Bridging Acoustic Emission Testing and Ultrasonics for FRP

Introduction

The use of Fiber Reinforced Polymer (FRP) for vessels and piping in the chemical processing industry (CPI) started in the 1960s. From early in the use of FRP for corrosion resistant equipment, challenges have presented themselves as engineers had to develop new design and construction methods to suit the behavior of the material. Early design approaches included use of metal vessel design standards, material design that did not correctly consider the reinforcement, quality control of fabrication and assembly, transportation damage, and damage from over-stressing at proof testing following metal vessel procedures, coupled with in-service abuse, process upsets, and other structural damage. Unfortunately, the early legacy of these challenges was numerous FRP failures, sometimes injuring workers. In spite of the undeniable industrial benefits of FRP to the CPI, in the early 1970s some significant owners of FRP vessels placed a moratorium on its use until its reliability could be improved.

Since that time, engineering efforts have resolved the early design flaws and have resulted in good design and construction standards. ^{[1] [2] [3] [4]} These standards are dynamic documents with systematic review and updates. 

To remove the moratorium and continue using FRP vessels, a non-destructive testing (NDT) method was required to evaluate the structure of the FRP and ensure that the final commissioning steps of hydrotesting and proof testing did not cause any damage. In the 1970s, investigation started of Acoustic Emission (AE) as a test method. This investigation was completed by the Committee on Acoustic Emission from Reinforced Plastic (CARP), including participation from material suppliers, users, test equipment suppliers and FRP fabricators. CARP worked as fast as they could to produce a solution, which also meant that they worked independently of the existing AE community and without independent critical review that Universities can provide.

The result of the CARP studies was an approach that focused on three effects: 

1. The “Felicity Ratio” is the ratio of the stress at which a specific level of acoustic emission occurs compared to the previously applied stress where the same level of acoustic emission occurred. This describes how damage to FRP from a testing load - such as water fill - can alter the acoustic emissions of the FRP when the load is removed and then reapplied. A Felicity Ratio less than 1.0 can often be encountered. 
2. Acoustic Emissions that continue when loading is held constant, and
3. High amplitude emissions.

Since 1983, AE has been adopted by many FRP users and has also been incorporated into relevant codes and standards. ^{[1] [2] [5]} Some jurisdictions require that FRP storage tanks only remain in service if they meet the acceptance criteria from periodic AE testing.

Each AE result is unique and does not depend on past results, nor does it predict future changes in the FRP.

This article is written to provide a brief case study of an AE test of an FRP storage tank. It begins with a description of the FRP tank being tested and the results of the test, which are used to illustrate how AE results can be combined with an attenuation-based ultrasonic technique that has been shown to provide reliable prediction of changes to FRP from service conditions. We then go on to provide further background of acoustic emission testing and relating the background to the test results. Ultrasonic testing was used to investigate the tank further along with conclusions about the cause of the AE results. Finally, a method is proposed to combine acoustic emission with the ultrasonic method described in my previous IJ article ^[6] to ensure reliable long term operation.