Non-Destructive Evaluation (NDE), Non-Destructive Testing (NDT), Zerstörungsfreie Prüfung (ZfP), or Ensayos No Destructivos (END) identifies different methods to examine objects without affecting their functionality (meaning without damaging or destroying them during testing - in contrast to the normal destructive material testing). Therefore, an examination of each component is possible before initial operation or also in service. Several of those methods are known from medical diagnostics.

Non-Destructive evaluation methods can be differentiated in volume and surface testing methods. Surface testing methods are used for the detection of incontinuities at the surface or close to the surface. On the other hand, volume testing methods are used for the detection of incontinuities in the complete volume (in most cases volume testing methods are "blind" at the surface). For all those different methods different physical effects (like elastic, electromagnetic or "thermal" waves) are used at different wave lengths.

The non-destructive testing methods are key enablers for cutting edge

In the following the most common methods:

Ultrasonic Testing (UT): Usually a volume testing method where a probe introduces an elastic wave into the object under examination. At acoustic impedance changes the wave is reflected, diffracted or changed otherwise. This can be detected with a probe (this can be the same probe as the sending probe). Other excitation forms for elastic waves exist - like laser or EMAT (Electromagnetic Acoustic Transducers) - and other detection possibilities, like vibrometer. Usually short pulses are used for ultrasonic testing. Using probes with several sending and/or receiving elements, so-called Phased-Array probes, the sound beam can be pivoted, focused and shifted electronically. Ultrasonic testing is suited for automation. Using reconstruction methods (SAFT: Synthetic Aperture Focusing Technique or TFM: Total Focusing Method) a 3 dimensional image of the volume can be calculated, similar to computed tomography in radiographic testing.

Radiographic Testing (RT): Also a volume testing method which uses X-Rays or Gamma-Rays (both being electromagnetic waves) to penetrate the object under examination. Shadowing effects due to different materials in the object can be visualized using film or digital detectors. Just as in medical technology a mathematical method can be used which combines different viewing angles to generate a Computed Tomography (CT).

Visual Testing (VT): The surface testing method which should be conducted in parallel to any other method is visual testing. Meaning just take at least a "look" onto the component under examination. Mirrors, magnifying glasses, microscopes and similar tools shall be used if necessary. Using photogrammetry, which usually combines pictures from different viewing angles, a 3D dimensional measurement of the surface can be determined. Moreover, photogrammetry can be used for the documentation of a visual or penetrant test.

Penetrant Testing (PT): For the detection of cracks (at the surface) a visual test is mostly not the best joice as cracks are often hard to see - therefore different methods are necessary. For penetrant testing the capillary action of cracks is utilized. Meaning a liquid (a color) is put onto the surace and penetrates the cracks. By using developers, the cracks are visualized afterwards.

Magnetic Leakage Flux Testing (MT): Magnetic leakage flux testing is another surface crack detection method - however just for ferromagnetic objects. For those however it is (in general) far more sensitive compared to penetrant testing. During leakage flux testing the effect is used that magnetic field lines are bent by cracks and leave the object. The magnetic field is either introduced by using magnetic yokes or by inductive coupling (magnetic field penetration) or directly created in the object by current (electric current magnetization). The current is applied by conductive (galvanic) coupling into the object. The effect of cracks on the magnetic field can be detected by magnetic particles (dry or in suspension) or by eddy-current sensors.

Eddy Current Testing (ET): A method where eddy currents are generated inductively at the surface in the object under examination by using an excitation coil. In the depth the alternating current decays exponentially due to the skin effect. Due to the alternating current in the object a current is induced into a receiver coil (which can be the same as the excitation coil). By comparing amplitude and phase between excitation and receiving coil information can be gained about different effects in the object - like conductivity, distance from the object, cracks, magnetic permeability. Similar to ultrasonic testing several coils can be used parallel (Phased-Array) and also this method is fitted for automatization.

Thermal Testing / Thermography (TT): Infrared radiation (heat radiation) is usually visualized in passive thermography using a special camera. Just by doing so a lot can be learned about the object (in service). By using an active excitation thermography is becoming one of the most versatile non-destructive testing methods. The excitation can be conducted by current (Induction / Eddy-Current or Conduction Thermography), light (Flash / Pulse or Laser Thermography), acoustic waves (Acoustic / Ultrasonic Thermography) or heat (Hot Air Thermography). Depending on the excitation the method is fitted for the detection of cracks (close to the surface), delaminations or wall thickness. For signal processing pulse-phase or lock-in post processing can be used.

 

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