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Basic principles
Published in M.K. Hurst, Prestressed Concrete Design, 2017
The idea of prestressing, or preloading, a structure is not new. Barrels were, and still are, made from separate wooden staves, kept in place by metal hoops. These are slightly smaller in diameter than the diameter of the barrel, and are forced into place over the staves, so tightening them together and forming a watertight barrel (Fig 1.1). Cartwheels were similarly prestressed by passing a heated iron tyre around the wooden rim of the wheel. On cooling, the tyre would contract and be held firmly in place on the rim (Fig. 1.2), thus strengthening the joints between the spokes and the rim by putting them into compression.
Active acoustic leak detection in a pressurized PVC pipe
Published in Urban Water Journal, 2020
Marshal Deep Kafle, Sriram Narasimhan
The acoustic source is a compact Class I barrel-stave flextensional omnidirectional transducer with a relatively low fundamental frequency flexural mode at 1.6 kHz (Figure 3). This fundamental mode is below the plane wave cut-off frequency which makes this device suitable for this application. However, the actual frequencies used in the ensuing experiments are much lower and as seen later the device is able to produce sufficient acoustic energy in the low region of the spectrum as well. The second mode is approximately 9.5 kHz, which is an axial mode, corresponding to the piezo stack. In the fundamental mode, all the surfaces expand and contract in phase, whereas in the second mode, the end plate and stave are out of phase. The modes of this source are excited using a piezo-ceramic stack, which is excited using a signal from the waveform generator (Keysight 33511B), amplified through a power amplifier (APS 125 with a maximum power of 450 VA@4) and a step-up transformer with a turns ratio of 1:22.7 and primary-rated voltage of 115 V RMS (to inject energy much below the fundamental barrel-stave mode of the source). Similar devices have previously been used for underwater navigation, ocean mapping and for tracking freely drifting floats (Fleming, Kwiecinski, and Jones 2008; Savoia, Mauti, and Caliano 2015); however, its use for leak detection is being reported for the first time. The source is oriented longitudinally within the pipe, and this is the desired orientation even in field settings. As noted by one of the reviewers, this may pose implementation challenges and will potentially require careful placement considerations and additional hardware design.