Vincent Bott, summer semester 2018


Air-coupled non-destructive testing (AC-NDT) is an alternative to the conventional contact non-destructive testing. The main difference between these both approaches lies in the coupling of a test specimen to the associated testing system. An inspection method without any need of physical coupling to the tested material is called AC-NDT. In the context of NDT the term “air-coupled” is used to describe contactless testing. Many applications can be realized with Unmanned Aerial Vehicles in Non-Destructive Testing.

AC-NDT motivates itself through its high potential of investigating materials which have sensitive surfaces (e.g. high porosity) or such materials that posses a lack of accessibility for sensors which are used in contact testing.

Applications of Air-coupled NDT Systems

To give an idea about the possible use of AC-NDT systems, the following lists demonstrate some of the most common applications in mechanical engineering and civil engineering, respectively.

Applications in Mechanical Engineering:

Type of AC-NDT-SystemExamples in ApplicationWorking Principle
Air-coupled Ultrasonic Inspection [1]Analyzing sensitive composite materials (e.g. detection of interfaces or internal cracks, delamination testing etc.).Phased-Array Air-coupled Inspection [2].
Thermal Camera (active and passive Thermography) [3]Detection of delamination or inner structures close to the surface of composite materials (active thermography). A further application is process monitoring (passive thermography).Detection of thermal radiation (For active thermography an active source is necessary e.g. detection of thermal radiation induced by a halogen spotlight [3]).
Laser vibrometer Laser-vibrometerMeasurement of vibration frequencies of a material surface (e.g. surface vibration of a car body).Based on Doppler-Effect.

Comparison of the laser-beam reflected at the surface of an object and the original laser beam w.r.t. the phase shift [4].

Microphone (Acoustic Testing)Vibration analysis.Conversion of a recorded acoustic signal into an electronic signal.
Human eye (Visual Testing)Inspection of a welt.

figure 1: Infrared-Thermography

Applications in Civil Engineering

Type of AC-NDT-SystemExamples in ApplicationWorking Principle
RADARDetection and ranging of rebar layers in concrete. Collect information about higher depth structures of materials [5] [6].Measurement of electromagnetic waves reflected by materials with characteristic permittivity [6].
Thermal Camera (passive Thermography) [3]Analyzing thermal insulation of a building.Detection of thermal radiation emitted by the inspected object [3].
Laser-vibrometerVibration analysis (e.g. of a wind turbine tower)Based on Doppler-Effect.

Comparison of the laser-beam reflected at the surface of an object and the original laser beam w.r.t. the phase shift [4].

„PS200 Ferroscan“ [7]Detection of rebar in concrete.Use of a magnetic field to induce an electrical field into the rebar [8].
Human eye (Visual Testing)Crack testing on concrete.

Advantages and Disadvantages of Air-coupled NDT-Methods

Contact free testing of materials is one outstanding advantage that all air-coupled NDT-methods have in common. Nevertheless, there are some other advantages as well as disadvantages of the several testing methods, shortly described in the following paragraph.

Infrared Thermography

Advantages: In general, there are two different applications of Infrared Thermography. On the one hand, passive Thermography can be used to measure emitted thermal radiation of an object. On the other hand active Thermography describes the detection of thermal radiation (previously) induced by an “active“ source [3]. For the measurement there is no need for a computer. The result can be directly observed at the screen of the infrared camera. The passive Thermography is very fast [3]. A big area can be investigated.

Disadvantages: Unfortunately, professional equipment is very expensive. The interpretation of the images requires experience [3]. Only the heat flux near to the surface of a component can be analyzed [9]. For high quality pictures, Lock-In Thermography might also take a lot of time.

Laser Vibrometry

figure 2: Possible applications of a laser vibrometer

above: Testing vibration frequency of a wind turbine tower

below: Testing vibration frequency of a tuning fork

Advantages: The air-coupled system is extremely sensitive and can be used to examine a relatively large spectrum of frequencies. Laser vibrometry can be used in a huge range of applications. It is important to note that exterior electromagnetic waves do not have an impact on the testing method, i.e. the testing method is somehow resistent against environmental effects.

Disadvantages: The Laser-vibrometer is a very expensive tool. The surface of the examined material must be sufficiently reflective. Point measurement only represents „one point“ of the whole surface [4]. An alternative contact NDT method is the so called sound emission analysis.

Acoustic Testing

Advantages: There exists a high probability to find the location of the source of an acoustic signal. The necessary equipment is relatively cheap and measurements can be done very fast [10].

Disadvantages: Measurements can be disturbed by noise coming from the environment. The source of the acoustic wave could be correlated with a damage in the material. The penetration depth strongly depends on material properties. Geometric effects can distort signals and therefore destroy the measurement.

RADAR

Advantages: Radar is a very fast method to scan and analyze ground and wall. The high penetration depth is a further aspect that plays an important role in civil engineering [5].

Disadvantages: The resultant measurements require high education of the inspector to interpret them correctly [5]. RADAR technique is based on electromagnetic waves. The waves reflected due to a high contrast in impedance of materials are received by the RADAR-System and transformed into a picture. As a consequence, wet surfaces of materials can disturb the transmitted waves.

„PS200 Ferroscan“

Advantages: Possibility of fast localizing of rebar by using a template [7]. Large areas can be scanned.

Disadvantages: The surface of a given material has to be flat (at least less curvature). The Ferroscan is a very expensive tool.

Visual Testing

Advantages: The visual testing is a cheap and fast method for simple applications. It requires no extra equipment.

Disadvantages: Only imperfections at the surface of a material can be found.

Literature

  1. Sonotec, "Sonotec". www.sonotec.de. [Date: 29.08.2018]
  2. Sonotec, "Sonotec ultrasonic incpestions" www.sonotec.de. [Date: 29.08.2018]
  3. Roth, Anita: "Thermography". www.wiki.zfp.tum.de, 2015. [Date: 29.08.2018]
  4. J.G.: "Laser vibrometer". www.wiki.zfp.tum.de, 2015. [Date: 29.08.2018]
  5. Grosse, C.: Grundlagen der zerstörungsfreien Prüfung. Skript, Lehrstuhl für Zerstörungsfreie Prüfung der TU München. München 2015, p.91-104.
  6. Moser, Thomas: "RADAR". www.wiki.zfp.tum.de, 2015. [Date: 29.08.2018]
  7. Hilti Corporation: "PS200 Ferroscan". www.hilti.de, 2009. [Date: 29.08.2018]
  8. Hilti Corporation: Bedienungsanleitung PS200 Ferrorscan, Liechtenstein, 2009
  9. Grosse, C.: Grundlagen der zerstörungsfreien Prüfung. Skript, Lehrstuhl für Zerstörungsfreie Prüfung der TU München. München , p.105-111.
  10. ZfP-Wiki: "local acoustic resonance spectroscopy". www.wiki.zfp.tum.de, 2012. [Date: 29.08.2018]

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