Acoustic emission generated by composite pressure vessels, in theory, should be relatively straight forward to analyze and to draw conclusions from if damage is present and/or damage is progressing by measuring the amount of acoustic emission generated and/or the rate at which acoustic emission is detected. A high amplitude acoustic emission hit from the same source to sensor distance should, in theory, be from a more severe source than a lower amplitude hit. Similarly, a composite pressure vessel failure mechanism that emits AE at an exponentially increasing rate is likely more than a severe than a source emitting AE at a constant rate. A properly instrumented pressure vessel in conjunction with correctly established AE data acquisition settings will produce repeatable acoustic emission test results that qualitatively and quantitatively align with fundamental AE principles like the Kaiser Effect and Felicity ratio.
Figure 1: Acoustic emission data from a damaged composite cylinder showing AE intensity increase and AE activity increase.
The simple correlation of acoustic emission test data to composite pressure vessel failure mechanisms, and steel pressure vessel failure mechanism for that matter, is depicted eloquently in Figure 1. As vessel pressure increases as depicted by the blue curve, there is no acoustic emission activity, acoustic emission is eventually initiated, acoustic emission increases at a constant rate, and then finally the AE rate increases exponentially. This is qualitatively observed by the increase in green dot, or AE hit, density as pressure increase. Similarly, the intensity or amplitude of AE hits occurring is also increasing. Very clearly, the occurrence of AE activity increases in intensity as the pressure increases and the composite damage progresses. Qualitatively, it can be concluded that failure is likely progressing at an exponential rate and is forthcoming.
Diving deeper into the data, some additional useful observations can be made. AE activity doesn’t start until about 3500 PSI. As pressure increases, AE activity does not resume until the previous maximum pressure is exceeded. This is classic Kaiser Effect [1,2] defined as the absence of detectable AE at fixed sensitivity level, until previously applied stress levels are exceeded.
Eventually at higher pressure, detectable AE begins to be observed at pressures below the maximum previous pressure which is classic Felicity Effect [2,3] which is defined as the presence of detectable acoustic emission at a fixed predetermined sensitivity level at stress levels below those previously applied. The Felicity Ratio is defined as the ratio at which AE is detected, to the previous applied maximum load. During the last three pressurization sequences this ratio decreases confirming that the inserted failure mechanism is increasing.
Figure 2: Cumulative acoustic emission activity correlating maximum activity to pressure.
With the advancement of acoustic emission hardware sensitivity, digital processing speeds and capabilities, conventional acoustic emission testing procedures are emerging as an alternative to modal acoustic emission (MAE) that traditionally serves the recertification of Type III and IV composite pressure vessels [4]. MAE is defined as branch of acoustic emission (AT) focused on the detection, capture and analysis of the sound waves generated by acoustic events due to fiber tow breakage, cracking, crazing, rubbing, delamination or fracture of structural components. Some recent studies have established feature based AE as an excellent alternative to complex process driven, an non-intuitive, MAE process [5,6]. Additionally, there are many well established standards and emerging ISO standards that outline general AE procedures and research and development steps to develop innovative acoustic emission COPV testing procedures for testing of hoop wrapped (Type 2) and fully wrapped (Types 3 and 4) composite transportable gas cylinders and tubes of water capacity up to 3000 l, with aluminium-alloy, steel or non-metallic liner or of linerless construction (Type 5), intended for compressed and liquefied gases under pressure [7].
In most cases deeper analyses are required than those presented in Figure 1 to quantitatively assess composite pressure vessel damage propagation with acoustic emission. For example, the Felicity Ratio does not decrease as anticipated across the top range of pressure excursions. Figure 2 is an alternative view that displays cumulative acoustic emission activity correlating maximum activity to pressure. In this view the AE maximum activity for each pressure excursion is correlated to pressure for easy reference.
AE data will be generated by matrix splits, matrix cracks, fiber breaks, and matrix chirps due to fracture surface fretting, and fiber/matrix debonding [8]. Data may be filtered to eliminate any external noise such as electromagnetic interference (EMI), mechanical rubbing, flow noise, etc. Acoustic emission noise events may be identified by shape, spectral characteristics, or other information known about the test such as a temporally associated disturbance due to the pressurization system or test fixturing. EMI is characterized by a lack of any mechanical wave propagation characteristics. Mechanical rubbing frequencies are usually very low and can be determined by experiment. There should be no flow noise. If the vessel, or a fitting, leaks, this will compromise the data as AE is very sensitive to leaks. Robust AE processes and algorithms are required to differentiation between relevant and non-relevant acoustic emission sources. Additionally, advanced analyses are required to discover the data trends that allow AE data to be quantitatively linked to damage progression.
Summary
Acoustic emission testing of steel and composite pressure vessels is relatively straight forward on a quantitative level but requires robust and proven strategies to quantitatively link AE activity and intensity to damage progression.
References
1) J. Kaiser: ‘Untersuchung über das Auftreten von Geräuschen beim Zugversuch’, Dr.-Ing. Dissertation, Fakultät für Maschinenwesen und Elektrotechnik der Technischen Universität München (TUM); 15.2.1950.
2) ASTM E1316-20, Standard Terminology For Nondestructive Examinations
3) Fowler, T. J., “Acoustic Emission Testing of Fiber Reinforced Plastics”, Preprint 3092, ASCE Fall Convention and Exhibit, San Francisco, 1977.
5) HyPactor Project. http://www.hypactor.eu/
6) USE OF ACOUSTIC EMISSION FOR INSPECTION OF COMPOSITE PRESSURE VESSELS SUBJECTED TO MECHANICAL IMPACT EWGAE 2018 Use of Acoustic Emission for inspection of various composite pressure vessels subjected to mechanical impact (ndt.net)
7) ISO/FDIS 23876 Gas cylinders — Cylinders and tubes of composite construction — Acoustic emission examination (AT) for periodic inspection and testing.
8) ASME BPVC.X-2015, MANDATORY APPENDIX 8 CLASS III VESSELS WITH LINERS FOR GASEOUS HYDROGEN IN STATIONARY SERVICE
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