As the use of fiberglass and carbon fiber composites in manufacturing has grown, so has the need for reliable nondestructive testing, both in the initial fabrication stage and while in service. Traditional fiberglass is commonly used in tanks, pipes, boat hulls, wind power blades, structural panels, and similar products. Because of their laminar layup structure, these materials are potentially subject to cracking parallel to the surface, either because of applied stresses or weaknesses resulting from manufacturing anomalies. These hidden internal cracks can have a significant impact on structural integrity and are normally not detectable by radiography or NDT techniques other than ultrasonics. Fortunately, ultrasonic testing provides a ready and well-established technique for locating and documenting internal flaws.
Ultrasonic flaw detection and thickness gaging are based on a simple principle of wave physics. A high frequency sound wave that has been generated by a small probe called a transducer and coupled into a solid medium like fiberglass or composites will travel in a straight line perpendicular to the surface until it encounters a material boundary such as a far wall, another material interface, or a lamination. At that point, the sound wave will be reflected in a predictable way. Thickness gages measure the round-trip transit time of the sound pulse and then use the programmed speed of sound in the test material to calculate thickness. Ultrasonic flaw detection analyzes echoes through a comparative process in which the echo pattern generated by a good part is compared with the echo pattern from a test piece. Since sound waves will reflect from voids or cracks, changes in the echo pattern indicate changes in the internal structure of a part. In testing fiberglass and composites, the instrument typically looks for the presence of echoes within a marked gate or window that represents the interior of the test piece. While the inhomogeneous nature of fiberglass and composites can generate scatter noise reflections even from solid material, cracks whose area approaches the diameter of the sound beam typically return strong localized indications that will be recognized by a trained operator.
The test frequency and probe size are selected based on the material being inspected and critical defect parameters. In general, higher frequencies and smaller beam diameters are required for resolution of smaller defects. Lower frequency probes are used to penetrate deeper into materials and offset scattering and attenuation of sound in materials with lower density or inhomogeneous structures. Probe selection and instrument setup should always be optimized for the job at hand.
Fiberglass parts and structures
Fiberglass is most commonly tested with traditional ultrasonic thickness gages and flaw detectors using low frequency single element transducers, commonly at frequencies of 2.25 MHz and below, typically as low as 0.5 MHz when thicknesses exceed approximately 0.5 inch or 12.5 mm. Specialized low frequency transducers utilizing impedance-matching delay lines techniques can optimize both penetration and near surface resolution. Thickness gages designed to provide a direct readout of total material thickness are simple to use and require little operator adjustment following initial setup. Conventional flaw detectors display a pattern of sound reflections referred to as an A-scan, which will change as material conditions change and which is interpreted by a trained operator to identify anomalies. Ultrasonic thickness measurements are particularly useful with fiberglass mat/roving lay-ups where variations in layer thickness make it necessary to periodically check thickness during manufacturing, and crack detection is of particular importance in the marine surveying industry to check for possible hidden hull damage in older boats.