Reduction in capacity of steel wire rope slings subjected to forced bending

Abstract

Wire rope slings are commonly used lifting tools in all sorts of engineering applications. Usually, slings are either loaded axially or they are bent around a load bearing element such as a shackle or pipe. In the latter case, the combination of bending and axial loading introduces additional stresses within the wire rope that cause a reduction in capacity. In the offshore industry, this reduction is currently taken into account using a safety factor recommended by standardization societies such as DNVGL and IMCA. However, the expression to derive this factor solely depends on the rope’s curvature around the bend. Given that steel wire ropes can come in many different geometric configurations and their behavior is nonlinear, it is questionable to assume that this reduction is only based on a single parameter. Consequently, the main research question of this thesis is:

How does the wire rope capacity reduction due to forced bending recommended by DNVGL and IMCA standards compare to analytical and experimental results obtained for wire ropes of different sizes and configurations?

The approach to answering this question starts with a literature study, creating a predictive analytical model and then performing experiments for validation. Findings from the literature study uncover the main parameters affecting wire rope behavior such as lay angles, wire diameters and stress-strain relations. With these insights, an analytical model is created to predict the reduction in capacity of steel wire ropes subjected to forced bending. The model analyzes individual wire behavior and then applies a failure criterion to determine when the entire rope cross-section would fail. Finally, small and full-scale experiments are conducted that test different rope configurations (6x25F-IWRC & 6x36WS-IWRC) with different rope sizes (∅20 mm & ∅77 mm).

Knowledge gained from the literature study, the analytical model and the experiments was combined to come up with conclusions pertaining to the main research question. It was found that rope configurations, specifically wire to rope diameter ratios, played a vital role in the capacity reduction of steel wire rope slings. Additionally, results indicate that the capacity of slings increases with higher lay angles. The results also indicated that expressions recommended by the standards are not conservative enough for certain curvature ranges and ropes. Building on the research question, several observations pertaining to rope ovalization and consistency of failure location raised ideas for further investigation.