Anna Margareta Bär, 07.2011
Using electromagnetic waves, it is possible to obtain information about the internal structure of a body or to locate/detect hidden components. As the specimen would not be damaged by such waves, they play a major role in non-destructive testing. Key applications are microwave-moisture measurement, various x-ray methods and the infrared thermography. To achieve precise and efficient results possible, it makes sense to combine the methods with one another. In timber construction, it is particularly important to detect internal damage caused by cracks, insect infestation, moisture or even rot and above all to determine the extent of the damage.
Properties of electromagnetic waves
Electromagnetic waves are, as the name implies, oscillations of electric and magnetic fields that are coupled. The oscillations of the electric and magnetic fields are perpendicular to each other. The known as wave-particle duality applies for the electromagnetic wave. This means that electromagnetic waves display the properties not only of waves but also of particles. The frequency f of the wave corresponds with the energy h * f of the particles (photons).
An additional characteristic of these waves is that in contrast to sound waves, they do not need a medium to propagate. However, a very important property of non-destructive testing is that electromagnetic waves can penetrate non-conductive objects. The penetration depth depends on the conductivity or the dielectric properties of the test material and the frequency of the wave. It generally applies, that the lower the frequency, the higher the penetration depth. (One exception is high-energy radiation such as X-ray radiation for example, in which the penetration depth is determined by the quantum physical transitions of electron shells.) The resolution of the test method in every case lies within the range of the wavelength, which according to λ = c / f is inversely proportional to the frequency . The separation of electromagnetic waves is determined by the frequency within the frequency spectrum.
Test methods with electromagnetic waves
In order to test the components, they have to be irradiated with electromagnetic waves. Through interactions of the waves with the material, its density, moisture and also (in)homogeneities can be detected using known specifications. Waves directed at the body are partially reflected, absorbed and transmitted. The reflection occurs at material borders and absorption occurs inside the material. The intensity of these effects depend on the dielectric properties of the materials to be tested.
Either reflected waves (reflective measurement) or transmitted waves (transmission measurement) can be detected. The wave component reflected from the back wall or from the intra-material transitions arrives later and is probably diphase-shifted at the receiver. If the propagation velocity inside the material is known, the depth of material boundaries (e.g. of internal threaded rods or flaws) can be measured using a run-time measurement .
If the complete amplitude response is measured, a frequency spectrum can be determined using the Fourier transformation. On the basis of this, more precise conclusions can be drawn about the structure of the specimen.
Using suitable programmes, measurement data are rendered into an image or a diagram and can thus be more easily interpreted.
Microwaves and radar waves oscillate in the frequency range of about 0.3 to 30 GHz (= 109 Hz). The wavelength and therefore the measurement accuracy is within the range of centimetres. Besides analysis of the structural composition of a component, microwave measurement is also used for the detection of moisture in timber construction.
For moisture measurement using microwaves, mobile hand-held devices have already been developed. Following application of a measuring head or a mechanically guided measurement equipment, the component is scanned row by row or column by column (hf sensor GmbH, 2008). Differences in moisture are indicated by different conductivity inside the wood. A penetration depth of up to 30 cm can be measured. With newer devices it is possible to scan an area with a single measurement run and store several thousand measurement results inside the device. The data are transferred to a computer and evaluated with suitable software. The results can even be depicted multidimensionally (hf sensor GmbH, 2010).
In addition to mobile systems there are also stationary systems. These are applied in production processes in which they monitor the structure and moisture for example in the production of silencers.
The measurement device is also guided over the component to carry out a structural analysis. Radargramms are produced with travel-time measurement. Internals such as internal threads for example, typically display a hyperbola ).
The advantage of microwave-based systems is that mobile systems are relatively handy and the data can be transferred to a laptop on-site (in cases where a partial evaluation has not already been done in the device itself). Thus measurement errors and calibration errors can be detected early. However, particularly in the case of radargramms, a lot of experience is required to ensure a correct interpretation.
The field of THz is still in the development phase. Following the breakthrough of passive operating THz body scanners at security checkpoints, the research has accelerated. Perhaps in a few years the research and technology will have come so far that the technology can be applied to the field of timber construction. Due to the shielding effect of water, it is questionable if the systems can be applied directly to the structure .
Infrared radiation is within the range of about 1013 to 1014 Hz. Infrared is used in civil engineering in infrared (IR)thermography. In particular, it is used in building physics for the detection of thermal bridges or generally for structural analysis of plaster, that is applied underneath thin layers of hidden components. However, sub-surface defects can also be detected .
Infrared thermography is principally a non-contact thermal measurement. In contrast to radar and X-ray measurements, it can be carried out passively. The infrared proportion of the thermal radiation is detected with a thermographic camera. Due to their different reflections, various materials can be detected in the component. If the temperature differences are too small, the specimen is actively heated and the process of heating as well as of cooling down is assessed. Thus, contrasts are stronger and can be easily measured .
The major advantage of IR thermography is that it can be relatively easily used for large area. Due to the low penetration depth of about 10 cm it is only useful for flaw detection in thicker components if it is clear that the defect is sub-surface. However, due to the low wavelength, a high resolution can be achieved.
X-rays are known from medicine and are in the range of about 1016 to 1019 Hz. Their wavelength is in the nanometer range and thus so short that they penetrate the molecular structure and thus can reach relatively high penetration depths of 50 cm . In timber construction it can be used for the detection of foreign objects, branches or flaws. In this case, access to the object from both sides is necessary as the testing is only possible in radiography.
The ionising effect of X-rays is problematic. Due to their high energy, X-rays can change molecular structures and thus are harmful. This has a greater influence on the measurement of components as high-energy radiation is necessary to make deeper measurements. Thus, for the use of X-ray tubes the adherence to the radiation protection regulations is necessary.
Larger stationary devices are used in the production process of timber construction. For example, sawmills radiograph the wood for foreign materials before sawing to prevent damage of expensive machines by hard inclusions such as stones or metal pieces, for example, nails . Here it is easy to install the devices in a specially shielded room so that personnel are protected from the radiation. Medium density can also be determined using the diameter and the amount of branching And therefore radiography can be used in the timber industry for mechanical sorting and for quality assurance.
Handy and mobile X-ray flash tubes are used for examination of an existing structure . The X-ray flash tube measures differently to continuously radiating tubes, with constant radiation pulses. The X-ray flash tube has a lower performance and the energy spectrum shows lower energies than systems operating with direct current. This is not sufficient for the exposure of common lead sheets Thus more sensitive detector foils e.g. based on TFT  are used. As a result, components can be tested on-site on rot, infestation of insects and flaws.
In the meantime, in the course of laboratory tests there has been some research on reflectometry using fast neutrons. With reflectometry, high-resolution images can be taken and even the distribution of glue and resin pockets can be very well detected. For the time being, these measurements will remain comparison measurements as at present neutrons can only be generated in a nuclear reactor.
Radiography as well as radar and IR thermography is an important non-destructive testing method. Due to the harmful radiation and along with it, the regulations and the necessity of access from both sides, the priority is now on other methods .
Summary / sensible combination of non-destructive testing methods
In most cases, it is appropriate to combine different testing methods. This ensures results can be verified and problematic cases interpreted more easily. Although evaluation software and testing devices are getting better, there are still many influencing factors affecting measurement which are often only interpretable with considerable experience. Particularly with wood and its inhomogeneous structure, an unambiguous interpretation of the photograms is difficult. It needs a lot of practice and background knowledge.
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