Surface defects of A500 rectangular tubes include folds, tube pits, pockmarked surfaces, and bridge skin. This article discusses various methods for detecting these surface defects.
Detection Methods for Surface Defects in A500 Rectangular Tubes
Eddy Current Testing:
Application: Suitable for detecting subsurface defects up to 6-7mm deep.
Types: Includes the placed coil method and the through coil method.
Mechanism: Defects distort the electrical characteristics of the eddy current, indicating their presence.
Limitations: Does not directly display the size and shape of defects; it mainly identifies their position and depth.
Radiographic Detection:
Radiation Sources: Uses X-rays or γ-rays.
Mechanism: Internal defects affect the radiation intensity passing through the tube, forming a radiographic image.
Visualization: Defects can be recorded on radiographic film, observed in real-time via a fluorescent screen, or detected by a radiation counter.
Advanced Techniques: Microfocus X-ray systems and digital imaging improve image clarity and signal-to-noise ratio.
Liquid Penetrant Testing:
Application: Detects surface-opening defects.
Methods:
Coloring Test: Involves applying a colored penetrant, which penetrates defects, followed by a revealing agent to visualize the defect.
Fluorescent Penetrant Testing: Uses ultraviolet light for higher sensitivity detection.
Mechanism: The penetrant enters defects and, after excess removal, the revealer highlights defect shapes, sizes, and distribution.
Ultrasonic Testing:
Application: Detects internal defects by reflecting sound beams within the tube.
Mechanism: Reflected acoustic energy varies based on defect characteristics and is used to locate defects, measure wall thickness, and determine defect depth.
Advantages: High sensitivity for detecting small cracks and significant penetration for thick sections.
Magnetic Particle Testing:
Application: Suitable for surface defects and those several millimeters below the surface.
Mechanism: DC magnetization equipment creates a magnetic field, and magnetic powder reveals defects through leakage magnetic fields.
Process: Magnetized areas with defects attract magnetic powder, displaying the defects.
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