Cyclic tests of pressure vessels

A pressure vessel with a steel liner, for storing hydrogen was subjected to cyclic tests at room temperature in accordance with the Draft ECE Compressed Gaseous Hydrogen Regulation, 2003. The vessel was loaded with pressures in the maximum range of 20-875 bar on a test rig in the Institute of Materials Science and Applied Mechanics at Wroclaw University of Technology.

a)

Cyclic tests of pressure vessels

b)

Подпись: Fig. 9. Hoop strain characteristic registered by fibre Bragg gratings in different places of vessel (a) and Bragg wave shift registered by sensor FBG1 for different internal pressures (b) (Blazejewski et al., 2007)

Optical fibre sensors (FBG) for local strain measurement were used for the monitoring of the structural health of the vessel. In addition, acoustic emission sensors were employed for reference measurement purposes (Blazejewski et al., 2007). In the course of the basic tests (20-875 bar) about 700 cycles simulating vessel operation (filling-emptying) were carried out. The diagram in Fig. 9 shows exemplary vessel hoop strains registered by sensors: FBG1, FBG2 and FBG3 during cycle loading and the variation in the length of the Bragg wave registered by the FBG1 sensor in a pressure range of 0-875 bar. It should be noted that the vessel’s circumferential strains for the pressure of 875 bar are relatively high (ranging from ~4000 to 4500 pe depending on the type of sensor), (Gersior et al., 2007).

Figure 10 shows the maximum strains (at internal pressure Pmax=875 bar) registered by the FBG sensors installed in the circumferential direction, versus the number of vessel loading cycles. It should be noted that an increase in the number of cycles results in a local increase
in registered strain values (enlarged fragment A in Fig. 10) leading to the failure of the vessel. This is due to the fatigue (cyclic softening) of the steel from which the liner was made. The bursting of the vessel was preceded by rapid increase in locally registered circumferential strains (enlarged fragment B in Fig. 10). By analyzing the slope of the strain – number of cycles curve in its linear region one can determine how long the vessel can safely remain in service, i. e. predict its life. This is particularly important in the case when pressure vessels are to stay in service for many years.

Cyclic tests of pressure vessels

Fig. 10. Maximum circumferential strain (for P = 875 bar) versus number of vessel loading cycles and close-ups of marked areas, (Blazejewski et al., 2007)

Figure 11 shows signals registered by the particular sensors (the circumferential FBG sensors, the strain gauges and the acoustic emission sensors) at the moment of failure of the vessel (the broken line in Fig. 11). It is apparent that the failure was preceded by the previously mentioned rapid increase in strains (in the order of 1500 + 3000 рє depending on the sensor and its distance from the defect) registered in the circumferential direction. Moreover, there is good correlation between the measured changes in the strain field on the vessel’s outer surface and the acoustic signals registered by the AE sensors. At the moment of failure a considerable increase in the root mean square (RMS) acoustic emission signal occurs, which indicates a high degree of damage to the composite layer (Gpsior et al., 2007). Figure 12 shows an analysis of local circumferential strains for the last 8 cycles of the tests to which the pressure vessel was subjected. The analysis was carried out in accordance with the procedure described in sect. 5.2, consisting in the determination of the ABS coefficient. Its value was calculated as the difference between the directional coefficients for the strain – pressure lines obtained from the reference measurement and the next measurements. The first of the measurements was adopted as the reference point. Figure 12 shows a marked jump in ABS coefficient values between cycles 3 and 4 for all the analyzed measuring points. Moreover, the differences between the particular ABS coefficient values increase, which is a symptom of nonuniformity of the strain field distribution on the external surface of the vessel. This situation is indicative of the strong development of defects in the load-bearing structure of the composite pressure vessel (Blazejewski et al., 2008).

Cyclic tests of pressure vessels

Fig. 11. Local circumferential vessel strains registered by FBG sensors (upper diagram) and strain gauges (middle diagram), and acoustic emission signal (bottom diagram) at instant of vessel failure (G^sior et al., 2007)

Cyclic tests of pressure vessels

Fig. 12. Analysis of measurement results for final vessel loading cycles (detection of defects in structure of pressure vessel), (G^sior et al., 2007)