Non-Destructive Testing

Non-destructive testing (NDT) is a method of evaluating a material’s integrity for both surface and internal flaws without destroying the sample. It analyses the metallurgical condition of a sample without making any interference with its structural and compositional properties. There are different types of NDT techniques that can be used according to the type of material and its application. NDT can identify and evaluate multiple defects or irregularities such as cracks, holes, flaws, dimensional changes, and impurities without opening up the whole machinery or part. NDT can also be applied to coatings to find surface or internal voids, delamination, cavities, or premature failure cracks. It compares the testing of parts with relevant standards to find the exact location of the flaw. It can investigate the overall machinery or a part of it in the quality control system. At times, there is a need for more than one NDT technique to ensure the exact location of an error. Therefore, multiple techniques with particular attributes are available in this domain to locate the flaw accurately and precisely. A variety of techniques are used for NDT applications of metals, plastics, composites, and ceramics. NDT is performed after a calculated time interval (depending on the application, such as after each flight, aeroplane parts would be tested) to prevent failure and improve the reliability, safety and usage of the concerned entity.

Why non-destructive testing is important

NDT is one of the most crucial and important parts of the quality control and quality assurance department. It finds immense importance in the applications such as aerospace, bridges, pipelines, heavy machinery units of industries, refineries, ships, and others. NDT finds its importance in the applications where the flaw can be detected without opening up the whole machinery. It not only saves the time of engineering personnel but also saves costs and resources. NDT techniques are also available in a portable form that makes it more convenient to be used in difficult positions. Following are the aspects that make NDT more crucial:

  • It ensures whether the materials or components are fit for use or not. It decides the need for maintenance or repair. Thus, it saves catastrophic failure of machinery in industries that deal with enormously high pressures or temperatures.
  • Regular checks through NDT can assure confident levels of safety and security of equipment and also for the operators.
  • In order to comply with various safety and security rules and certifications, industries use NDT as a cost-effective method.
  • NDT provides an immediate and quick way to find the flaws in machinery that allows it to be done at repeated intervals. Moreover, the part to be tested is not taken out of service making it easier to be applied in difficult positions and scenarios.
  • NDT data is recordable which means it can be used for continuous inspection and study of equipment usage.

Non-destructive testing methods

There are a number of NDT techniques that can be used for the detection of flaws in multiple applications, some of these are mentioned below:

1. Visual Inspection (VT)

Visual inspection or visual testing (VT) is the oldest yet most widely used technique. It is the process of examining any component or part of machinery with the naked eye. Optical aids such as mirrors, borescopes, optical aids, and magnifying lenses. Digital image analysers, cameras, and computer systems are used to save the images and analyse them at any time. VT is usually used for the routine inspection of pressure vessels, storage tanks, pipelines, and related equipment. It is a less technological technique with a number of advantages over other techniques. It is cost-effective and easy to apply the technique. Further advancement is being processed such as remote visual testing (RVT) and unmanned aerial vehicles (UAVs) commonly referred to as drones.

2. Ultrasonic Testing (UT)

In this technique, ultrasonic waves are sent to the desired object or part. Short pulse waves of frequency from 0.1 to 15 MHz are sent to detect the flaws. It can measure the thickness of objects when assessing pipeline corrosion or related objects. UT has two components for the detection of flaws namely transducer and detector. Normally, a piezoelectric transducer is connected to a flaw detector and this transducer is moved over the surface being examined. A couplant is used to efficiently transmit the ultrasonic waves and enhance the accuracy. But this couplant is not present in the advanced techniques of UT including electromagnetic acoustic transducer (EMAT) or laser excitation. It has two types including pulse-echo and through transmission. In the pulse-echo technique, the same transducer acts as an emitter of ultrasonic waves and receives the echo signals back for analysis. The transmission uses a transducer at one end and a receiver at the other end.

3. Liquid Penetrant Testing (LPT)

It is also known as LPT, in which a fluorescent or colour dye is applied to the surface under examination. Aerospace and medical industries usually use this technique to visualise the defects in machinery components. It needs a clean surface so prior to application of this technique, the surfaces are cleaned from any kind of dust, rust, or scale. In this technique, you will apply any colourful liquid, fluorescence, or visible liquid to the part under examination. After removing the excess penetrant after a certain time, a developer solution is applied that works according to the principle of capillary action. Under ultraviolet radiations (1,000 micro-watts per centimetre squared intensity), the fluorescent is examined where the flaws are now visible. After development, one shall not wait for too long as the developer may over bleed and lead to misguiding detection.

Dye Penetrant vs Liquid Penetrant

Fluorescence Penetrant Inspection uses fluorescence as a liquid for inspection while Dye Penetrant Inspection uses dyes of different colours as a penetrant liquid for inspection. Dyes can be of various colours, one of the most common colours is a red dye.

Standards for FPI

ASTM E1417 FPI standard is normally used for the medical and aerospace industry. Kemet offers FPI systems with the flexibility to perform these tests manually or with automatic procedures. It comes with an integrated system of wastewater handling and extraction that helps in saving the chemicals. Thus, it saves the cost of labour, energy, and chemicals. For example, Kemet has applied automated FPI systems for aircraft systems, turbine blades, the framework of aircraft, and medical implants. But they use manual systems for the inspection of smaller parts. Thus, a smart combination of automated and manual systems can create a reliable solution for a durable, long-term, user-friendly, and safe FPI system. The dipping penetrant technique is employed for mass production. It uses large-sized baths for pre-cleaning, dipping in penetrant solution, dust-storm developer solution, and post-cleaning solution. Automated rotation, data handling, batch tracking, and time management are used for efficient FPI processes in industries.

Applications of Liquid Penetrant Inspection (LPI)

LPI can be widely used for a number of applications ranging from the aerospace industry to the medical equipment manufacturing industry. Its major applications are:
  • Turbine blades
  • Parts of the engine
  • Aerostructures
  • Brake parts
  • Wheel hubs
  • Landing gears
  • Automotive spark plugs
  • Castings and forged components
  • Rotor discs and blades
  • Machined parts

non destructive testing

4. Eddy Current Testing (ET)

Eddy current testing is used for electrically conductive materials as it uses the electromagnetism principle for flaw detection. It uses a coil through which an alternating current is passed. This coil is placed near the surface to be tested. Changing the magnetic field of AC helps in the production of eddy currents in the specimen. Changes in phase and magnitude of these eddy currents are used to detect the crack, void, or irregularity. The greater the density of eddy currents, the higher will be the chances of determining flaws to greater depths of the conductive specimen.

5. Magnetic Particle Inspection (MPI)

Also known as MPI, it is the technique that uses magnetic particles to detect linear flaws near the surface of ferromagnetic particles, usually used for surface examination. First, the specimen to be examined is magnetised using a yoke magnet. The magnetic lines will deform or change shape where the defect (crack, void, or leakage) is present. When magnetic particles are sprinkled over the surface, they will assemble and bridge the gap over the flawed area. This technique is known for its high resolution and is applied generally to iron specimens.

6. Acoustic Emission Testing (AET)

This testing will allow you to monitor the ultrasonic stress waves emitted from a deformed material under stress. You will have to mount a sensor to the part under examination and this sensor will convert those stress waves into electrical signals. An external stimulus is needed for this test such as high temperature, pressure, or load. You can use this technique to study the structural integrity and health of components. You can apply this technique to find defects such as cracks and breakages.

Impact on cost and quality of NDT by Automated NDT Systems

The aerospace industry is having a shift from manual procedures to automated systems. It needs a number of protocols to convert the existing NDT systems into automated ones. No doubt, it enhances the efficiency of systems but it adds cost to the measurement. Two main advantages of shifting towards automated NDT are:

  • Reproducibility
  • Man-power reduction

Automated NDT will always be reproducible independent of the operator’s condition, temperature and humidity level, or any external factor. The results will always be reproducible for a specified time interval. Increased accuracy can be seen in results as the signal strength to be detected is much lowered when it comes to automatic detection as compared to an operator detecting a high-level echo in the case of UT. We can see a decrease in manually operating costs such as labour cost, risk management cost, failure cost and inexperienced analysis costs are reduced. Therefore, if we compare the overall advantages and disadvantages of shifting to automated NDT, there is a way of progressing with less danger and more compatibility.

Non-Destructive Testing - Conclusion

NDT is a very important tool for quality control and maintenance. The aerospace industry and medical equipment manufacturers need a consistent NDT service to ensure the correct usage of equipment and safety measurements. It is needed to lower the risks of catastrophic failures and big losses. NDT includes visual testing, ultrasonic flaw detection, acoustic emission testing, magnetic particle inspection, liquid penetrant testing, and eddy current testing. Liquid penetrant testing is used with three steps including surface cleaning, liquid (dye or fluorescent) application, and developer solution application. Automated NDT is next step in industries to improve the performance of machinery.

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