Possible shortcomings of the calibration methods for certain non-destructive monitoring devices for helically wound steel cables

Coupled extensional–torsional behaviour of axially pre-loaded helically wound steel cables (wire ropes and/or spiral strands) under specific forms (i.e. unit-step, triangular, and half-sine) of impact loading are considered in some detail. The final closed-form formulations can handle both the no-slip and/or the traditionally used full-slip coupled extensional/torsional constitutive equations for helically wound cables, and describe the various characteristics of the resulting pairs of axial or torsional waves at any location along the cable with one end fixed against movement and the other end subjected to impact loading. By using extensive numerical results, which cover the full range of current manufacturing limits for the lay angle (with this being the sole controlling geometrical parameter as far as the axial/torsional stiffnesses are concerned), it is shown that significant differences exist between a number of axial/torsional wave characteristics, depending on whether the no-slip or the full-slip version of the constitutive relations is used in the analysis. It is demonstrated that modest increases in the magnitudes of the lay angles can lead to significant increases in the differences between the no-slip and the full-slip wave propagation characteristics. The present findings may have significant practical implications in relation to the currently adopted techniques used by industry for calibrating the electronic boxes, which are subsequently used as permanently installed devices, for the in situ detection of individual wire fractures under, say, fatigue loading associated with cable-supported structures.