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
Depending on the structure and presence of flaws, hundreds, thousands, or even millions of AE hits can be detected by the data acquisition system. The acoustic emission activity must be analyzed in parallel with load information to determine if the flaw is inactive, active, and the rate at which it is growing.
The basic input for all analysis is the acoustic emission “hit” is shown in Figure 2. An AE hit is generated inside the cylinder at sufficient amplitude to travel to and be detected at the sensor. The amplitude of the hit is displayed on the vertical axis in Volts. The arrival time of the hit at the sensor is shown on the horizontal Time axis in microseconds (usec). A
The AE energy is also a common feature used to determine flaw activity in a pressure vessel. The AE energy is calculated for each hit by integrating the rectified voltage signal over the duration of the AE hit. The corresponding units are Volts-microseconds (V-usec). As fracture propagates, the energy released by the flaw increases.
Figure 2: Acoustic emission hit received at an AE sensor.
Acoustic emission wave propagation in composite pressure vessels is very complex due to anisotropy, multi-material, multi-layer, and cylindrical geometry. Typically, acoustic emission testing of cylinders is performed using sensors in the 100 to 500 kHz range which are sensitive to the lowest order extensional and flexural wave modes present in COPVs wall thicknesses used in industry. Temporal, frequency, and statistical analysis of acoustic emission waveform features can identify likely damage sources. Matrix cracking acoustic emission has been documented in the 50 to 200 kHz range and fiber rupture acoustic emission has been largely documented at higher frequencies in structures with comparable thickness and construction. The acoustic emission frequency spectra of the different damage will overlap and are not delineated clearly.
The acoustic emission generated from composite matrix cracking is commonly low amplitude at moderate time durations. Comparably, acoustic emission originating from fiber rupture occurs at higher amplitude, longer time duration, and higher energy levels. Similar to the frequency spectra, however, there will be toverlap between these acoustic emission parameters for matrix cracking and fiber breakage. A multi-feature analysis approach, combined with a robust database of acoustic emission waveforms from known composite damage mechanisms, is the most reliable approach to correctly characterize the source(s) of acoustic emission in composite pressure vessels.
An example acoustic emission setup on a composite cylinder is shown in the video below. The cylinder was instrumented with two acoustic emission sensors and one strain gage during the pressurization sequence. The transducers were placed towards the end of the cylinder. Broadband acoustic emission sensors were used. Acoustic emission calibration and source location was performed using a simulated acoustic emission source. The acoustic emission sensitivity and source location tested during this calibration process using 1-D or 2-D location algorithms depending on the size of the composite cylinder and number of acoustic emission sensors used.
The acoustic emission generated by three difference cylinders that incurred an increasing amount of impact damage are shown in the video below. The amount of acoustic emission energy released by the cylinders correlated to the severity of impact damage. Acoustic emission generated by matrix cracking was detected over a lower frequency range compared to fiber rupture acoustic emission. The amplitude and duration of the acoustic emission hits from fiber breaks were generally higher and longer that those from the matrix cracking. Catastrophic failure and an exponential increase in acoustic emission activity was observed from the cylinder that incurred the greatest degree of impact damage.