Ultrasound is commonly used to test materials for nondestructive evaluation of defects and properties. Though the value of ultrasound in industry is obvious, its modus operandi is severely limited by the physical coupling of ultrasonic transducers with the test medium by applying liquids gels, grease, glycerin water, etc. For the past 20 years, few attempts have been made to change this mode of coupling to non-contact mode. A variety of ultrasound generation mechanisms has been developed by utilizing piezoelectric, capacitive, laser, and electromagnetic phenomena. In this informal note we describe the evidence of phenomenally high air/gas transduction piezoelectric transducers in the 100kHz to 5MHz frequency range. We also introduce an ultrasonic non-contact analyzer, characterized by >150dB dynamic range and ±1ns time-of-flight accuracy. Based on the combination of the new transducers and the analytical system, feasibility of several Non-Contact Ultrasound (NCU) applications and measurements are also presented.

NCU TRANSDUCERS (International and U.S. patents pending)

FREQUENCIES:100 kHz, 200 kHz, 500 kHz, 1.0 MHz, 1.5 MHz, 2.0 MHz, 3.0 MHz, 4.0 MHz, and 5 MHz. Higher frequencies transducers, such as 7 to 10MHz have also been produced, however, they have not been seriously investigated for NCU materials characterization.

BANDWIDTHS: Narrowband (typically, 30 to 40% of bandwidth center frequency @ -6dB). Broadband (typically 50 to 80% of bandwidth center frequency 2-6 dB). Bandwidths are directly related to the active area dimensions at a given frequency.

DIMENSIONS: 1 mm. 1.5 mm, 2.0 mm, 3.0 mm, 6.3 mm, 12.5 mm, 19.0 mm, 25 mm, 38 mm, 50 mm, 76 mm, and 100 mm diameters. Squares and rectangular shapes have also been produced.

FOCUSING: Focal lengths yielding Numerical Apertures of 0.5, 0.25, 0.125, and 0.06 have been successfully produced for a wide range of NCU transducers.

ENVIRONMENT OF OPERATION: Temperature, -20°C to 70°C. Humidity: <90%RH. Air or other transmission operating gas pressures: From RTP to at least up to 80 bars.

EXAMPLES OF TIME, FREQUENCY, AND SENSITIVITY OF NCU TRANSDUCERS, Figures 1, 2, 3, and 4


Fig 1: NCT-55NB (nominally 500kHz and 12.5mm active diameter) separated by 10mm ambient air in direct transmission mode. Transmitter excitation: One burst of 16V sine wave. Receiving transducer amplification: NONE Peak F: 500kHz. BW: 150kHz. Sensitivity: -50dB.

Fig 2: NCT-510WB (nominally 1MHz and 12.5mm active diameter) separated by 10mm ambient air in direct transmission mode. Transmitter excitation: : One burst of 16Vsine wave. Receiving transducer amplification:NONE. Peak F: 0.96MHz. BW: 0.65MHz. Sensitivity: -55dB.

Fig 3: NCT-520WB (nominally 2MHz and 12.5mm active diameter) separated by 5mm ambient air in direct transmission mode. Transmitter excitation: One burst of 16V sine wave. Receiving transducer amplification: NONE. Peak F: 2.0MHz. BW: 1.0MHz. Sensitivity: -60dB.

Fig 4: NCT-540WB (nominally 4MHz and 12.5mm active diameter) separated by 5mm Ambient air in direct transmission mode.Transmitter excitation: One burst of 16V sine wave. Receiving transducer amplification NONE. Peak F: 3.4MHz. BW: 1.8MHz Sensitivity: -66dB.

PROOF OF HIGH FREQUENCY TRANSDUCTION IN AIR – see Figures 5, 6, 7, and 8.
IMPORTANT NOTE: The function of this exercise is to demonstrate NCU propagation through complex materials with very low energy excitation of transducers.

Fig 3a: 500kHz transmission in 12mm multilayered PHENOLIC composite. T and R are approx. 10mm away from material surfaces. Excitation: One burst 16V sine. Ampl: 70dB

Fig 4a: 500kHz transmission in 12mm multi-layered PHENOLIC composite. T and R are approx. 10mm away from material surfaces Excitation 200V spike. Ampl: 70dB.

Fig 5: 1MHz transmission in 5mm multilayered GFRP composite. T and R are approx. 5mm away from material surfaces. Excitation: One burst 16V sine. Ampl: 70dB.

Fig 6: 1MHz transmission in 5mm multi-layered GFRP composite. T and R are approx. 5mm away from material surfaces Excitation 200V spike. Ampl: 70dB.

Fig 7: 2MHz transmission in 1.6mm multilayered GFRP composite. T and R are approx. 4mm away from material surfaces. Excitation: One burst 16V sine. Ampl: 70dB.

Fig 8: 2MHz transmission in 1.6mm multilayered GFRP composite. T and R are Approx. 4mm away from material surfaces Excitation 200V spike. Ampl: 70dB

NON-CONTACT ANALYZER, THE NCA 1000 (U.S. patent pending)

The NCA 1000 is also a unique development in ultrasound. This system is characterized by >150dB dynamic range and nano-second time-of-flight measurement accuracy even under ambient environments. In its full two channel – four path – operation this system provides all the information about the medium that ultrasound is capable of measuring. The NCA 1000 provides the following significant material information and is characterized by unique features.

1. Simultaneous measurement and display of test material thickness and velocity (density, elastic properties, etc.).
2. Measurement of true transmissivity (transmission co-efficient) in materials for texture, phase analysis, and defect detection.
3. FFT for frequency-dependence of ultrasound attenuation for texture, microstructure, and other subtle detections in materials.
4. Phase analysis for even more subtle detection in materials.
5. Characterization of material surfaces by measuring the reflectivity or by ultrasonic spectroscopy.
6. The NCA 1000 provides two trend plots by the availability of computer-generated x-axes, which can be synchronized with the motion of the test material or of the transducers. By doing so one can plot characteristics such as thickness, velocity, time-of-flight, attenuation, etc. as a function of different locations on the test material. This can be done for either internal, or its external features of the test medium. It is obvious that this system already has a built-in B-scanning mechanism.
7. The NCA 1000 is fully equipped with Statistical Quality Control Module, which facilitates numeric measurements of all measured parameters.
8. The NCA 1000 is also equipped with C-scan software and can be integrated with external transducer or material translational systems.
9. The NCA 1000 can be operated in its popular chirp synthesis mode, as well as in the standard pulser mode, also provided with the system.
10. Besides the operation of the transducers and the NCA 1000 in direct transmission mode, they can also be used from the same side to generate and measure both bulk and surface characteristics. Same side mode has been successfully applied to polymers, composites, and even metals!
11. Besides the NCU operation, the NCA 1000 can also be used in direct contact or in immersion modes. In the later, its time-of-flight accuracy is better than +/-100ps!
12. While the NCA 1000 is an extraordinarily sophisticated system ever developed in the field of ultrasound, it is still very easy to operate and can be configured for on-line applications, as well.
13. NCU transducers and the NCA 1000 have been successfully and gainfully applied for the analysis of wide range of materials and for wide range of applications. These are: Green ceramics, powder metals, and composites for velocity/density relationships and for defect detection in the early stages of materials formation; porous ceramics, foams, etc. for porosity and thickness control; early stage polymer, composites, prepregs and analysis for heterogeneity disbonding, etc.; multi-layered similar and dissimilar materials for bond/disbond detection; Rubbers and tires for multiple applications; natural and processed food for multiple applications; lumber and construction materials for multiple applications; and even for metals such as, aluminum, steel, etc. for thickness and corrosion measurements!

APPLICATIONS AND MEASUREMENT EXAMPLES

HONEYCOMB STRUCTURES, Fig. 9, 10, and 11


Fig 9: NCA 1000 trend plot showing the direct transmission through a GFRP composite bonded honeycomb structure at various points. Regions showing the sharp drop in transmission are indicative of defects such as, delaminations. Transducers: NCT 55 transducers, nominally 500kHz and 12.5mm active area diameter. T and R are separated by ~40mm ambient air from the material surfaces.

Fig 10: NCA 1000 trend plot showing the T-R reflection – same side -- through a GFRP composite bonded honeycomb structure at various points. Regions showing the sharp drop in transmission are indicative of defects such as, delaminations. Transducers: NCT 55 transducers, nominally 500 kHz and 12.5 mm active area diameter. T and R are separated by ~10mm ambient air from the test material surface.

Fig 11: NCA 1000 trend plot showing the direct transmission through an ALUMINUM PLATE bonded ALUMINUM honeycomb structure at various points. Regions showing the sharp drop in transmission are indicative of defects such as, delaminations. Transducers: NCT 55 transducers, nominally 500kHz and 12.5mm active area diameter. T and R are separated by ~40mm ambient air from the material surfaces.

FIBER RE-ENFORCED PLASTIC COMPOSITES, Figures 12 and 13.


Fig 12: Transmission (IR) image of a mild-impact Damaged 6.4mm multi-layered GFRP composite. Scan area shown: 38x38mm.Transducers NCT-510 with 1mm aperture.

Fig 13: First thickness reflection image of Fig. 12 specimen. Scan area shown: 38x38mm. Transducers NCT-510 with 1mm aperture


GREEN POWDER METALS COMPACTS, Figures 14 and 15.


Fig 14: IR Map (dB) - Transmission (IR) image of a 14mm thick green powder iron compact. Scan area: 50x50mm. NCT-55 transducer with 2mm aperture.

Fig 15: VELOCITY map (m/s)- Velocity image of Fig 14 specimen. Scan area: 50x50mm NCT-55 with 2mm aperture.


THICKNESS AND VELOCITY MEASUREMENT, Fig. 16.

Fig 16: Test Material Example: 22mm Porous Sintered Ceramic (NZP) Frequency: 1.0MHz. Mode of Operation: Transmission

CONCLUSIONS

In this paper we have provided a documentary and applications-oriented evidence of extremely high transduction piezoelectric transducers for non-contact ultrasonic evaluation of materials. A similar introduction has been given for a multi-functional non-contact analyzer.

Non-contact ultrasound mode is among the most significant developments for characterization and analysis of materials. Though we have provided selected examples of its applications, there is no doubt that the users of this technology will further enhance the causes of materials quality and process control in our increasingly complex world.

© NDT.net - info@ndt.net

|Top|