Non-Destructive Testing: Types and Applications

Non-Destructive Testing (NDT) allows inspectors to collect data on a product without damaging it. It is used to detect internal defects and assess deterioration without disassembling or destroying the object.

While the terms Non-Destructive Testing (NDT) and Non-Destructive Inspection (NDI) are often used interchangeably, strictly speaking, NDT refers to the testing process itself, whereas NDI implies a pass/fail judgment based on specific standards or criteria.

Japanese Industrial Standards (JIS) Z 2300 defines Non-destructive testing as "Testing to determine the presence or absence of flaws, their location, size, shape, distribution state, etc., without destroying the material or product". And Non-destructive testing is defined as "a judgment based on the non-destructive testing result according to standards or criteria.

In some cases, however, non-destructive inspection is similarly used to NDT. As the methods of the non-destructive inspection in this section also includes NDT's, we recommend separating the use depending on your applications and purposes.

The most two NDT purposes are:

Quality Assessment :

Checks for problems with manufactured products and parts. For example, it is used to check shrinkage cavities in casting, welding defects, etc.

Lifetime evaluation :

Confirms safe usage in the product operation. It is available to check for abnormalities in structures and infrastructures that will be used for a long period.

Non-destructive testing offers safe and effective ways of inspecting objects as follows.

The non-destructive testing is particularly accurate effective because it can identify internal defects of the objects without destroying them. The method is the same as an X-ray examination that shows broken parts that are difficult to discern externally.

The NDT is available for the inspection of products before shipment since the method does not contaminate or damage them. This makes it helpful in ensuring better-inspected products by all inspections, increasing product reliability. In some cases of the NDT, however, you might require many preparation processes that are relatively expensive.

Several techniques are used in non-destructive testing, selected based on the material properties and the type of defects being inspected.

Radiographic Testing (RT)

Radiographic testing (RT) uses X-rays or gamma rays to penetrate objects. By analyzing differences in material thickness and density appearing in the images, RT detects internal defects. Computed Tomography (CT) is an advanced imaging method providing cross-sectional and 3D images, allowing for detailed analysis of internal structures.

Note: As RT utilizes radiation, appropriate safety measures and strict adherence to regulations are required during operation.

Applications: Internal analysis of lithium-ion batteries and PCBs; inspection of pipes and welds in power plants and infrastructure.

Ultrasonic Testing (UT)

Ultrasonic testing (UT) transmits ultrasonic waves into an object. By measuring the reflection of sound waves from internal boundaries or anomalies, UT inspects the internal condition of the material. It is a safe and widely used method for detecting internal flaws in homogeneous materials.

Limitations: Inspecting irregularly shaped materials or rough surfaces can be difficult.

Applications: Detecting internal defects in rolled metal products and composite materials.

Eddy Current Testing (ET)

In Eddy Current Testing (ET), a coil carrying an AC current is placed near the object's surface. This induces eddy currents within the conductive material. Disruptions in the eddy current flow caused by surface cracks or defects are then measured. ET is ideal for automated inspections as it typically requires no surface preparation.

Limitations: ET is only effective on conductive materials.

Applications: Surface crack detection, thickness measurements, and manufacturing line inspections.

Penetrant Testing (PT)

Penetrant Testing (PT) refers to the process using the capillary phenomenon where a penetrant is used to coat an object and fill the inside of the defect.

After the process, the surface penetrant is removed.

The penetrant that has entered the defect cannot be washed out and remains, so by supplying a developer, the defect is absorbed and becomes visible. The PT is suitable only for surface defect inspection. It needs longer processes and greater time to complete, unsuitable for internal inspection.

It is used to inspect turbine blades of jet engines and automobile parts.