What Is NDT

What is NDT？

Testing is an integral part of equipment maintenance. It is critical to evaluate the materials, components, design, and structure of products and assets. Programs can be classified as destructive or non-destructive based on the state of the component under test after the detection is complete.

If the component is damaged during detection, the detection method used is called destructive detection. In contrast, nondestructive testing is carried out without causing damage to the equipment under test.

In this paper, we will focus on the different applications of nondestructive testing methods.

What is nondestructive testing?

Testing methods that do not compromise the structural integrity of the part under test are called nondestructive testing (NDT). NDT employs a variety of inspection techniques to evaluate components individually and collectively. It uses different principles from the fields of science (physics, chemistry, and mathematics) to examine components. NDT can also be called nondestructive assessment (NDE) or nondestructive testing (NDI).

Let's imagine a piston running inside an engine being tested for defects or material degradation. The piston can be cut open to check for defects inside. However, once tested, even if the piston is found to be free of defects, the piston can no longer be used in the engine. This is a form of destructive detection.

The piston can be examined by radiography instead of cutting it open. We can use ionizing radiation (X-rays, gamma rays) to detect defects in components or material degradation. If it passes the test, the component is still usable. This is a form of nondestructive testing.

Where is nondestructive testing used?

NDT is used to test the quality and machine condition of components before or during use. Nondestructive testing is used for condition assessment and quality control in a wide range of industries, including (but not limited to) :

• Aerospace - Testing castings

• Automotive -- Tests the durability of piston heads

• Manufacturing -- Testing the quality of components before they are put into production

• Medical devices - Test the durability and composition of the stent

• Military and Defense - Ballistic detection and analysis

• Packaging - Testing for packaging structure and leakage opportunities

• Marine industry - Identifying corrosion

• Power generation - Detection of defects associated with welding

• Waste Management - Identifying recyclable metals in waste

• Petrochemical industry - Testing pipelines used to transport oil

Why use nondestructive testing?

A significant advantage of NDT is the reusability of the components under test. Most importantly, non-destructive testing can usually be done on components that are still in operation. The equipment and testing equipment used to carry out most nondestructive testing methods is compact and portable. This makes it easier to detect components in a working machine.

Other benefits of nondestructive testing are listed below:

Nondestructive testing can ensure the safety of working parts. Components suffer wear and tear, which inevitably leads to breakdowns and failures. NDT helps detect early signs of degradation and helps determine the cause of equipment failure. Maintenance teams can use this information to perform corrective maintenance and adjust their preventive maintenance efforts. All this increases the reliability of assets.

The purpose of quality assurance. The NDT method can be used to ensure the quality of the product output. The quality assurance team will be able to quickly analyze whether the product is within the tolerance range.

Evaluate the remaining service life of the machine. The wear and tear of machines is a natural byproduct of their operation. They must be replaced at the end of their useful life. But all machines degrade at different rates due to different operating conditions and other factors. NDT checks can help estimate how long the machine will last before you can better buy a replacement.

Nondestructive testing method

A large number of non-destructive detection options are available. Which method you will use depends on the type of part you are testing and what defects you want to find.

Some NDT methods apply only to specific categories. Next, we discuss the most common NDT methods that have broader applications.

1) Visual detection

Visual testing is by far the simplest nondestructive testing method. It is usually classified as part of routine maintenance. Maintenance professionals use it daily to check for common signs of wear and tear. Depending on its application, it may or may not occur while the machine is running.

Robots and drones equipped with cameras can be used to perform visual inspections remotely in cases where direct access to test subjects is not possible.

In its most advanced applications, visual detection is combined with machine learning algorithms. This only applies to product quality tests where a large number of standardized components need to be examined.

2) Ultrasonic detection

Ultrasonic detection is based on the propagation and reflection principle of high-frequency sound waves. It can be used for defect detection/assessment, dimensional measurement, material characterization, etc. Ultrasonic receivers and transmitters are used for detection.

Ultrasonic sound waves are transmitted through the material under test. Sound travels through the assembly and reflects off a rigid surface located at the other end of the transmitter. Measure the time it takes to transmit and receive sound waves. Time differences between different parts of a component can be used to identify defects in the material.

Different types of ultrasonic detection modes can be used to identify different defects, cavities, material deterioration, etc. The mechanical parts with a heavy workload should be inspected by ultrasonic regularly. A good example of ultrasonic testing is the detection of defects and deformations in the wheels and axles of railway cars.

3) Vibration analysis

Vibration analysis is a common method to monitor the condition of rotating parts in operation. The basic principle of vibration analysis is that different materials have different vibration characteristics.

In addition to vibrometer devices, different types of sensors can be installed to measure vibrations. They are designed to measure displacement, velocity and acceleration, misalignment, loosening, and similar failures that rotating equipment may encounter.

As with all the other techniques we discuss here, vibration analysis provides valuable data for condition monitoring and predictive maintenance.

4) Magnetic particle detection MT

Magnetic particle detection is used to detect near-surface defects in ferromagnetic materials. The specimen is held between the two magnetic poles of the electromagnet and a suspension of magnetic particles is poured over the specimen. The test method is based on the effect of a magnetic field on ferromagnetic material.

When magnetic particles gather near defects and cracks, defects on the surface of the material will be highlighted. For a better visibility, use ultraviolet light to see defects.

Magnetic powder inspection can be performed using a hand-held device such as a wet horizontal machine or a magnetic yoke. Specifies that MT may be used to check the following items:

• The inside and outside surfaces of boilers and pressure vessels

• Components that have suffered fire damage

• Locomotives and historic boilers

• Yankee Dryer

• Cargo hold

• Liquefied petroleum gas service ships

• Welding repair and pressure items change

5) Penetration detection

In cases where magnetic particle detection is not feasible, penetration detection can be used. A clean working surface is required for penetration testing.

During the penetration inspection, the liquid dye penetrant is sprayed over the area to be tested and left unchanged in the open air. The time required for the penetrant to work on the surface (also known as retention time) can range from 10 minutes to an hour. It depends on the properties of the material under test.

Use a dry lint-free cloth to remove the liquid penetrant from the work surface. Spray a small amount of developer solution onto the working surface of the test. If the surface under test is defective, the liquid dye is brought to the surface after the developer is applied.

Liquid penetration tests are commonly used to test welded surfaces and work on the principle of capillary action.

6) Eddy's current detection

Eddy's current testing is a common nondestructive testing technique used in manual and automatic test scenarios. It is based on the principle of electromagnetic induction.

When a voltage is applied to the coil, it creates a strong magnetic field. When metal is introduced into the coil, the magnetic field fluctuates and the current flowing through the circuit increases. This is due to the eddy flow inside the metal.

Current consumption increases when there are defects or holes in the material. Eddies must travel longer distances -- increasing resistance, which manifests as increased current consumption. The difference in current consumption between different cross-sections of the material can be used to identify the location and size of defects.

This type of non-destructive testing is performed using eddy current testing equipment, including electromagnetic probes, current flaw detectors, ECT conductances, and other accessories. These tools are used to perform different types of electromagnetic testing, such as surface scanning, subsurface testing, weld testing, fastener hole testing, tube testing, heat treatment verification, and metal grade classification.

7) X-ray testing and industrial computed tomography

X-rays and other tomography techniques are widely used in medicine. However, some of the same techniques are also used in industrial applications as part of nondestructive testing.

X-rays and CT scans can be used for industrial radiography to view detailed images of the material under test. X-rays pass through the components and the images can be printed on film or viewed in real-time using a computer.

Ct can also color code various objects based on composite metals or the presence of cavities. X-rays can be sent on the test object from different angles to obtain images with higher detail. X-ray tests and computed tomography fall into the broader category of radiographic tests in which different types of ionizing radiation can be used.

Other applications

The seven NDTS described in the previous sections are commonly used in different industries. However, more NDT technologies are being adopted in science laboratories and industry operations, such as:

Guided wave detection: Defect identification by controlled excitation of multiple ultrasonic waves sent in different directions.

Laser detection: Laser beams are used to detect defects in materials. The three laser testing techniques used are holography, shear photography, and profilometry.

Leak detection: Leaks are tested by various methods such as bubbles, pressure changes, halogen diode, and mass spectrometer tests.

Magnetic flux leakage: Identification of defects from different magnetic flux patterns in ferrous materials.

Neutron radiographic detection: low-energy neutrons pass through the working surface instead of X-rays.

Thermal/infrared detection: Map surface temperatures based on emitted infrared radiation.