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Composite Pressure Vessel Testing using Acoustic Emission Testing

Acoustic emission (AE) technology has been applied to steel pressure vessels, including U.S. DOT and ASME high pressure cylinders, for over 40 years.Over the last few decades, the acoustic emission technology has also been applied to composite overwrapped pressure vessels (COPVs).Compared to other non-destructive testing (NDT) techniques, acoustic emission testing is unique because it detects active flaws in steel and composite pressure vessels.The concept is shown in Figure 1 for U.S. DOT steel cylinders. Two sensors are placed on both ends of the cylinder and connected to AE instrumentation using coaxial cable.The tube is pressurized from 50% to 110% of its maximum allowable working pressure (MAWP).If a fatigue crack, or corrosion related flaw, is present in the pressure vessel and grows during pressurization, it will emit a sound wave, or acoustic emission, which is detected by the sensors. The fatigue crack is located through analysis of the relative time-of-arrivals of the acoustic emission at each sensor.A cylinder passes or fails the test based on how much acoustic emission is detected over the course of the test. This is a summary of the basic procedure used in all U.S. DOT Special Permits that allow for acoustic emission testing in lieu of hydrostatic testing.

Figure 1: Typical setup for an acoustic emission retest under a U.S. DOT special permit

Acoustic emission was developed as an alternative retest method to hydro-static testing for steel and composite cylinders for the following reasons:

· In-service testing: The cylinders can be tested in-service on the trailer using the gas product.

· No contamination: The water used during hydro-static testing contaminates the cylinder and accelerated corrosion on the inside.

· Remote inspection: The entire cylinder is inspected using only two sensors.

· Sensitive to active flaws that grow under pressure

· Inspection is fast and economical

While the relationship between acoustic and fracture in steels is well understood it is still being investigated in the broad range of composite load bearing structures that are available on the market today. The current research, in general, is focused on the characterization of acoustic emission generated by the sources in Table 1. Qualitatively speaking, this means correlating how much acoustic emission is generated from damage mechanisms (AE hits, energy, etc), how often acoustic emission is generated (AE rate), and frequency over which it is generated (AE source).

Table 1: Sources of Acoustic Emission in Steel and Composite Pressure Vessels

Steel Pressure Vessels Composite Pressure Vessels

Fatigue cracks Fiber breakage

Corrosion fatigue cracks Matrix cracking

Stress corrosion cracks Fiber pullout from matrix