A forensic approach to wire rope defects16 February 2006
Detecting faults in wire ropes involves a great deal of sophisticated detective work, as Roland Verreet and Dr Isabel Ridge explain
If you have ever read a Sherlock Holmes story, you know the pattern: A dead body is lying on the floor. Was it murder or suicide? If it was murder, who killed the person? Why did they do it? And how did they do it? Sherlock Holmes will inspect the room with his magnifying glass, collect a limited number of seemingly trivial and unrelated details, and then present a surprising, but undisputable answer to all these questions.
The work of a wire rope detective has a lot in common with Sherlock Holmes. A steel wire rope has failed and the accident has caused a lot of damage. The rope user, and maybe also a judge in court, will want to know whether the rope was “murdered” (by external factors) or “committed suicide” (for example, in multi layer spooling on the drum). They will want to know how it happened. Why? One reason, of course, is curiosity. Another is that they need to know what caused the accident in order to prevent similar occurrences in future. A third reason is money. Wire rope failures are often associated with expensive damage, and someone will have to pay the bill.
But you do not always need a wire rope failure. The analysis of a discarded rope can also give you valuable information about your crane, the way it operates, and the rope you have been using. In the hands of an experienced inspector, this information might lead to a better crane or wire rope design, or to an improvement in maintenance procedures and safety.
The tools of wire rope detectives are a little more sophisticated than those of Sherlock Holmes. The fingerprint powder has been replaced by groove gauges, digital calipers, accelerometers, goniometers, digital cameras, and laptops, and the magnifying glass has been replaced by the scanning electron microscope.
There are several causes of wire rope failure, and what follows in this article and the second article next month is a brief description of each.
Mechanical wear in steel wire ropes is the removal of material due to mechanical abrasion. It can be reduced by lubricating the rope and, on multi layer drums, by choosing a suitable rope design. For example, the rope should be Langs lay, and it should have compacted outer strands. A swaged rope surface offers even greater reliability.
Due to wear against sheaves, drums, or neighbouring rope wraps, a rope diameter will initially reduce at a high rate. With increasing wear, however, the bearing surface of the wire rope will increase and the rope diameter reduction will slow correspondingly.
As long as the rate of diameter reduction due to wear is higher than the fatigue crack propagation rate, ropes will not develop fatigue wire breaks. Once the rate of diameter reduction slows down, fatigue breaks will appear.
Mechanical wear must not be confused with plastic wear. Plastic wear is the deformation and displacement of material (without or with only little material loss).
Bending fatigue breaks
Rope bending fatigue is caused by running over sheaves or on and off single layer drums. A fatigue crack normally starts at the points of contact between the outer wires and the sheave or drum surface, or at crossover points between individual rope wires. It then proceeds as more bends occur, finally creating a fracture which is perpendicular to the wire axis.
Fatigue breaks occur more often on the inside of the bend (at the point of contact with the sheave) than on the outside of the bend (at the points of highest bending stresses).
The fatigue resistance of steel wire ropes generally increases with increasing number and decreasing diameter of the outer wires of the rope. This improvement goes along, however, with a reduction in the rope's resistance to wear.
Wire rope endurance can also be increased by increasing the sheave or drum diameter or by reducing the line pull.
Wear or corrosion might increase the rate of crack formation and crack propagation. Good wire rope lubrication and relubrication during service, however, will reduce the friction between the rope elements and therefore improve steel wire rope fatigue resistance.
Corrosion is the reaction of metal with oxygen. In steel wire ropes, we distinguish between atmospheric corrosion (producing uniform "rust"), and more local forms of corrosion such as pitting corrosion (creating deep pits in areas where the protective coating is damaged or missing).
Corroded steel wire rope will lose its strength and flexibility. Corroded wire surfaces will form fatigue cracks much faster than protected surfaces. If high local stresses help propagate these cracks, we call this mechanism stress corrosion.
The amount of corroded metal is a function of the surface which oxygen can attack. Steel wire ropes have an exposed surface about 16 times larger than a steel bar of the same diameter and will therefore corrode correspondingly faster.
The amount of corrosion can be reduced by reducing the exposed surface. This can be done, for example, by galvanising or heavy galvanising the rope wires. A steel core can also be protected by a plastic coating. An internal and external lubrication will also reduce or prevent corrosion.
Steel expands when it corrodes. Therefore, sometimes an increase in rope diameter over time might be an indication that the rope is corroding internally. Static ropes (suspension ropes or rope sections lying over a saddle or an equalizer sheave) are more likely to corrode than running ropes.
Tensile overload breaks
Tensile overload breaks are created when the axial load in the individual wire exceeds the wire's breaking strength. Tensile overload breaks are generally associated with a ductile wire diameter reduction at the point of break and the formation of the typical "cup and cone" ends.
Every wire rope failure will be accompanied by a certain number of tensile overload breaks. The fact that tensile overload wire breaks can be found therefore does not necessarily mean that the rope failed because of an overload. The rope might have been weakened by many fatigue breaks over time. The remaining wires are then no longer able to support the load, leading to tensile overload failures of these remaining wires.
Only if the metallic area of the tensile overload breaks and shear breaks (see below) combined is much higher than 50% of the wire rope's metallic cross section is it likely that the rope failed because of an overload.
Shear breaks are caused by high axial loads combined with perpendicular compression of the wire. Their break surface is inclined at about 45° to the wire axis. The wire will fail in shear at a lower axial load than pure tensile overload.
If a steel wire rope breaks due to overload, a great percentage of the wire breaks will be shear breaks. This is because under high axial load, the wire rope will reduce in diameter and wires will be compressed at, for example, crossover points.
If a steel wire rope breaks as a consequence of jumping a sheave or being wedged in, a majority of wires will exibit the typical 45° break surface.
With a shear break, the fracture surface is fairly smooth in appearance. There is a small reduction in diameter at the edge of the fracture surface, but it is small compared with the necking associated with a ductile tensile cup and cone failure. Quite often a wire shear failure will be associated with wear, plastic wear, or fatigue.