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What is nondestructive testing? What are the various NDT methods?

Release time：2026-03-27

Nondestructive testing (NDT) refers to a quality-control technique that, without compromising the structural integrity, service performance, or internal microstructure of the test object, employs physical or chemical methods to perform qualitative, locational, and quantitative inspections for surface and subsurface defects—such as cracks, porosity, slag inclusions, lack of penetration, and looseness—in materials, components, and welded structures.

I. What is Nondestructive Testing:

A nondestructive testing technique that uses physical methods—such as sound, light, magnetism, electricity, and radiation—to detect internal or surface defects (e.g., cracks, porosity, slag inclusions, lack of penetration) without damaging, impairing, or altering the workpiece/material’s original properties, shape, or service condition. In a nutshell: no damage, detect both internal and external flaws.

II. Five Major Nondestructive Testing Methods

1. Ultrasonic Testing UT

Principle : High-frequency sound waves are emitted toward the material; when they encounter a defect, they reflect back as an echo.

The location and size of defects are determined by analyzing the echo.

→ It’s like performing an ultrasound on the workpiece.

2. Radiographic Testing RT (X/γ Rays)

Principle : The X-rays penetrate the workpiece, and they pass through defects more easily.

Contrast between light and dark is formed on the film or detector, allowing direct visualization of defect images.

→ It’s like taking an X-ray of the workpiece.

3. Magnetic Particle Testing MT

Principle : After ferromagnetic materials are magnetized, defects generate leakage magnetic fields.

Magnetic particles are attracted and aggregated, directly revealing the crack location.

→ Only detects surface and near-surface cracks in ferromagnetic materials.

4. Penetrant Testing PT (Dye Penetrant / Fluorescent)

Principle : Utilizing capillary action, the penetrant seeps into surface-opening cracks,

After cleaning, use a developer to “draw out” the traces.

→ Only surface-opening defects are inspected; applicable to both metallic and non-metallic materials.

5. Eddy Current Testing ET

Principle : The probe is energized to generate a magnetic field, which induces eddy currents in the conductive workpiece.

Defects alter the eddy currents, and the instrument detects these changes.

→ Suitable for conductive materials and rapid automated inspection.

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