Thermal Behavior of UHMW-PE
A better way to assess residual life expectancy
Recent years have seen a massive growth in the use of ropes made with Dyneema® to replace steel wire ropes. One by one, hurdles have been overcome, new standards set, and new applications made possible. And now, a further challenge is starting to be defeated. It comes with promising research into better ways to assess the in-use condition and residual life expectancy of synthetic ropes made with UHMWPE fiber.
Synthetic ropes now go where only steel has gone before
The combination of less weight at the same strength and diameter compared with steel wire rope, plus better fatigue resistance, safety, chemical and corrosion-resistance, have seen ropes made with Dyneema® win favor in a host of high-end application fields. These include marine and offshore, commercial fishing, hoisting, transport, and leisure and sports.
These are challenging, demanding applications, which makes it essential to be able to assess the condition of a rope in service. Typically, this involves periodic visual inspections and tracking how the rope in question has been used. This methodology is based on experience with steel wire rope, where it is relatively easy to spot broken or corroded wires and to assess overall condition.
“A visual inspection is always a bit subjective and it needs a certain craftsmanship”
Growing interest in sophisticated inspection
Synthetic ropes and hybrid cables made with Dyneema® can also be inspected visually, but because of the high number of UHMWPE fibers compared to the thicker steel wires, it is harder to assess their condition accurately. The result is that decisions about retirement dates are often based on highly conservative models rather than reality.“A synthetic rope is based on a high number of fibers and although this increases the level of redundancy in a positive way, it makes also more difficult to come up with a very clear retirement criterion for unexperienced users,” says Paul Smeets, Application Development Manager at DSM Dyneema. “A visual inspection is always a bit subjective and it needs a certain craftsmanship to come with a good number. With synthetic ropes increasingly replacing steel wire ropes in demanding situations, there is growing interest in having more sophisticated ways to predict residual life next to visual inspection.”
Recognizing that a more sophisticated methodology would benefit end customers, researchers at DSM Dyneema are exploring more scientific testing methods. Among these is one that involves the use of thermal analysis to evaluate the structural integrity of synthetic ropes and synthetic/steel hybrids under bending fatigue conditions.
A new technique for inspecting UHMWPE ropes
The initial findings have been documented in a study called New Approaches for Studying UHMWPE Thermal Behavior that is available as a free download. The study explores the use of various calorimetric techniques – isothermal, conventional and fast scanning calorimetry – to rank and monitor the structural integrity of UHMWPE ropes and hybrid cables made of steel wire and UHMWPE.
"As is the case with steel wire, the crystalline structure of Dyneema® changes with use. With DSC (Differential Scanning Calorimetry) it is possible to capture these modifications for HMPE” says Daniel Istrate- Scientist Morphology- Thermal Analysis at DSM, who is one of the study’s authors. “Not only the DSC peak basic characteristics (e.g. temperature, area, height, shape, etc.) show a relationship to material history, but also its dependency on the heating rate applied.” And knowing this makes it possible to design a tool for performing either in-service or post-service analysis of ropes to achieve performance feedback or to understand any deterioration levels.
Turning a known unknown into a known known
It’s long been known that the melting behavior of UHMWPE can change with thermal history, but the idea to use this to measure fatigue and residual life expectancy was only picked up some years ago when developing ropes with Dyneema® for high-end applications . It was this that led the team to make the connection between use, melting behavior and remaining life expectancy. “We knew of the properties but had never connected them to lifetime,” says Smeets. “That was the new step.”
While promising, there remain a number of challenges to the thermal analysis approach. “It’s a time consuming test method,” says Smeets, “and we need to fine-tune some aspects . We need to perform more testing to further check if the calibration line is statistically sound. You also need a lab and qualified persons, which means it probably cannot be done e.g. offshore or during operation but preferably at service intervals . We need to look further at these issues.”
But even if thermal analysis is not there yet – and it’s just one of several possible methodologies that DSM Dyneema is exploring – the results do at least confirm that it is possible to develop more sophisticated residual life tests for high-performance synthetic ropes. The question is no longer if, but when.