Today composite pressure vessels are used in many fields, such as: the car industry, the aerospace industry, rescue services, etc. They owe their popularity to the considerable reduction in the weight of the vessel in comparison with steel vessels, and to their high mechanical strength. Also their excellent resistance to corrosion is not without significance. In order to illustrate the subject, a case of a composite hydrogen vessels is presented below. The use of hydrogen to feed fuel cells and for combustion in conventional engines requires a safe and expensive fuelling method. Storing hydrogen in pressure vessels of the CH2 (compressed hydrogen) type has become the predominant technology, especially in cars. The latest (4th generation) designs are pressure vessels with a nominal working pressure (NWP) of 70 MPa. The weight of the vessel itself should be low enough in order for the latter to be installed in a car without the necessity to reduce the boot space. For this reason, wholly composite vessels (Fig. 8) must be installed. The load-bearing wrapping of such a vessel is made from high-strength carbon fibres (CFRC) while the liner (ensuring tightness) is made of high-quality plastics. The burst test pressure (BTP) at a safety factor of 2.35 (for CFRC) has to be above 164.5 MPa. In the case of glass composites (GFRC) the safety factor increases to 3.65 whereby BTP is higher.
Because of the stringent requirements the CH2 vessel is an integrated high-tech product which can withstand extreme strains, and all its building components, i. e. the materials, the calculations, the design, the lining and wrapping technologies and the test methods, represent the cutting edge of research and engineering. The problem of SHM for such a vessel is discussed below. The discussion is based on the present authors’ experience gained in carrying out research projects, among others, within the 6th EU Framework Program (Storage of Hydrogen – StorHy) and the 7th UE FP (Integrated GAS Powertrain – InGas).
The combination of the highly-strained composite load-bearing structure of the vessel and the hazardous substance (which compressed hydrogen fuel is) necessitates the monitoring of the structural health of such objects. It is estimated that the danger area in the case of explosion of a full car methane tank is 250 m large in diameter and in the case of hydrogen it may be as large as 1000 m in diameter (Mair, 2007). Therefore it is of critical importance to ensure operational safety over the entire ten-year service life.
At the moment the standards require that pressure vessels be subjected to routine tests certifying them fit for further use, but it is not necessary to permanently install SHM systems. Because of such regulations pressure vessel manufacturers prefer to increase the amount of the material reinforcing the structure to increase its safety than to optimize the the vessel design. This leads to higher production costs and a higher final product price. However, currently efforts are made, e. g. (Sulatisky et al., 2010), to exert pressure on the legislative bodies to introduce regulations reducing the current high safety factors for the manufacture of pressure vessels when a continuous structural health monitoring is employed. As already mentioned, standard NDE methods are not suitable for the on-line monitoring of such objects and can be used only for periodic inspections (Foedinger et al., 1999; Degrieck et al., 2001). A system of sensors should ensure the assessment of the technical condition of the high-pressure cylinder during both its production and (above all) its long use.
The research in this field, e. g. (Foedinger et al., 1999; Degrieck et al., 2001; Kang et al., 2006; Hernandez-Moreno et al., 2009; Sulatisky et al., 2010; Blazejewski et al., 2010; Glisic & Inaudi, 2004; Frias et al., 2010), shows that the best of the offered solutions is to use optical fibre sensors for this purpose, since they are characterized by resistance to electromagnetic interference, non-sparking safety, ease of integration with the structure of the composite material and high sensitivity in a wide measuring range. Such sensors, permanently installed on the surface of the vessel or incorporated into the structure of the composite material, enable the continuous monitoring of the structure’s technical condition over its whole service life.
In one of the first works (Foedinger et al., 1999) dealing with the application of optical fibre sensors to the monitoring of the structural health of standard testing and evaluation bottles (STEBs), the authors used FBG sensors which were integrated with the composite loadbearing layer as it was being wound during its manufacturing. The sensors were to measure the strain and temperature of the vessel during the curing of the epoxy resin and quasi-static pressure tests. A micrographic analysis of the cross sections of the composite load-bearing shell containing the optical fibre was carried out and showed good interaction between the two as well as no defects in the areas incorporating the sensors. The latter were located in several areas of the pressure vessel, e. g. in its cylindrical part and in its bottoms. Each time they were positioned in the direction consistent with the direction of the reinforcement, which means that they were wound at different angles to the principal axis of the vessel. In addition, resistance strain gauges and thermocouples were employed as the reference sensors. The strain gauges were installed on the vessel’s outer surface (after resin curing) in the neighbourhood of the fibre Bragg gratings. The investigations showed good agreement between the sensor measurement results and the strain gauge measurement results (depending on the winding direction and the location, the difference between the FBG sensor results and the strain gauge results ranged from 1.2% to 24%) and the numerical FE model results.
In (Degrieck et al., 2001), the authors investigated the possibility of using fibre Bragg gratings to monitor the strains of components made of composite materials, including laminates and pressure vessels made by winding. The research was carried out in several stages. First tensile tests and temperature tests were carried out on a bare optical fibre with an FBG sensor. Then carbon-epoxy laminate with a fibre Bragg grating placed between the layers was made. The laminate was subjected to the static three-point bending test. Finally, a pressure vessel was made by winding a glass fibre, in which an optical fibre together with a Bragg grating was wound (in the circumferential direction) with the final reinforcement layer, parallel to the latter. The vessel was then subjected to tests. In the first test it was statically loaded with an internal pressure of 0-16 bar and then cyclically with 0-3 bar. In both tests a change in pressure resulted in a shift of the Bragg wave reflected from the sensor incorporated into the structure of the composite load-bearing layer. It was found that being located directly in the material structure, FBG sensors are ideal for measuring strains in composite materials and in addition, they represent a promising technique for nondestructive testing and evaluating the structural health of composite elements (Degrieck et al., 2001).
The suitability of optical fibre (FBG) sensors for monitoring composite pressure vessels, especially taking into account the possibility of integrating such sensors with the loadbearing layer during the manufacture of vessels by winding, is the subject of the paper (Kang et al., 2006). The authors describe the difficulties they encountered while embedding measuring heads (made by welding FBG sensor together) in a composite material. Only one of the four measuring heads in a pressure vessel with the sensors prepared in this way survived the manufacturing process (winding and high-temperature curing). It turned out that because of their low resistance to lateral stresses in the weld, the welded fibres are poorly resistant to the conditions prevailing during vessel manufacturing whereby the risk of optical fibre damage during reinforcement winding increases considerably. Measuring heads consisting of several sensors produced directly in one optical fibre segment additionally coated with a thin protective layer proved to be much better. In the latter case, the percentage of sensors which after the vessel manufacturing process were still fit for measurements increased to ~79% (11 out of the 14 sensors were available). By comparing the reflection spectrum for the pressure vessel before annealing with the spectrum for the ready vessel one could determine the local residual strain. In all the measuring points local compression amounting to several hundred microstrains (pe) was registered, but the strains at the vessel bottoms were relatively larger (Kang et al., 2006).
Then the vessel was subjected to the pressure test until failure. For reference purposes, strain gauges were installed on the vessel’s surface. The gauges were damaged before the vessel burst. The maximum registered strains amounted to 0.95% for the pressure of 2900 psi (~200 bar). Also the splitting of the Bragg wave into several peaks was observed – probably due to the displacement of a sensor relative to its optimal position fixed during winding (the sensor was not situated parallel to the reinforcement fibres), (Kang et al., 2006).
The reliability and safety of composite vessels with an integrated SHM system is also the subject of the research conducted by Hao and others (Hao et al., 2007). They proposed to create a system which would monitor the circumferential and longitudinal strain of a tested vessel made of glass-epoxy composite, by means of three FBG sensors installed on its surface. In addition the authors used two strain gauges to locally measure strains in the circumferential and longitudinal directions. The tested vessel (with a 3.2 mm thick loadbearing wall) was statically loaded with a pressure of 0-200 bar. The obtained measurement results showed good convergence for the strains registered in the longitudinal direction, particularly in a low pressure range (up to 8 bars), and large scatters for the circumferential strains. The differences between the FBG sensors and the strain gauges amounted to as much as 60%, even though they were separated by a distance of maximum 12 mm. According to the quoted authors this is due to the different distances of the sensors from the defect around which the vessel burst (Hao et al., 2007).
The use of fibre Bragg gratings for the monitoring of the state of stress of a composite pressure vessel was also the subject of research carried out by Frias and others (Frias et al., 2010). In their paper the authors describe a measuring system based on six FBG sensors and two piezoelectric polyvinlyidene fluoride (PVTF) sensors, integrated with the composite vessel structure. The sensors were located between the steel liner and the load-bearing layer made of glass fibre in thermoplastic resin. The liner was made by welding the two bottoms and the circumferential FBG sensors were located in the direct vicinity of the weld. The other sensors (two Bragg gratings and two piezoelectric sensors) were located in the cylindrical part, and registered longitudinal strains. The vessel was subjected to cyclic testing within which 9 test series with 30 cycles in each were carried out. The upper cycling pressure amounted to 5-40 bar with a 5 bar step in the successive series. The test results showed good convergence for the different measuring systems. Moreover, the liner in the weld region was found to plasticize quickly under the cyclic loading, as indicated by the circumferential FBG sensors. The latter showed that the vessel behaved nonlinearly in the circumferential direction in the weld area and linearly in the longitudinal direction outside this area (Frias et al., 2010).
Glisic and his co-workers in (Glisic & Inaudi, 2004) presented the application possibilities of a system for monitoring the structural health of 4th-type composite (the so-called full composite, a polymer liner and carbon-epoxy reinforcement) pressure vessels for storing compressed methane in cars, with a nominal working pressure of 350 bar. The proposed measuring system is based on interferometric SOFO® sensors. The latter are characterized by relatively long measuring arms which average the measured strain over their length. The aim of the research carried out within the EU ZEM Project (the 5th EU Framework Program) was to develop a topology of sensors distribution on a vessel, and a proper algorithm analyzing the measurement data, enabling the detection of local defects which could endanger the safety of the users (Glisic & Inaudi, 2004).
SMARTape® sensors were integrated with the vessel structure and located between the last carbon fibre layer and the first glass fibre layer. The interferometric SOFO® sensors measured strains in the longitudinal direction (four sensors spaced at every 90° on the circumference) and in the circumferential (helical) direction (2 sensors with opposite lead angles).
become more deformable (under a constant pressure), which meant that the slope of the strain-pressure line would change. It was also assumed that a local defect could result in local deformations (e. g. ovalization, bending) of the vessel, which would be registered by pairs of SOFO® sensors distributed symmetrically on its surface. Thanks to the latter assumption the correlation of measurements between selected sensors could be analyzed. In the case of a defect, the linear dependence between the strains registered by one sensor and the ones registered by another sensor will be disturbed and will change (Glisic & Inaudi, 2004).
In order to prove the above assumptions a series of static and quasi-static tests were carried out on new pressure vessels and vessels with designed flaws (cuts differing in their geometrical dimensions and delaminations), subjected to a pressure of 0-350 bar. The authors of (Glisic & Inaudi, 2004) show that an increase in such parameters as deformability or the relative compliance coefficient and a change in the reciprocal linear dependence between the particular sensors are indications of vessel damage.