Fiber waviness in laminated composite material is introduced during manufacture because of uneven curing, resin
shrinkage, or ply buckling caused by bending the composite lay-up into its final shape prior to curing. The resulting
waviness has a detrimental effect on mechanical properties, therefore this condition is important to detect and
characterize. Ultrasonic characterization methods are difficult to interpret because elastic wave propagation is highly
dependent on ply orientation and material stresses. By comparison, the pulsed terahertz response of the composite is
shown to provide clear indications of the fiber waviness. Pulsed Terahertz NDE is an electromagnetic inspection method
that operates in the frequency range between 300 GHz and 3 THz. Its propagation is influenced by refractive index
variations and interfaces. This work applies pulsed Terahertz NDE to the inspection of a thick composite beam with fiber
waviness. The sample is a laminated glass composite material approximately 15mm thick with a 90-degree bend.
Terahertz response from the planar section, away from the bend, is indicative of a homogeneous material with no major
reflections from internal plies, while the multiple reflections at the bend area correspond to the fiber waviness. Results of
these measurements are presented for the planar and bend areas.
Pulsed Terahertz NDE is being examined as a method to inspect for possible corrosion under Space Shuttle Tiles. Other
methods such as ultrasonics, infrared, eddy current and microwave technologies have demonstrable shortcomings for
tile NDE. This work applies Terahertz NDE, in the frequency range between 50 GHz and 1 THz, for the inspection of
manufactured corrosion samples. The samples consist of induced corrosion spots that range in diameter (2.54 to 15.2
mm) and depth (0.036 to 0.787 mm) in an aluminum substrate material covered with tiles. Results of these
measurements are presented for known corrosion flaws both covered and uncovered and for blind tests with unknown
corrosion flaws covered with attached tiles. The Terahertz NDE system is shown to detect all artificially manufactured
corrosion regions under a Shuttle tile with a depth greater than 0.13 mm.
Metallic surface roughness in a nominally smooth surface is a potential indication of material degradation or damage. When the surface is coated or covered with an opaque dielectric material, such as paint or insulation, then inspecting for surface changes becomes almost impossible. Terahertz NDE is a method capable of penetrating the coating and inspecting the metallic surface. The terahertz frequency regime is between 100 GHz and 10 THz and has a free space wavelength of 300 micrometers at 1 THz. Pulsed terahertz radiation, can be generated and detected using optical excitation of biased semiconductors with femtosecond laser pulses. The resulting time domain signal is 320 picoseconds in duration. In this application, samples are inspected with a commercial terahertz NDE system that scans the sample and generates a set of time-domain signals that are a function of the signal reflected from the metallic surface. Post processing is then performed in the time and frequency domains to generate C-scan type images that show scattering effects due to surface non-uniformity.
Terahertz NDE is being examined as a method to inspect the adhesive bond-line of Space Shuttle tiles for defects. Terahertz signals are generated and detected, using optical excitation of biased semiconductors with femtosecond laser pulses. Shuttle tile samples were manufactured with defects that included repair regions unbond regions, and other conditions that occur in Shuttle structures. These samples were inspected with a commercial terahertz NDE system that scanned a tile and generated a data set of RF signals. The signals were post processed to generate C-scan type images that are typically seen in ultrasonic NDE. To improve defect visualization the Hilbert-Huang Transform, a transform that decomposes a signal into oscillating components called intrinsic mode functions, was applied to test signals identified as being in and out of the defect regions and then on a complete data set. As expected with this transform, the results showed that the decomposed low-order modes correspond to signal noise while the high-order modes correspond to low frequency oscillations in the signal and mid-order modes correspond to local signal oscillations. The local oscillations compare well with various reflection interfaces and the defect locations in the original signal.
Wire integrity has become an area of concern to the aerospace community including DoD, NASA, FAA, and Industry. Over time and changing environmental conditions, wire insulation can become brittle and crack. The cracks expose the wire conductor and can be a source of equipment failure, short circuits, smoke, and fire. The technique of using the ultrasonic phase spectrum to extract material properties of the insulation is being examined. Ultrasonic guided waves will propagate in both the wire conductor and insulation. Assuming the condition of the conductor remains constant then the stiffness of the insulator can be inferred by measuring the ultrasonic guided wave velocity. In the phase spectrum method the guided wave velocity is obtained by transforming the time base waveform to the frequency domain and taking the phase difference between two waveforms. The result can then be correlated with a database, derived by numerical model calculations, to extract material properties of the wire insulator. Initial laboratory tests were performed on a simple model consisting of a solid cylinder and then a solid cylinder with a polymer coating. For each sample the flexural mode waveform was identified. That waveform was then transformed to the frequency domain and a phase spectrum was calculated from a pair of waveforms. Experimental results on the simple model compared well to numerical calculations. Further tests were conducted on aircraft or mil-spec wire samples, to see if changes in wire insulation stiffness can be extracted using the phase spectrum technique.
Aging wiring has become a critical issue to DoD, NASA, FAA, and Industry. The problem is that insulation on environmentally aged wire becomes brittle and cracks. This exposes the underlying conductive wire to the potential for short circuits and fire. The difficulty is that techniques to monitor aging wire problems focus on applying electrical sensing techniques that are not very sensitive to the wire insulation. Thus, the development of methods to quantify and monitor aging wire insulation is highly warranted. Measurement of wire insulation stiffness by ultrasonic guided waves is being examined. Initial laboratory tests were performed on a simple model consisting of a solid cylinder and then a solid cylinder with a polymer coating. Experimental measurements showed that the lowest order axisymmetric mode may be sensitive to stiffness changes in the wire insulation. To test this theory, mil-spec wire samples MIL-W-81381, MIL-W-22759/34, and MIL-W-22759/87 (typically found in aircraft) were heat-damaged in an oven, in a range of heating conditions. The samples were 12, 16, and 20 gauge and the heat-damage introduced material changes in the wire-insulation that made the originally flexible insulation brittle and darker in color. Axisymmetric mode phase-velocity increased for the samples that were exposed to heat for longer duration. For example, the phase velocity in the 20-gauge MIL-W-22759/34 wire changed from a baseline value of 2790m/s to 3280m/s and 3530m/s for one-hour exposures to 349 degree(s)C and 399 degree(s)C, respectively. Although the heat-damage conditions are not the same as environmental aging, we believe that with further development and refinements, the ultrasonic guided waves can be used to inspect wire-insulation for detrimental environmental aging conditions.
Although laser ultrasound is a promising technique for many applications, the signal-to-noise ratio of laser ultrasound is poorer than for contact ultrasonics. In this paper we present some of our current work in optimizing the excitation of laser ultrasound in composite materials. We are characterizing laser generated ultrasound as a function of laser parameters, composite material properties, and their interaction. Experimental results for a number of different laser wavelengths and pulse widths, for several materials, are presented.