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I. Overview of Radiographic Testing

Radiographic testing is one of the five conventional non-destructive testing methods. Its fundamental principle involves utilizing various types of radiation to penetrate objects. By observing differences in the absorption and scattering of radiation caused by internal structures or defects, it detects macroscopic geometric flaws within materials.

Common Types of Radiation:

Radiation with high penetrating power primarily includes X-rays, gamma rays, and neutron radiation.

Primary Applications: X-rays and gamma rays are most widely used, primarily for detecting internal defects in industrial products such as welds, castings, and composite materials. Neutron rays are employed in specialized scenarios.

Main Methods: Radiography is the most fundamental and widely applied technique, utilizing X-rays or gamma rays to penetrate test specimens, with photographic film serving as the recording medium.

II. Principles of X-ray Inspection Equipment

1. Generation of X-rays

X-rays are produced by an X-ray tube, whose core operating principle is:

Electron Emission: The cathode filament is electrically heated (to incandescence) in a vacuum, emitting electrons.

Electron Acceleration: An extremely high voltage (tube voltage, typically tens to hundreds of kilovolts) is applied between the cathode and anode. Electrons are accelerated in the electric field, gaining significant kinetic energy.

X-ray Generation via Target Impact: The high-speed electron stream collides with the anode (target material). This interaction with the target atoms' external electric field causes a portion of the electrons' kinetic energy to be abruptly converted into X-ray energy (the majority is converted into heat energy).

Key Parameters:

Tube Voltage: Determines the penetrating power (energy) of the generated X-rays. Higher voltage produces higher-energy rays capable of penetrating thicker objects.

Tube Current: Determines the intensity of the generated X-rays. Higher current produces more photons per unit time, improving the image signal-to-noise ratio.

2. Principle of Radiographic Testing

This principle relies on two fundamental characteristics of X-rays:

Attenuation: As X-rays pass through matter, their intensity gradually diminishes due to absorption and scattering. The degree of attenuation depends on the material's density, thickness, and atomic number. Thicker and denser materials cause greater attenuation.

Photographic property: X-rays can expose X-ray-sensitive film.

Workflow:

Setup: Position the X-ray machine on one side of the object. Place a dark bag containing X-ray film and an intensifying screen (typically lead foil for enhanced sensitivity) on the opposite side.

Exposure: Activate the X-ray machine to transmit radiation through the object. Areas without defects absorb radiation differently than defective regions (e.g., pores, cracks, inclusions), resulting in varying radiation intensity reaching the film.

Less Absorption at Defects: Areas like pores or shrinkage cavities transmit higher radiation intensity, causing greater film exposure.

Defect-free areas absorb more radiation: These regions transmit lower-intensity rays, resulting in less exposure to the film.

Darkroom Processing: The exposed film undergoes development, fixing, washing, and drying in a darkroom. This yields a negative film with varying densities (grayscale), known as the radiographic negative.

Film evaluation: The dried film is examined under a viewing light. Areas of greater density on the film correspond to regions within the object that absorb fewer rays (i.e., defects), while areas of lesser density correspond to regions that absorb more rays (i.e., intact areas). By analyzing the shape, size, and distribution of these density images, the evaluator can determine the type, size, quantity, and location of defects.