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Acoustic Emission Testing of Fiberglass Reinforced Plastic FRP Tanks

TECHKNOWSERV CORP. (TKS) is a leading supplier of fiberglass tank acoustic emission testing on new and in-service fiberglass reinforced plastic tanks.  Acoustic emission testing (AET) is used to inspect newly fabricated tanks during hydrostatic testing and in-service testing typically in 5-year intervals.  The object of acoustic emission testing of FRP tank are to find manufacturing and in-service defects that include delaminations, fiber breaks, matrix cracking, and fiber pullout.  Acoustic emission testing in accordance with ASTM E1067-07: Standard Practice for Acoustic Emission of Fiberglass Reinforced Plastic Resin (FRP) Tanks/Vessels.   ASTM 1067E is an advanced integrity test that quantifies the deterioration of reinforced plastic tanks. 

The process required installation of acoustic emission sensors at different tank elevations and a calibration process that confirms the detectability.  The total number of sensors depends on the acoustic properties of the tank and the number of critical areas to be monitored which may include known damage areas, stress concentration areas, and attachments like piping and flanges.  The tank is progressively filled to 50%, 75%, 87% and 100% capacity.  At each interval, the tank is acoustically isolated and sensors listen for acoustic emission activity.  The acoustic emission data is analyzed determine how intensity and activity.  The intensity and activity are measures of the severity of the defect in the fiberglass tank.  

Acoustic emission in steel pressure vessels generated by fatigue cracks is well understood and is used on other steel structures including bridges, pipelines and many other steel and aluminum load bearing structures.  Over the last 30 years, AE has also been applied to composite structures for the same reasons noted above, mainly to rapidly screen structures for active flaws.  For pressure vessels especially, it is very difficult to match the cost benefit of inspecting with AE.  Table 1 compares the sources of AE in steel and composite pressure vessels.

Table 1 AE sources in steel and fiberglass tanks 

 

Source of AE in steel PVs

Source of AE in composite PVs

Fatigue cracks

 

Fiber breakage

Corrosion fatigue cracks

 

Matrix cracking

Stress corrosion cracks

 

Fiber pullout from matrix

 

There are two fundamental principles to follow when testing pressure vessels with acoustic emission.  The first is the Kaiser Principle in steel.  The Kaiser Principle states that acoustic emission is not generated in a material until load levels that have been previously exerted on a material are exceeded.   The cumulative acoustic emission as a function of load is shown in Figure 1.  

 

Starting at Point A there is no pressure and therefore no AE generated.  AE is initiated at the yield strength of the material.  As the stress/pressure is increased from A to B, the amount of AE also increases.  From B to C, the stress/pressure is decreased and there is no increase in AE.  Acoustic emission only is initiated at load levels greater than the pressure at Point B.  Consider this scenario in terms of a DOT-3A 2400 PSI steel pressure vessel with a fatigue crack.  Assume that a fatigue crack is present and grows when the vessel is loaded to 2100 PSI.  According to the Kaiser Principle, the fatigue crack will not grow, nor will AE be observed, until the pressure exceeds 2100 PSI.  The Kaiser Principle is the basis for pressurizing these tubes to 110% of their service pressure during the 5-year AE retest.  

Composites respond differently to stress than steels and damage may propagate at stresses below the previous load level.  This phenomenon is also shown in Figure 7 at D-E-F.  After the pressure reaches D it is decreased to E with no increase in AE observed.  As the pressure is increased to F, AE is initiated at pressures lower than D.  Note that this is inconsistent with the Kaiser Principle in steels.  This phenomenon is typical of composites and is called the Felicity Effect.  In composite material research, the Felicity Ratio is used to characterize damage.  Consider this scenario in terms of an FRP tank  with fiber breaks and matrix cracking due to impact damage.   Assume that this pressure vessel is loaded to 75% capacity and AE is observed.   Now if the pressure vessel is pressurized again and AE is observed at 50% capacity then the damage was propagated at a lower pressure.  

 

Figure 1: Kaiser Principle and Felicity Effect.

Figure 1: Kaiser Principle and Felicity Effect.

 

 

The Felicity Ratio (FR

The Felicity Ratio (FR

 

Where Stress 1 and Stress 2 are the respective loads at D and F.   If damage is present and/or propagating in an FRP tank, the Felicity Ratio is commonly less than 1.   The magnitude of the Felicity Ratio is used to characterize FRP tank damage.   

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).

 

Figure 2: Acoustic emission hit received at and AE sensor.

Figure 2: Acoustic emission hit received at and AE sensor.

 

Depending on the structure and presence of flaws, hundreds, thousands, or even millions of AE hits can be detected by the instrumentation.   The AE 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 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.  This is represented by the blue shaded area in Figure 3.   The corresponding units are Volts-microseconds (V-usec). As fracture propagates, the energy released by the flaw increases.  

 

 

Figure 3: Acoustic emission hit received at and AE sensor.

Figure 3: Acoustic emission hit received at and AE sensor.

 

TECHKNOWSERV CORP. has inspected hundreds of fiberglass tanks, piping systems, and pressure vessels over the last decade.  In addition to acoustic emission testing of fiberglass tanks, TKS offers External visual inspection will be performed to identify any areas containing defects as defined ASTM D2563-08: Standard Practice for Classifying Visual Defects in Glass-Reinforced Plastic Laminate Parts.  The tank wall, pipe penetrations, flanges and foundation will be inspected as part of the external visual inspection.

Internal Visual Inspection:  Internal visual inspection will be performed to identify any areas containing defects as defined ASTM D2563-08: Standard Practice for Classifying Visual Defects in Glass-Reinforced Plastic Laminate Parts.  The tank wall, pipe penetrations, the floor and lower knuckle will be inspected as part of the internal visual inspection.  When personnel access to the interior of a tank is not possible a remote video camera is used.

Ultrasonic Thickness Testing:  Ultrasonic Thickness Testing (UTT) is performed on the tanks to identify the corrosion rate of the tank and estimate the residual life expectancy.  

Corrosion Barrier Hardness Testing:   The hardness of the inner and outer corrosion barriers will be taken as outlined in ASTM D2583: Standard Test Method for Indentation Hardness of Rigid Plastics by Means of a Barcol Impresser.  These values will be compared to the published maximum and minimum hardness values of the resin manufacturer.  A hardness that varies from the manufacturer’s published maximum and minimum hardness values can indicate non-fully cured resin and inappropriate resin thickness.  Internal hardness testing will not be performed on internal surfaces that are inaccessible due to lack of manways.

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