Increased Fatigue Life Of Wire Rope Through Periodic Overloads
Authors: Smith H.L. , Stonesifer F.R. , Seibert E.R. 
Published: October 2008

Periodic overloads have shown pronounced improvements in the fatigue endurance of wire rope, typically limited in service use to loads one-fifth of static breaking strength. This enhanced life is attributed to overload crack retardation and to readjustment-or stresses within the rope by movement of contact points between wires during overloads. The optimum overload is thought to be that which just begins to result in rope yield. To optimize rope life one must also look at number of overload cycles, frequency of overload application and rope material and construction as well.

Wire breaks, for the ropes investigated, do not become significant until about 50% of rope life is spent. The static strength of a rope-was generally found to correlate with break density of its component wires. Break density was a maximum at intermediate applied stresses, with few breaks at higher and at lower stresses, indicating wire break up, at service use loads, may not necessarily be a good indicator of remaining cyclic life.

Salt water exposure decreases fatigue life of wire rope, depending on load cycle duration, but overload benefaction still occurs.

Scanning electron microscopy shows individual wire failure to be by subcritical fatigue crack growth.

The phenomena of overload crack retardation, well known in aerospace structural and machine elements, has not been shown in wire rope prior to the work at NRL.

INTRODUCTION

Until this year, periodic recertification of weight-handling equipment for servicing U.S. Navy submarine-launched missiles included an overload proof-test. Tasked to evaluate this rather costly procedure, the Naval Research Laboratory mounted an investigation which showed that if not too frequent, periodic overloading can be beneficial to the endurance of wire rope to tensile fatigue failure.l,2 This critical study has served as partial reason for relaxing the proof test requirement for this service, as an exception to the general requirement of Navy ordnance handling manuals OP4 and OP5. However, a more far reaching applicability of this effect is perceived, applications to other Naval service as well as to civilian activities such as in the offshore technologies of present conferring. Accordingly additional studies have been sponsored. This paper reviews findings of the initial investigation in the context of additional results obtained to date.

EXPERIMENTAL PROCEDURE

The earlier NRL study1,2 made use of a "typical" lifting sling consisting of a spreader-beam, connected from its ends to a single point by two convergent wire-rope pendants, Fig. 1. Later work employed two pendants in parallel array connected by load equalizing gussets. In each case identical loading was applied to duplicate pendants. The pendants were terminated with an eye splice at either end, around a galvanized steel thimble, with the free rope end splayed around the body of the pendant and clamped with two compression sleeves. The central region of the wire rope, free of the end clamps, was approximately 0.44 meters (17.5 in.) in length.