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Anna Margareta Bär, 07.2011

Artikel auf Deutsch


Impact-echo (IE) is a non-destructive, acoustic method that has been used in civil engineering since the mid-1980s. Currently, it is used in solid masonry construction for building component thickness measurement, and locating structural elements and flaws. As in ultrasonic detection, (multiple) reflections of waves caused by impact-echo with a short-duration mechanical impact at material margins are measured. It is technically a relatively simple method that requires, as with almost all types of non-destructive material testing, some expertise in analysing the measurement results. [1] [2]

Testing procedure

The impact-echo method is based on the measurement of acoustic waves. These waves are actively generated by a short mechanical impact, typically with a small metal sphere (impactor, see 1.2) at the component surface close to a detection sensor. The waves propagate into the component and are reflected at external surfaces of materials with different acoustic impedances for example, at the rear wall of a component, at material changes or flaws. The reflections of the sound wave i.e. its oscillation amplitudes are recorded on the time axis (see Fig. 1.2, A-diagram) and digitalised (see Fig. 1.1). [3]

The frequency spectrum (see Fig. 1.2) of this measured signal can be determined using the Fourier-Transformation. Multiple reflections lead to relatively high signal strengths with individual frequencies which can be allocated to the depth levels of reflectors in the component in a performance-frequency diagram. The thickness i.e. the depth \begin{array}{l}d\end{array} is determined by the geometrical relationship \begin{array}{l}d = \frac{v}{2 * f}\end{array}, whereas \begin{array}{l}f\end{array} stands for frequency and \begin{array}{l}v\end{array} for velocity of sound inside the material.

For this, the sound propagation velocity must be known or as in 1.2, determined. It has proven effective to measure along a line or a grid. The individual results can then, depending on the measurement, be shown in tabular form or graphically and definitively assessed. [1] [2][4]

Fig. 1.1: Schematic illustration of the impact-echo method [3]

Von DGZfP-Merkblatt B11: Merkblatt über die Anwendung des Impakt-Echo-Verfahrens zur zerstörungsfreien Prüfung von Betonbauteilen über CC BY-ND http://www.ndt.net/article/dgzfp2012/papers/p6.pdf

Fig. 1.2: Time signal (left) and frequency spectrum (right) of an impact-echo measurement [3]

Von DGZfP-Merkblatt B11: Merkblatt über die Anwendung des Impakt-Echo-Verfahrens zur zerstörungsfreien Prüfung von Betonbauteilen über CC BY-ND http://www.ndt.net/article/dgzfp2012/papers/p6.pdf


Measurements and their assessment

The evaluation of the measurement results is task-dependent.

Measurement of component thickness of a homogeneous construction (e.g. a tunnel wall) should give a main reflection and therefore a frequency peak (see Fig. 1.3 left). This can be used to determine the position of the back wall i.e. the component thickness. Measurement grids are particularly advantageous in large-area measurements. Most are a grid of 40 - 80 cm (16 - 31 in) and only if flaws are expected is a smaller grid of 15 - 30 cm (6 - 12 in) used. <ref name="DGZFP" /> The individual thickness can then be depicted with a colour code or in greyscale (see Fig. 1.4).

A flaw/cavity or an object inside the component is shown, depending on its size, as a second frequency peak (with narrow horizontal dimension of the reflector) or as an individual peak instead of the back wall signal (see Fig. 1.3 right). One cannot therefore say whether there is material behind the flaw or object or not.    

Fig 1.4: Part of a diagram of thickness measurement of a tunnel wall. [3] Narrowing of the measurement grid at the top centre due to low component thickness

Von DGZfP-Merkblatt B11: Merkblatt über die Anwendung des Impakt-Echo-Verfahrens zur zerstörungsfreien Prüfung von Betonbauteilen über CC BY-ND http://bauwerksdiagnose2012.dgzfp.de/Portals/bwd2012/Dokumente/bb/p04_kufa_poster.pdf


Impactor and testing equipment

A small steel sphere is typically used as impactor (see Fig. 1.5 left). The smaller the sphere, i.e. the contact surface, the shorter the contact periods achieved and the sound wave consists of more high frequencies. This is particularly applicable for the detection of near-surface reflections. Relatively small reflections can also be detected. Larger impactors create correspondingly low frequencies which penetrate deeper into the component. As a result, the detectability and verification possibilities will be limited. To correctly determine the entire thickness of a component, it is useful to carry out several measurements with different impactors. [3]

With manual impacts, it requires practice to obtain a constant impact so that measurements can be made, though there are also devices that can ensure controlled release and guiding. These (electromechanical) impactors are often contained in a housing and already connected to the sensor so that release information can be transferred to the measurement computer (see Fig. 1.5). These devices have the disadvantage that they require a larger and flat surface, which means they have difficulties performing on curved surfaces.

Fig. 1.5: (left) measurement unit with impactor in the form of a metal sphere (manual), (right) combined measurement unit with integrated electromechanical impactor [3]

Von DGZfP-Merkblatt B11: Merkblatt über die Anwendung des Impakt-Echo-Verfahrens zur zerstörungsfreien Prüfung von Betonbauteilen über CC BY-ND http://www.ndt.net/article/dgzfp2012/papers/p6.pdf

Determination of the sound propagation velocity v of a material for the impact-echo method

There are several possibilities to determine the velocity of sound in the body to be measured. [3]

One possibility is to carry out an impact-echo measurement on a homogeneous site with known thickness and using the measured values for frequency, \begin{array}{l}f\end{array}, the back wall reflection and the thickness, \begin{array}{l}d\end{array}, to calculate the velocity of sound, \begin{array}{l}v\end{array}, using \begin{array}{l}v=2∗d∗f\end{array}.

If this is not possible, the velocity of sound can be determined using calibration blocks or drill-cores of the components. These should be shaped as a circular cylinder with an height h that is at least double the diameter. If this is not the case, a correction factor has to be factored in, otherwise the result will be distorted by wave conversions. The calibration of the specimen should be carried out under the same conditions as measurements on the component to be tested.

An additional possibility is the determination of the longitudinal wave velocity on the component’s surface. In this case, the measurement is either carried out with two sensors or a triggered impactor and a sensor, and the travel time of a wave between two definite measurement points is measured. As only the velocity of sound of the longitudinal wave is measured here, a correction factor has to be factored in. For the first two possibilities, it makes sense to carry out an additional measurement.

In exceptional cases, the velocity of sound can also be determined by empirical values. [3] [4]

Application in civil engineering

The impact-echo method is mainly used in solid masonry construction for thickness measurement of components accessible from a single side. Particularly in tunnel construction, it is used to check the thickness of the tunnel wall. Additionally, structural components such as pre-stressing tendons or cavities can be detected. Cavities that resulted from poor compression or other structural flaws can be detected as well as delaminations and interlocking disruptions. [4] [3]

Limitations of the method

Detection

Detection relates to the size of the frequency intervals and is device dependent. In general, detection increases with the depth of measurement. The thickness measurement uncertainty in components of 10 - 80 cm (4 - 31 in) is 2 - 5% and at least 1 cm (0.4 in). If the measurement of the distortion inside of a specimen is smaller than the wavelength, a confirmation is difficult. In addition, the lateral dimension \begin{array}{l}D\end{array} of the distortion should be larger than 1:3 in relation to its depth \begin{array}{l}d\end{array} (see Fig. 1.3 right).

\begin{array}{l}\frac{D}{d} > \frac{1}{3}\end{array}

Thickness measurements are relatively simple to carry out, whereas for example the detection of jacket tubes can only be carried out under good conditions and ultrasound measurement is recommended. [5]

Penetration depths

The penetration depth is dependent on the wavelength \begin{array}{l}λ\end{array} i.e. the velocity of sound \begin{array}{l}v\end{array} in the material and the frequency of the wave \begin{array}{l}f\end{array} as the wavelength is determined by \begin{array}{l}\lambda = \frac{v}{f}\end{array}. In concrete for example, the velocity of sound is about 4000 m/s (13123 fps), this results in a wavelength of 2000 to 100 mm (79 - 4 in) in a frequency range of 2 to 40 kHz. [3]

In general, the possible measurement depths increase with decreasing frequency. Under good measurement conditions and a suitable frequency, in concrete, a measurement depth of about 80 - 100 cm (31 - 39 in) can be achieved. As very short contact periods and thus high frequencies are difficult to generate, a minimum thickness of about 5 cm (2 in) is necessary.

Measurement obstacles

In the case of rough surfaces, the impacts are no longer or difficult to reproduce and the measurement will be distorted. Very rough surfaces are therefore smoothed beforehand. [3]

It is generally assumed that the reflector is the boundary surface of material to air. Air possesses a low acoustic impedance. For material transitions from low to high acoustic impedance, the equation of the geometrical relation (see Fig. 1) has to be adapted. For steel e.g. \begin{array}{l}d = \frac{v}{4f}\end{array}. However, this is only noticeable with thicker layers. With small obstacles such as reinforcement or similar, this can be ignored.

In the case of small diameter measurement objects, not only can back wall echoes and reflections of objects within the component affect the measurement, but also distorting reflections from the side walls. Thus objects close to the component’s boundaries are difficult to detect.

Advantages and disadvantages of the impact-echo method

Advantages

One advantage is that the impact-echo method is a relatively uncomplicated method, which can, if necessary, be automated. As no couplant is required, normally, the surface of the component will not be damaged.

The assessment of the A-diagram is quick, thus after each single measurement, the partial result can be checked for plausibility and if necessary, repeated at that point, external influences can be sought or the measurement grid reduced in the damaged area.

Another major advantage is the high penetration depth, thus large component thickness can be penetrated completely even though it is only accessible from one side. Additionally, metallic objects reflect less acoustic waves than electromechanical waves such as in radar, thus objects behind reinforcement are clearly visible.

Disadvantages

In the case of manual impact of the impactor, it could be that the transmitted waves differ slightly with each single impact. A little practice is required to ensure good, constant and reproducible results.

Due to the weak absorption of the wave, meaningful measurements from the outer sections cannot be determined due to reflections from the side walls. Test components must also have an appropriately large diameter due to the relatively large wavelength to be detected.

Summary

The impact-echo is a relatively simple and technically uncomplicated method, the evaluation of which requires specific expertise as with almost every non-destructive test method. It is particularly suited for the detection of cavities and thickness measurement. However, to reach a better detection, it is recommended to combine it with higher frequency test methods. Thus, also environmental influences on the measurement can be more easily identified and equalised.

It is important that measurements are carried out under the supervision of qualified personnel in order to avoid erroneous measurement and misinterpretation.

Literature

  1. Beutel, R.; Finck, F. et al.: Untersuchung der inneren Struktur einer Spannbetonbrücke mit Hilfe des Impact-Echo- und des Radar-Verfahrens. DGZfP-Berichtsband 94-CD, Plakat 9. Rostock, 2005.
  2. Wiggenhauser, H.: Impact-Echo. In: Bauphysikkalender 2004. S. 358 ff. Cziesielski, e.(Hrsg.), Berlin: Ernst & Sohn. Berlin, 2004.
  3. Merkblatt über die Anwendung des Impakt-Echo-Verfahrens zur zerstörungsfreien Prüfung von Betonbauteilen. DGZFP-IE, DGZfP. 2011.
  4. Große, C. U.: Impakt-Echo-Verfahren. In: Grundlagen der Zerstörungsfreien Prüfung. Arbeitsblätter im Rahmen der Vorlesung, S. 45 ff. Lehrstuhl für Zerstörungsfreie Prüfung der TU München. München, 2011.
  5. Taffe, A. et al.: Zerstörungsfreie Prüfverfahren im Bauwesen (ZfPBau). Filderstadt, 2010.