What is the classification of non-destructive testing?

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1. Ultrasonic Detection Principle

Ultrasonic non-destructive testing (NDT), also known as ultrasonic testing or UT, is a technique that uses high-frequency sound waves to examine the thickness and internal structure of a material. As the ultrasonic wave travels through the material, changes in its acoustic properties and internal structure affect how it propagates. By analyzing the behavior of these waves—such as their reflection, transmission, and attenuation—engineers can identify flaws, measure thickness, or assess the integrity of the material. This method is widely used in industries like aerospace, automotive, and construction for quality control and safety assurance.

Currently, several advanced techniques are employed, including the pulse echo method, Time-of-Flight Diffraction (TOFD), and Phased Array technology. Each method has its own advantages and is selected based on the specific application and material being tested.

2. X-ray Detection Principle

X-ray detection, also known as radiographic testing, works by using the intensity reduction of X-rays, gamma rays, or neutron beams as they pass through materials. The varying density and composition of the material cause different levels of absorption, which creates an image on X-ray film or digital sensors. This allows inspectors to visualize internal structures without damaging the object. It is especially useful for detecting cracks, voids, and other internal defects.

This type of testing is categorized into various methods such as radiography, fluoroscopy (now commonly used in industrial television systems), dry plate photography, tomography (like industrial CT scans), and digital imaging technologies. These methods vary in resolution, speed, and application scope.

3. Magnetic Particle Detection Principle

Magnetic particle inspection (MPI) is a non-destructive testing method used to detect surface and near-surface discontinuities in ferromagnetic materials. The process involves magnetizing the component and then applying fine magnetic particles to its surface. These particles are attracted to areas where the magnetic field leaks due to cracks or other imperfections, making them visible under proper lighting conditions. MPI is often used in the inspection of welds, castings, and forgings.

4. Penetrant Detection Principle

Penetrant testing, or dye penetrant inspection (DPI), relies on the capillary action of liquids to detect surface-opening flaws. A liquid penetrant is applied to the surface of the material, where it seeps into any existing cracks or pores. After removing the excess penetrant, a developer is applied, which draws the remaining penetrant out of the defect, creating a visible indication. This method is ideal for inspecting non-porous materials such as metals, plastics, and ceramics.

5. Eddy Current Detection Principle

Eddy current testing is a non-contact method that uses electromagnetic induction to detect surface and subsurface flaws in conductive materials. A coil generates an alternating current, which induces eddy currents in the test piece. Discontinuities in the material disrupt the flow of these currents, and the resulting changes in the electromagnetic field are measured. This technique is highly effective for detecting corrosion, fatigue cracks, and material thinning, especially in tubular components and heat exchangers.