4. Ultrasonic Testing Techniques for Nondestructive Evaluation of Fiber-Reinforced Composite Structures

The recent years have been characterized by significant breakthroughs in the development and utilization of ultrasonic-based NDT techniques for the detection and characterization of various types of flaws in a wide range of fiber-reinforced composite materials. However, ultrasonic-based NDT techniques have also been the subject of intense criticisms due to the limitations that they inflict on the plants and the cost of the maintenance downtime while inspecting in-service composites structures and/or components. Attempting to address these challenges, major modifications have been implemented including the improvement of the methods of sending ultrasonic waves into the test structure (e.g., by contact between ultrasonic probes and test structure, immersion or ‘water delay line’ testing, air-coupled ultrasonic, laser ultrasonic testing, electromagnetic acoustic transducers, etc.). Each of these approaches presents its own advantages and disadvantages depending on the application, size of the test structure, and accessibility (i.e., access to one side or both sides of the test structure). Typically, localized or point testing can be achieved using water, gel, or grease coupling but achieving and maintaining constant coupling via these substances does add considerable time to the successful completion. Although an air-coupled ultrasonic system can be applied while the test structure remains in use and does not require additional acoustic couplants, overcoming the high impedance mismatch between the transducer/air and subsequent air/component interfaces is not a trivial success, and requires several modifications. Apart from the methodologies of sending ultrasonic waves into the test structures, additional improvements include the speed of the scan, improved spatial resolution, and advanced flaw detection capabilities of ultrasonic systems that are constantly being achieved, which prompted the design of fully portable systems that can be used in the field even in structures with complex geometries where accessibility is perceived to be challenging. Perhaps one of the most interesting advances is the recent development of the FlawInspecta system, which features high-speed ultrasonic array imaging fixtures that use a relatively low-cost ultrasonic array driver platform for operation. The FlawInspecta system also allows for the simultaneous scanning of a large area and includes additional fixtures to help users to overcome the problems of coupling. In general, ultrasonic NDT systems remain excellent tools for use in assembly lines during the manufacturing process of composite structures to ensure high-quality structures are manufactured and guaranteed their predicted life span while in-service. Nevertheless, a few challenges such as the difficulty of set up makes the system difficult to operate by the general NDT practitioners and requires a high level of skills and/or training for operators to be able to accurately scan the composite and interpret the test results. Although the capabilities of ultrasonic-based inspection techniques have been demonstrated at certain frequencies, studies employing appropriately-sized deeply buried flaws are still warranted to determine the reliability of use in thick fiber-reinforced composite structures. To date, significant advancements in ultrasonic inspection are still limited by the lack of sufficient lower-frequency, higher power evaluation systems which are likely to be overcome soon with the constantly observed cost reduction of phased array ultrasonic probes.