Hidden Damage in Lifting Slings: Corrosion and Ultraviolet Light

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St. John Shipyard,
New Brunswick

 

Advertorial--Every safety inspector charged with validating the safety of rigging gear should be trained to identify hidden damage to wire rope slings and synthetic slings. Broken wires, cuts, weld splatter; corrosion and other forms of visual indicators of strength loss may lead a safety inspector to assume that there is also some form of hidden damage that cannot be easily identified by the naked eye. Wire rope strands should be opened to inspect the interior and the core. Synthetic materials used for lifting slings and running ropes are not subject to hidden damage from rust and corrosion, but they are weakened in much the same way by exposure to ultraviolet light. Ultraviolet (UV) light is the light at shorter wavelengths than visible light, past the violet end of the spectrum. UV light degrades synthetic slings by transferring energy into the fibers. This energy can cause damage by creating heat or its energy can actually break molecular bonds in a fiber’s structure. The main source of UV light is the sun, but some UV light is also created by welding arcs and Xenon light, which is now used in automobiles as a brighter headlamp.

 

Here’s one example. In 1989, a presentation was made on heavy lifting slings to a rigging group at St. John Shipyard in New Brunswick, Canada. Their facility was located adjacent to the Bay of Fundy, a body of salt water. They were building modules weighing up to 500 tons inside a building and then moving the modules outdoors to be lifted into a dry dock for final assembly of a finished ship. To move those heavy lifts, the shipyard had been using a set of 4-inch diameter by 90 foot long wire rope slings. Because these lifting slings were so heavy and awkward to handle, they were stored outdoors beside the dry dock.  This subjected them to the elements that included cold, heat, salt air and rain. After two years of this exposure, the lifting slings were replaced because of the effects of corrosion from outdoor conditions.

 

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Mianus Bridge, Greenwich, Conn.

 

At that time, Slingmax® had a relatively new high-performance fiber Twin-Path® synthetic sling. The benefits explained to the riggers included light weight and ease of handling compared to the heavy wire rope slings. Surprisingly, the shipyard team identified another benefit - which our Twin-Path® synthetic slings could be stored inside after use and would not be subjected to the weather. Based on this, they were willing to spend extra money on Twin-Path® synthetic slings with the same capacity as their 4-inch wire rope slings. The Twin-Path® synthetic slings were less expensive in the long term because they don’t rust and don’t have to be replaced every two years.  Ultimately, that set of synthetic slings continued working for the shipyard for 15 years.

 

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Silver Bridge, West Virginia

 

There are methods to slow steel corrosion such as painting, galvanizing, plating, and covering with heavy grease, but nonetheless, many accidents have occurred because the hidden damage was not identified by normal safety inspection. Several bridges have fallen because strength bearing members had hidden corrosion that went unnoticed by safety inspectors. In 1967, the Silver Bridge between West Virginia and Ohio collapsed, killing 46 people. In 1983, a section of the Mianus Bridge on Interstate 95 in Connecticut collapsed, killing 3 people. Both of these tragedies were blamed on the failure of safety inspectors to identify corrosion. Crane ropes and wire rope slings have failed from environmental exposure and lack of proper safety inspection.

 

With flat webbing slings, a 2003 research study by the Web Sling & Tie Down Association (WS&TDA) confirmed what industry experts already suspected.  Long term UV exposure reduces web sling breaking strength. Polyester web slings lost up to 30% of their strength during the first 12 months of exposure, after which the strength loss leveled off and Nylon web slings showed a strength loss of up to 50%-60% after 36 months of exposure with no indication of leveling off. This leads to an important question:  Should flat web slings be given a maximum life expectancy similar to the five-year shelf life of synthetic fall protection gear? 

 

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UV Tests of 2-In. Nylon

 

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UV Tests of Polyester

 

The other most common synthetic sling is called a round sling. These products have a strength bearing core inside a protective cover or jacket. Since the late 1970s when the round sling was invented, it was assumed that the round sling cover, no matter the thickness or color, protected the load bearing core from UV degradation because it appears to be opaque. Opaque synthetics are materials that do not allow light to pass through. In an opaque textile the light has to break down the outer layer before it can pass down to the inner section. Thus darker colors are more resistant to UV damage, because they reflect more UV light. Thicker materials resist UV light better than thinner. Thicker materials allow the surface to suffer some UV degradation while retaining the strength in the inner core material.

 

Only recently has it been discovered that UV not only can cause significant damage to the breaking strength of round slings, but there is a wide variance in the protection levels provided by different roundsling covers. Testing has found that while most covers provide adequate protection, others do not. Since ultraviolet light can cause loss of strength and is essentially hidden damage similar to the corrosion of steel, the inspector needs to recognize the issues involved.

 

The following chart displays test results of UV degradation to varying round sling cores and covers used by riggers around the world.  This testing was conducted independently by Murdock Webbing Company in the United States and DSM N.V. in The Netherlands.

 

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N/A indicates that K-Spec load bearing core fiber is never used inside these types of covers.

 

A study of the above chart shows the vast difference in the protection afforded by various types of round sling covers. The green Covermax® is much thicker than other round sling covers and therefore, the sling loses negligible strength. Round slings protected by the thinner lighter colored covers lost the most strength. Also notice the breaking strength between the core yarn types when unprotected. Polyester lost 300% more strength than K-Spec® core yarn.

 

A simple field test of any cover may indicate the level of UV protection. Hold a flashlight against the cover and see how much light shines through the material. The more light that penetrates the cover, the less UV protection you have on that roundsling 

 

Is there a mechanical way to determine “hidden damage” in a round sling?  The answer is yes. In 2010, the United States issued a patent to Slingmax® Rigging Solutions (US #7,661,737) for a pre-failure warning indication system for all round slings. It’s called the Check-Fast® Inspection system.

 

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See the EWI

 

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EWI is missing

 

Here’s how it works. Every round sling is made up of multiple wraps of a particular load bearing fiber. A round sling made with the Check-Fast® Inspection System adds an extra winding of fiber called a “sacrificial strand”, which is independent of the load bearing core yarns.

 

The ends of this independent strand are equally tensioned among the other load bearing core yarns via a “weak link”. The weak link material has a calculated breaking strength that is lower than the sling’s core yarn.  Also, the weak link degrades faster than the core yarn when exposed to UV light.

 

If the round sling is exposed to damaging UV rays, the weak link is designed to break before the load bearing core yarn. The weak link is attached to an External Warning Indicator (EWI) cord which is pulled inside the round sling cover. This system also reacts when the sling is subjected to an overload above its working load limit. With the Check-Fast® inspection system, there is an objective way to determine if a round sling is fit for use.

 

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Slingmax Covermax round sling tubing

 

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Exposed webbing faded to white after 334 hours of UV exposure.

 

In the course of a lift, if a round sling is severely overloaded or serious degradation has occurred, the Check-Fast® weak link is designed to fail first, before the load bearing core yarns and whip the EWI cord inside the cover with a pop. The patented Check-Fast® Inspection system is also designed to detect damage caused by yarn on yarn abrasion, fatigue, heat and chemical damage. The Slingmax® website (http://www.slingmax.com) has a video containing a detailed explanation on how the Check-Fast® Inspection System works and a break test.

 

Knowledge of hidden damage is paramount for any safety inspection program. Length of service, exposure to the elements, and any other potential cause of concealed damage should be a dominant consideration no matter the sling type or material. Safety Inspectors need the training and education necessary to realize the potential for hidden damage.

 

Article courtesy of Dennis St. Germain, Jr.,  Executive Vice-President Slingmax® Rigging Solutions

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