Discloses methods to perform magnetic testing of tensioning elements in a
pre-stressed concrete cylinder, such as a pipe or water reservoir and
testing apparatus. The apparatus includes magnetic flux production means
and detector means disposed proximal to a surface of the cylinder in a
plane in common with the magnetic flux production means that is orthogonal
to an axis of the cylinder. The apparatus operates over a range of low
frequency signals, for example, between 20 and 300 hertz or a pulse.
Output of the inspection apparatus includes a signal and distance plot
showing the results of testing a cylinder at one or more frequencies. In
accordance with another method of analysis, a characteristic of the phase
of the output over distance is plotted, including the phase or
representations of the in-phase or quadrature components of the received
signal in relation to the driving signal.
