R.E. Hobbs

THE FATIGUE OF STRUCTURAL WIRE STRANDS

Abstract This paper presents the results of a limited series of large scale in-line and bending fatigue tests on socketed structural strands typical of those used as stays for guyed masts and for suspension bridge hangers. The strand, consisting of a group of hard drawn galvanized steel wires laid up helically about a common axis, is terminated by zinc filled conical sockets. The various failure mechanisms, concentrated on the wires close to the socket, are described and discussed. It is concluded that conservative predictions of the in-line fatigue lives of real strands should be possible although further work is needed on bending fatigue.

Torsion tests on large spiral strands

Abstract Carefully conducted experimental studies of the torsional characteristics of substantial structural strands are reported. The match between experimental results for an old and fully bedded-in 39 mm diameter, 91 wire spiral strand and some theoretical predictions is very encouraging. The theory, which has been reported elsewhere, treats the individual layers of wires in a strand as orthotropic sheets and, via established results in contact stress theory, takes full account of the frictional interactions between wires. Static and dynamic torsional stiffness and hysteresis data are presented for axially-preloaded strands. Strain gauging of individual wires showed significant scatter, but the match between theory and the mean of the experimental data for a position remote from the termination was good, using the same assumed value of the friction coefficient for all tests. For an old strand, random loading can significantly increase the level of torsional hysteresis above that found for regular cyclic loading, provided that the interwire force changes are large enough to overcome the contact friction. Tests on a new 41 mm diameter strand indicated that hysteresis measurements on newly-manufactured strands could be misleading for long term applications.

Asymmetric effects of prop imperfections on the upheaval buckling of pipelines

Abstract Submarine pipelines often carry products which are much hotter than the surrounding seawater. The potential thermal expansion is restrained by friction between the pipeline and the seabed, causing the development of large compressive axial forces in the line, which can lead to buckling of the pipeline. This paper takes a fresh look at the vertical buckling of a pipeline encountering a point irregularity on an otherwise perfectly flat seabed, the so-called ‘prop case’. Some approximations and assumptions in earlier work in this area are reexamined and their effects are calculated. Most importantly, the assumption that buckling is symmetric about the prop is tested. Asymmetric results are found, at a lower critical temperature than the symmetric mode, a fact which may have significant implications for design.

Analysis of axial fatigue data for wire ropes

Abstract Length, termination, and statistical effects in the axial fatigue of wire ropes are discussed. Considering experimental results in the light of theoretical estimates of the recovery length of broken wires in spiral strands and ropes, a specimen lenght of ten lay lengths is suggested as a desirable lower limit. Terminations similar to those to be used in practice should also be tested to identify the and effects. By analysing statistically significant population of test data obtained under nominally identical conditions, a Gumbel distribution is proposed as the most accurate representation of the distribution of the fatigue life for steel wire rope.

Tangential Compliance of Rough Elastic Bodies in Contact

A simple method is presented for determining the tangential force-displacement relationship between two elastically identical generally curved bodies when the tangentical force is not directed along either of the principal axes of the contact patch

Fatigue of wire ropes bent over small sheaves

Abstract The fatigue performance of heavily loaded wire ropes bent over rather small sheaves has been investigated. A selection of 28 and 35 mm diameter ropes of 6 x 36 construction was tested over sheaves with D / d ratios between 12 and 20. Three different groove profiles with diameters 2.5% less than the nominal rope diameter and mean loads of up to 65% of the rated axial strength of the rope were used. Under the conditions examined, a 28 mm independent wire rope core bright wire rope had the best fatigue life and a 28 mm fibrecore galvanized rope the worst. One of the three grooves employed severely decreased fatigue life.

Two-dimensional upheaval buckling of a heavy sheet

Abstract A large elastic sheet with a significant self-weight subject to biaxial in-plane compression while lying on a rigid horizontal plane may buckle. The buckles, circular areas which lift away from the rigid subgrade, are essentially a plate (two-dimensional) phenomenon, analogous to the widely discussed one-dimensional upheaval buckling of beams. It is shown that plate upheaval is relatively insensitive to subgrade friction, a fact which may simplify design against such buckling.

Bending fatigue in high-strength fibre ropes

Abstract Bending fatigue tests on parallel lay (Parafil) ropes of the aramid fibre Kevlar 49, and helically laid ropes of Kevlar 29 and high-modulus polyethylene (HMPE) are reported. The Parafil ropes were subjected to three different regimes: free bending-tension tests intended to produce failures at the mouth of a termination, sheave bending through 45° under varying axial load, and sheave bending through 180° at constant axial load. The Kevlar 29 and HMPE ropes were tested over a 180° sheave. Full descriptions of the various procedures are given, and more than 20 separate results presented. Although the free bending-tension tests were intended to produce failures at the mouth of the termination, most failures occured elsewhere: one test was stopped after one million cycles. The 45° sheave bending tests produced lifetimes of about 4000 cycles when tested at up to 50% of the static breaking strengh. The 180° sheave bending test gave lives of 157 cycles at 25% of the static breaking load (SBL) for the Parafil ropes, and 100 to 400 cycles at about 40% SBL for the helically laid Kevlar 29 ropes. The HMPE rope gave nearly 6000 cycles at a load of 40% SBL.

Pressure-sensitive ‘Yarns’ and In-rope Friction Measurements

This paper describes the development of a method for measuring pressures and friction within textile structures and its use on four ropes made from two different man-made fibres. The method employs a novel pressure-sensing ‘yarn’ element built (or pulled) into the rope and a separate measurement of the force needed to pull a short length of rope yarn out of the rope. The coefficient of friction is then simply found from the pull-out force divided by the pressure force acting on the surface area of the yarn. Two wire-rope-construction Kevlar ropes and two multirope polyester-fibre ropes were compared, at a variety of mean axial loads (1%, 5%, and 10% of the ultimate axial load), and on first loading as against the eleventh cycle of loading. By using built-in sensors, coefficients of friction were found to range from 0.10 to 0.14 for the Kevlar 960 and from 0.24 to 0.31 for the Kevlar 961 ropes, with variations at low loads. Because of technical difficulties with the pulled-in sensors used for the polyester...

Computer control of a complex block-loading test

Abstract This paper describes the rig used for the full-scale fatigue testing of an oilfield production riser connection. The complex block-loading spectrum and the need to co-ordinate data acquisition with the loading made a real-time, computer-based control system an attractive, perhaps an essential, solution. It is seen that in the integrated system of electronics, servohydraulics and rig hardware, the computer functioned as a man/machine interface. Aspects of safety to which the computer contributed are highlighted, but the importance of electro-mechanical devices is also recognized. It was found that a multiple checking technique was a valuable tool in avoiding nuisance tripping of the safety circuit. Finally, the main advantages of the computer-based control system are summarized.