The acoustic emissions are detected by piezo-electric transducers, which are either connected directly to the rope, or its termination(s) and these are subsequently converted into electrical signals. However, a careful study of published work suggests that only limited research has been carried out as regards the feasibility of using acoustic emission monitoring techniques for detecting the occurrence of wire fractures in helical steel cables (i. e. spiral strands and/or wire ropes).

Using previously reported orthotropic sheet theory it is now well established that for a given mean axial load, the torsional and axial stiffnesses of a helical cable are functions of the applied external load perturbations. The no-slip stiffnesses for small load changes have been found to be significantly larger than for large perturbations (where the full-slip stiffnesses operate), because small disturbances do not induce interwire slippage. The theory predicts the no-slip and/or full-slip bounds to the stiffnesses for both spiral stands and wire ropes.

With the coupled axial/torsional stiffness matrices for axially preloaded cables under both the no-slip and full-slip regimes determined in a closed form, it has, therefore, been possible to study the coupled axial and torsional wave propagation in helical cables subjected to various forms of impact loading (due to sudden fracture of individual helical wires). All the final solutions are in a closed form and (most importantly)the validity of the traditional methods of calibrating the so-called black boxes (acoustic emission monitoring devices) which are used for in-situ determination of individual wire fractures in structural cables have been questioned.

Instrumentation experts calibrate these devices by picking up what is called significant effects which are (under laboratory conditions) simulated by deliberately fracturing a dented wire in a newly manufactured and axially loaded cable at the end of which the black box signals (waves) are picked up. However, in old and fully bedded-in cables in practice, the cable structure is compacted in such a way that the amplitudes and speeds of axial and torsional waves are governed by the no-slip cable stiffnesses which are (as discussed previously) significantly different from the full-slip stiffnesses which govern the behaviour of newly manufactured cables originally used for calibrating the black boxes. It is, therefore, suggested that some degree of caution should be exercised in interpreting the data obtained from such devices under service conditions.