was moved. The movement caused the spar to jerk and the cable to become suddenly taut. The cable failed, the spar dropped, and someone was injured.

STAINLESS STEEL AND CORROSION

The Sherlock Holmes part involved special details of stainless-steel alloys. Stainless steel is resistant to corrosion. (More or less. Ultimately, rust always wins.) This is because the chromium content forms a skin of chromium oxide on the surface of the metal. Chromium oxide is a hard, very adherent coating, unlike iron oxide, (rust) which is soft and flakes off. However, the chromium oxide requires oxygen for its formation and maintenance. Without oxygen, the protective layer will be defeated. Worse, the areas where the coating is defeated will become “active”; less noble than the “passive” areas where the coating is intact, and the two regions form a corrosion cell, or a small battery. The differences in the galvanic potential between active areas will induce an electric current that drives corrosion at the active site. This is oxygen deprivation corrosion. 316, and some other stainless steels, have a small percentage of elements such as molybdenum, which greatly reduce this tendency.

Unfortunately, the cable was 302 stainless steel. 302 is frequently used for wire cable because it is strong and resists corrosion when exposed to a marine atmosphere. Unlike 316 stainless, which is also commonly used for wire, and is resistant to corrosion, 302 gains a lot of strength by being cold worked (during the process of drawing out the wire), so it makes stronger wire. Unfortunately, also unlike 316, 302 is very subject to oxygen deprivation corrosion.

The cable had been wrapped with tape near the terminal fitting, presumably to protect it. This deprived the wire of oxygen and held salt water (which includes chlorine ions that enhance corrosion of stainless steels) against the wire, so it corroded, especially by being galvanically driven by the remainder of the cable exposed to oxygen. When the object moved and the cable became taut, it failed under the sudden shock load. Therefore, rigging handbooks recommend that stainless steel wire, even 316 or other less sensitive alloys, not be covered in any way (unlike galvanized wire or bright steel wire).

CORROSION RESISTANT DESIGN

This is also what is called a “maintenance induced failure”, where a maintenance procedure was done with the best of intentions but was a root cause of the failure. Though this speaks to training maintainers, it would only be a very specialized issue except that it also speaks to a more important, more widely applicable feature of failure resistant design. There are four parts to corrosion resistant design: proper materials, proper protection (such as anodes), proper coating (or absence of it) and proper inspection. Proper inspection is critical in any system that is subject to deterioration, not only from corrosion, but also from fatigue, overload, or any other cause. The noted engineer and engineering historian, Dr. Henry Petroski, notes that the inability to inspect steel eyebar links was a cause of the Ohio Silver Bridge failure.

A key concept used in the marine industry is “leak before fail”. A crack or penetration from corrosion should be anticipated, and the structure should be designed robustly enough, so that it can tolerate a small failure that announces itself by leaking or by being readily visible on a scheduled inspection before it becomes critical to structural survival of the structure. Offshore and ship design codes link fatigue and corrosion damage tolerance to inspection processes for this reason. Robust design and systematic inspections are two legs of the process of ensuring the safety of ship and offshore structures.

In the case of the tape covered wire, it would have announced itself by forming “fishhooks”; broken ends of the wires making up the cable on its surface. Not only would they have been seen, but fishhooks are so named because they produce small annoying cuts on the fingers of anyone who encounters them by running their hand over the cable. Covering the cable with wire made it uninspectable, both to actual visual inspections and to accidental inspections producing cuts.

INSPECTION REGIMES

Any engineering system subject to possible deterioration should be designed to be inspected and an inspection regime that coordinates with the effects of a failure should be developed. In circumstances such as consumer products that can’t have formal inspection regimes, it should also fail gracefully, preferably by becoming unserviceable before it is subject to a critical failure (automotive engineers think about this a great deal), or at least by some readily visible, audible, tactile, or even olfactory announcement that would become apparent in an informal “inspection”. (How many times have callers to “Car Talk” made the “funny noise” that raised concern about their car?) In an even larger sense, it is important to remember that maintainers and operators become designers when they modify a system, even in seemingly small ways, and to ensure they understand what they are doing, which speaks to procedures, design, and the wide world of human factors, (much more later on this subject).

SHOCK LOADS

Another point is to respect the power of shock loads, even seemingly small ones. A shock load can occur when kinetic energy, mass times velocity, is absorbed in a short distance when a mass stops suddenly. The energy developed by the moving mass is released in a very short time as the mass decelerates rapidly. It goes from its initial speed to motionless in a small fraction of a second. Because force is mass times deceleration, force is large when an object stops suddenly. This, of course, is how a hammer works, but we can find lots of surprising sources of shock loads in many systems.

To come back to spars, antennas, masts, doors and similar objects, failures caused by rattling are also common, and the result of many small, repeated shock loads. The spar is slightly loose and moves freely for a short distance then stops suddenly against its socket, securing pin, or whatever holds it. It gradually hammers at its connection, further loosening, traveling farther, and producing larger shock loads. Eventually something breaks, either due to direct overload or by fatigue. Such loads not only hazard the moving part itself, but they have been known to cause failures in other parts of the structure that supports them, such as the surface they are mounted on. One key is to secure such things in a resilient mount that minimizes motion by being tightened against the item, and which also, under more extreme loads, absorbs the kinetic energy over a small distance, mitigating the force of shock.

ABOUT THE AUTHOR

Christopher D. Barry, PE, s an experienced naval engineer who has, over the years, gained extensive experience in the maintenance, overhaul, acquisition, design and construction of commercial and military ships and boats, offshore oil platforms and other floating equipment. He is well versed in the areas of hydrostatic, structural, hydrodynamic and mechanical engineering analyses of resistance, motion RAOs in waves, mooring systems and more. See Christopher’s full CV here.

RESOURCES

Brion Toss, The Complete Rigger’s Apprentice, International Marine, ISBN 0-07-064840-9, 1998
Henry Petrowski, To Engineer is Human, the Role of Failure in Successful Design, Vintage Books, ISBN 0-679-73416-3, 1992
AMPP, The Association for Materials Protection and Performance, www.amp.org