Ultrasonic readings taken from fiber reinforced composites, such as plastics, can be related to the elastic modulus, and hence the strength of the material. Research results dating back to the 1960's have shown that stressing fiberglass reinforced polymers (FRP) may result in decreasing the modulus of the material—thus reducing its strength. In service, the stresses applied to FRP typically have the same effect.
An early patent application regarding use of ultrasound for testing materials was made in 1940 by Dr. Floyd Firestone at the University of Michigan. This early patent, and most subsequent work using ultrasound identified that invisible inhomogeneities within materials could be detected. For most of the 74 years since this patent application, the focus of ultrasonic testing has been on metals. In the case of ultrasound, a pressure pulse is applied to a metal material and inhomogeneities are detected when a feature blocks some of the path of the ultrasounds—features that are parallel to the path direction are generally not detected.
Use of fiber reinforced composites for structural applications has been pursued since the 1930's, and has seen significant changes in the polymers and fibers available. With the growth of commercial aircraft starting in the 1960's, many investigations were conducted into use of ultrasound to detect flaws and defects in composites. Because many fiber reinforced composites are made in layers, interfaces between layers often interrupt the path of the pressure pulses and show as features or possible defects for most ultrasonic techniques. Ultrasound is generally considered to be the most common non-destructive technology used for composite materials.
In the early 1960's, use of ultrasonic testing (UT) was already showing reliable results for finding flaws in metallic structures. One of the desirable attributes of this technique is that reliable data could be generated if only one side of the material under investigation was accessible. This meant that in addition to finding flaws or defects, the same techniques could be used to produce thickness records of reasonable accuracy. At the same time, use of composite materials such as glass reinforced thermoset plastics was being explored for a number of structural and corrosion-resistant applications. Starting in the mid 1960's, researchers started to examine uses of ultrasound with these fiber-reinforced composite materials.
In one study, ultrasonic pulses were applied to composites and the responses were received using acousto-ultrasonic devices, thus mixing the principles of ultrasound with acoustic emission testing. This process is generally referred to as “acousto-ultrasonic” because the forces applied to the specimen are from ultrasonic pulses, whereas for acoustic emission, the forces applied to the composite are from mechanical loads, such as pressures and weights. In both cases, the responses are received in real time by acoustical equipment. This work showed correlation between the attenuation of the signal transmitted through the full thickness of a laminate—across its layers—and its tensile strength parallel to its layers. This technique is the subject of two American Society for Testing and Materials (ASTM) standards—ASTM E 1495 Standard Guide for Acousto-Ultrasonic Assessment of Composite, Laminates and Bonded Joints2 (ASTM E 1495) and ASTM E 1736 Standard Practice for Acousto-Ultrasonic Assessment of Filament Wound Pressure Vessels3 (ASTM E 1736).]
A method to employ these techniques is described in ASTM E 1736. In this Standard Practice, it is recommended that initial readings be taken from the vessel to be monitored after calibration to a reference standard and before it is put into service. After the unit has been in service for some time, the results of the initial readings are then compared to readings taken after the unit has been in service. Changes that have occurred in the modulus of the composite from corrosion, decay or mechanical loads will appear as changes in the results of the scan. While there is a relationship between acousto-ultrasonic results and the presence of detectable defects such as voids or delaminations and porosity, it is not certain as yet whether or not these defects are the cause of strength changes. Furthermore, in order to make use of any correlation, it is generally required that reference standards be available for each feature and condition that requires detection.
Many users of structural composites can report that the structural capacity of the composite has reduced while it has been in service. There have been numerous investigations into this phenomenon, including proposed models of the causes of these changes.
Embodiments of the system and method described herein are intended to address at least one of the drawbacks of conventional systems and methods.