Nondestructive analysis with THz imaging


Terahertz waves are extremely promising for nondestructive analysis of faults in advanced materials, including many types of composites and layered structures.

Polar substances such as water show a strong THz absorption, conductive materials are likewise absorptive and reflecting. In contrast, non-polar substances such as dust, plastic, leather, wood, and fabrics can be transmissive to THz waves. This indicates that THz imaging can be employed for quality control and analysis of internal structure of a wide range of technologically important materials. The high sensitivity to water makes THz imaging most suitable for dry materials. THz imaging can also be employed to identify fiber orientation in composites. In fiber-reinforced plastics, the orientation of the fibers inside the host medium can result in a birefringent behavior of the composite material for sub-mm waves.

Due to the wavelength of THz waves the lateral resolution of a THz imaging system can be better than 1 mm, and the depth resolution can be better than 50 µm due to the short duration of the probe signals. The imaging depth depends on the absorbing properties of the material under test, but several cm is possible.

If you are interested in learning more about THz nondestructive analysis of your composite materials, don’t hesitate to contact us!

A laboratory-type THz imaging setup is shown in Figure 1.

Figure 1: Experimental setup for THz reflection imaging.

In this setup the probe signal is an ultrashort THz pulse, with a duration of less than 0.5 ps, corresponding to a raw depth resolution of 150 µm. This signal is guided to the object plane and reflected from the sample. The temporal shape of the reflected signal is detected with high accuracy in the THz detector, and stored for analysis. The temporal shape of the reflected signal contains the required information about the internal structure of the sample. Each interface within the sample leads to a small reflection, and the depth of the interface in the sample is encoded on the signal as the delay of the particular reflection signature.

An few illustrative examples are given below, recorded by student Torben Kristensen as part of his Master project during 2010. Figure 3 shows two samples prepared for reflective THz imaging. The letters “DTU” and “Fotonik” were cut out of thin polymer foil (the “T” was cut out of aluminium foil) and sandwiched between layers of polystyrene foam.

Figure 2: Samples for 3D THz imaging of hidden objects in strongly scattering media. First sample: Letters “DTU” hidden in a sandwich of polystyrene foam. Second sample: Letters “Fotonik” sandwiched on top of each other between polystyrene foam layers.

Figure 3 shows that THz imaging clearly reveals the position and shape of each letter in the sample. In particular the curvature of the letters is clearly visible, and the shadow cast by the metallic letter “T” is seen on the substrate. A weak imprint of the the two other letters is seen on the substrate, due to the time delay and attenuation of the THz signal transmitted through the letters above the substrate.

Figure 3: THz 3D image of the letters “DTU” placed between styrofoam plates which are strongly scattering for infrared and visible light. The letters “D” and “U” are made of plastic, the letter “T” is cut out of aluminium foil.

Figure 4 shows the 3D THz image of the letters “Fotonik” stapled on top of each other. The depth resolution is sufficient to resolve each letter, and the signal strength is sufficient to resolve all seven interfaces between the letters and the polystyrene sheets, thus revealing a 3D imagery of the internal structure of the layered sample.

Figure 4: THz 3D image of the letters “Fotonik” stabled on top of each other, sandwiched between compact styrofoam layers.

Finally, Figure 5 shows an example of identification of delaminations at the glued interface between a woven polymer layer (thickness 2 cm) and a steel plate. The THz image to the left shows a sample with correct gluing, and thus no delaminations. The central THz image is recorded on a sample prepared with delaminations. The delaminated areas are clearly visible in the THz image. The time-of-fligh traces (right) recorded along the dashed line in the central image, reveals the thickness of the delamination, 530 µm.

Figure 5: THz imaging of delaminations in the interface between a 2-cm thick polymer layer and a steel plate. Left: No defects present. Middle: Defects present. Dashed line indicates 1D scan position. Right: 1D time-of-flight signals recorded along the dashed line, revealing the precise thickness of the delamination.

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© 2012 Our research is carried out at DTU Fotonik - Department of Photonics Engineering, Technical University of Denmark. Suffusion theme by Sayontan Sinha