1.3 Concept of composite pressure vessel monitoring system
The on-line monitoring of a pressure vessel is not limited to only the measurement of the mechanical parameters (e. g. strain fields) of its composite load-bearing layer. In order to determine the vessel’s structural health and to detect critical defects which may pose a risk to the safety of the users it is necessary to employ proper algorithms to analyze the acquired measurement data. An ideal solution to this problem is a hybrid method in which the acquired measurement data are compared with the numerical model of the monitored object. Moreover, a good numerical model should indicate the primary sensor locations to be taken into account when installing sensors. However, this approach is not always possible, usually due to the lack of sufficient information needed to create a reliable numerical model of the investigated object. In addition, the use of the hybrid method necessitates the use of separate numerical models for each type of vessel. Any change in the design must be properly taken into account in the modelling process.
In the case of pressure vessels for which there is no precise information on the geometry of the load-bearing wrapping or the materials used, the present authors proposed to use a method consisting in analyzing local changes in the strain field (measured in many places) during the service life of the vessel and comparing them for particular pressure values. Any deviations from the assumed uniform strain field distribution in the vessel’s cylindrical part indicate its potential damage (Blazejewski et al., 2008).
In order to create a fully functional system of monitoring the structural health of composite pressure vessels a special coefficient called ABS (absolute value) was calculated and compared with the threshold value. The ABS coefficient value was calculated as the absolute value of the difference between the coefficients of the directional strain-pressure lines for the successive vessel stress cycles. In addition, in order to locate possible defects the ABS coefficient values were compared between the particular measuring points (sensors). For a pressure vessel with no defects the dependencies between the particular sensors should be the same or very similar (considering the uniform state of stress in the load-bearing layer). When damage (e. g. a delamination or a crack) arises, the strain field is no longer uniform and the dependencies between the sensors are disturbed (Blazejewski et al., 2010).