A VERSATILE ULTRASONIC MEASUREMENT SYSTEM FOR FLAW DETECTION AND MATERIAL PROPERTY CHARACTERIZATION IN COMPOSITE MATERIALS

There is an ever increasing interest in the use of polymer-matrix composite materials in high performance automotive, marine, and aerospace structures. Ultrasonic NDE is a particularly effective method of insuring that such materials are fit for their intended purpose. Specifically, ultrasonic methods are inherently appropriate for internal-flaw detection and material-property characterization. Although flaw detection is certainly important, the real measurement needs are in the area of quantifying material attributes that influence compressive and interlaminar shear strength: fiber volume, porosity content, stiffness, and others. Flaw detection and material-property characterization in composite materials are often inhibited by measurement challenges which cannot be met by traditional flaw detection instruments. For example, ultrasonic phase and group velocities cannot be reliably determined using broadband-pulse, phase-spectrum techniques in materials that exhibit high frequency-dependent attenuation. On the other hand, high power broadband pulses with high sub-MHz harmonic content are needed for detection of internal flaws, particularly in thick sections. Traditional flaw detectors are not well suited for this role because their pulse-generation and reception circuits are generally optimized for operation in the 1-10 MHz frequency region and have less than 30 dB dynamic range. To address the special needs of polymer composites, we have demonstrated a versatile, computer-controlled measurement system. The system can operate in the 50 kHz -5 MHz frequency region either as a high-powered flaw detector or as a sensitive, variable frequency vector voltmeter. In this paper we describe the principles of operation and functions of the new instrument, including some unique methods of improving signal-to-noise ratio and minimizing instrumentation errors. We also discuss the results of flaw-detection experiments in thick-section (50 mm) epoxy/glass specimens and compare velocity measurements obtained with broadband-pulse and narrowband phase sensitive techniques. Contributions from the National Institute of Standards and Technology are not subject to copyright.