Table of Content
Non-destructive Testing (NDT) is a combination of various inspection techniques used individually or collectively to evaluate the integrity and properties of a material, component, or system without causing damage to it. In other words, the part that requires the use of one or more of those techniques can still be used once the inspection process is over. NDT is therefore often used for the detection, characterization, and sizing of inherent discontinuities, as well as those associated with damage mechanisms.
NDT is regulated by Codes and Standards according to the type of industry, country, and other criteria. ASME, ASTM International, COFREND, CSA, CGSB, and ASNT are well-known examples.
NDT Techniques Appellation
The names of those NDT Techniques generally make reference to a particular scientific principle or to the equipment used to conduct the inspection. For example, Ultrasonic Testing is based on the propagation of ultrasonic sound waves in a material, and magnetic particle testing uses very small particles that are affected by the application of a magnetic field.
What are the 5 Types of NDT Methods?
Many different NDT methods are available in the industry, each of them having its advantages and limitations, but five of them are most frequently used:
1. Ultrasonic Testing
2. Radiographic Testing
3. Magnetic Particle Testing
4. Liquid Penetrant Testing
5. Visual Testing
The 5 Types of NDT Methods are:
1. Ultrasonic Testing
Ultrasonic testing allows a view into the inside of a component. To do this, a probe is moved over the surface of a component and the ultrasonic waves emitted by it or their reflections are tracked on a screen. A phased-array imaging test can also be performed, which allows easier interpretation.
Both flat and voluminous imperfections can be inspected. In the case of surface imperfections, it is often superior to RT. It is used, for example, for Wall Thickness Measurement with vertical probes and simple geometries with angle probes.
2. Radiography Testing
Radiography non-destructive testing refers to an imaging test method that allows a view of the inside of a component. Depending on the component and size, an x-ray tube or radioactive material is used to irradiate a film. One advantage of this method compared to Ultrasonic testing is that the type of imperfections, e.g. whether pore or slag, can be determined more easily.
Since the handling of the preparations is subject to special safety regulations, the recording is often carried out by service providers. An evaluation of the imperfections can then be carried out separately. Digital Radiography using X-ray image intensifiers or radioscopy systems is becoming increasingly popular because images can be stored and evaluated digitally.
3. Magnetic Particle Testing
Magnetic Particle Testing is a method for testing magnetizable materials with a minimum of time. Magnetic particles are applied to the object to be tested with a liquid or powder. Inconsistencies are then revealed by a change in the magnetic field, which causes the particles to be aligned differently from the ‘good’ part of the object.
Magnetic Particle Testing of a Pipeline:
In this way, cracks or inclusions of non-magnetic materials can be quickly detected. Especially remarkable is the detection of small cracks with a width of 0.001 mm and a depth of 0.01 mm. For comparison: human hair has a thickness of 0.04 mm or more.
4. Liquid Penetrant Testing (LPT)
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Liquid penetrant testing is a flexible procedure that uses the capillary forces of cracks or pores. For this purpose, color or fluorescent penetrant testing (contrasted) is applied to a cleaned component, e.g. with a spray can, followed by a ‘developer’. The penetrant then ‘creeps’ into the smallest cracks and reveals the imperfections.
Important for the handling of the substances are environmental aspects, storage, disposal, and correct transport. For Dye Penetrant Testing, the colors red and white (contrast-developer) are usually used. Care should be taken with rough or brittle surfaces, as this can lead to so-called false readings, or with the color intensity, as these do not necessarily indicate the depth of a crack.
5. Visual Testing (VT)
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Visual Testing is an optical method to check objects for discrepancies. You can check with the naked eye as well as with tools like magnifying glasses or mirrors.
Visual Testing of Turbine Parts
Many imperfections, such as external cracks, can already be found with visual testing, which makes this type of inspection a simple but powerful procedure that can be used to check some imperfections of welds, fractured surfaces, corrosion phenomena, or grinding.
Other Types of NDT Techniques
In addition to commonly used methods like ultrasonic and radiographic testing, the automotive industry also utilizes eddy current, time-of-flight diffraction, full matrix capture, and magnetic flux leakage. These diverse NDT techniques ensure comprehensive assessment, enhancing the safety and reliability of automotive components.
1. Phased Array Ultrasonic Testing:
Phased array ultrasonic testing is based on the same physics as the Conventional Ultrasound inspection. The differences are mainly the probe technology and configuration as well as the acquisition of instrument electronics. Possible phased array configurations are dependent on what the probe and electronics are capable of. Each element is controlled individually, therefore allowing for the generation of a customized ultrasound beam using a defined delay.
2. Automated Ultrasonic Testing:
AUT consists of a motorized inspection system (the scanner), which manipulates the probes while tracking their position the entire time. In addition to Weld Inspection, the AUT technique is ideal for corrosion detection on difficult-to-access structures. It can also provide 100% coverage with an increased production of resulting data in comparison to traditional methods.
3. Conventional Ultrasonic Testing:
Conventional Ultrasonic Testing uses a probe comprised of a piezoelectric element capable of deforming and generating high-frequency acoustic waves that travel at a specific velocity dependent on the material. Conventional ultrasonic inspection is primarily used for thickness measurement, welding inspection, lamination, and corrosion detection.
3. Time-of-Flight Diffraction:
Time-of-Flight Diffraction is a technique based on an ultrasonic wave’s traveling time, or ‘time of flight’, and the diffraction produced by the extremities of the discontinuity. TOFD is recognized for its high level of accuracy and precision regarding sizing and is often used as a complement to the Phased Array method.
4. Full Matrix Capture:
FMC is an advanced data acquisition and reconstruction method using PAUT probes. FMC is based on the synthetic focusing principle and is processed by algorithms resulting in a picture-like visualization of the area under examination. The resulting matrix can be processed by algorithms to produce the image. This process is called the Total Focusing Method.
5. Conventional Electromagnetic Testing:
Eddy Current Testing inspection is based on the interaction between a magnetic field source, a coil, and the electrically conductive material to inspect. The result of this interaction is the induction of Eddy Currents (also known as Electromagnetic Induction). Discontinuities can then be detected by measuring and analyzing the intensity variations of the current.
6. Eddy Current Array (ECA)
Eddy Current Array (ECA) technology represents the evolution of the Conventional Eddy Current method. This technology offers a wider coverage and a greater sensitivity to potential flaws due to the multi-coil design. Eddy Current Array probes can be customized to best suit the application and required coverage; the number of coils and flexibility of the probe can be adjusted to inspect complex geometries, such as the teeth on gears.
7. Tangential Eddy Current:
Tangential eddy current inspection is another technique based on magnetic induction. The main difference between Tangential and Conventional Eddy Current is that the coils are oriented tangential to the surface. Considering that eddy currents are created perpendicular to the surface, this orientation improves the depth positioning and sizing of flaws.
8. Pulsed Eddy Current:
Pulsed Eddy Current inspection is a technology based on the magnetic field penetration through multiple layers of coating or insulation to reach the surface of a given material and induce eddy currents. These NDT Techniques are generally used to measure thickness and to detect corrosion on ferrous materials that are covered with an insulation layer, fireproofing, or coating.
9. Small Control Area Radiography:
Small controlled area radiography is distinguished by using a compact exposure device. This instrument improves the efficiency of radiographic operations by making it safer while maintaining or increasing productivity when compared with traditional exposure devices.
10. Magnetic Flux Leakage:
Magnetic flux leakage inspection is based on electromagnetism and the measurement of permeability variations. The magnetic flux leakage analysis confirms the presence of potential flaws due to wall thickness loss caused by corrosion or surface flaws such as cracks.
Key Takeaways
- With so many different NDT Techniques, each having its characteristics, some of them might be perfectly suitable for certain applications but utterly ineffective in other cases.
- For example, some NDT Methods are limited to surface examination while others allow a complete volumetric inspection.
- The different types of non-destructive testing are often complementary. As a result, we can exploit the advantages of combined techniques.
- Consequently, choosing the appropriate method is a very important step for optimizing the performance of an NDT inspection, it is thus essential to be well-advised when preparing the inspection plan.
References:
1. Nucleom
