Magnetic particle testing equipment is a type of non-destructive testing (NDT) device widely used for detecting surface defects in metals, and is particularly effective for identifying surface or near-surface anomalies such as cracks, voids, and inclusions. It primarily relies on the principles of magnetism: by applying a magnetic field to the metal surface and utilizing magnetic particles to reveal areas of magnetic flux leakage, it enables the detection of cracks or other defects. The following sections will detail how magnetic particle testing equipment detects cracks on metal surfaces through various operational steps and techniques.

I. Basic Principles of Magnetic Particle Testing
The working principle of magnetic particle testing equipment is based on the magnetization characteristics of ferromagnetic materials when subjected to an external magnetic field. When ferromagnetic metals (such as iron and steel) are exposed to an external magnetic field, they become magnetized, forming closed lines of magnetic flux. Typically, if the metal surface is continuous and free of defects, the magnetic field will distribute itself uniformly along the surface of the workpiece; however, if cracks, voids, or other defects are present on the surface, these anomalies will disrupt the distribution of the magnetic field, resulting in magnetic flux leakage. When such flux leakage occurs, magnetic particles will be attracted to and accumulate in these specific areas, thereby revealing the location and geometry of the cracks.

II. Detailed Steps of the Inspection Process
- Preparation of the Workpiece Surface
Prior to conducting magnetic particle testing, the metal workpiece must first undergo a cleaning process. If the workpiece surface retains oil stains, rust, metal shavings, or other contaminants, these impurities may interfere with the adhesion of the magnetic particles, leading to inaccurate inspection results. Therefore, the workpiece must be thoroughly cleaned to ensure a smooth surface free of any adhering substances. This is typically achieved through methods such as solvent cleaning, mechanical grinding, or sandblasting.

 
- Application of the Magnetic Field
Once the workpiece surface has been cleaned, a magnetic field must then be applied. Magnetic particle testing equipment generates this magnetic field using either electromagnets or permanent magnets. Electromagnets offer the advantage of adjustable field strength, allowing the magnetic field to be tailored to suit the varying sizes and shapes of different workpieces. The applied magnetic field may be direct current (DC), alternating current (AC), or pulsed, depending on the specific requirements of the inspection and the characteristics of the workpiece. Through this process, the entire surface of the workpiece becomes magnetized, establishing a closed magnetic flux circuit.

- Generation of Leakage Flux Fields
When defects such as cracks, voids, or spalling are present on the surface of a workpiece, these defects disrupt the continuity of the magnetic field, causing magnetic flux lines to leak out and form a leakage flux field. The leakage flux field is the central element of magnetic particle testing; the presence of a defect causes the magnetic field to deviate around the crack or flaw, thereby creating a visible phenomenon of magnetic flux leakage.

  
- Application of Magnetic Particles
After the magnetic field has been applied, the surface of the workpiece must be covered with magnetic particles. Magnetic particles are fine ferromagnetic granules that are attracted by the magnetic field and accumulate at the points where magnetic flux lines are leaking. When cracks or other surface defects are present, the magnetic particles will gather within these leakage flux regions, forming distinct indications of the defects. The magnetic particles may be applied as a dry powder or as a liquid suspension (when using fluorescent magnetic particles, ultraviolet light illumination is typically required to enhance visibility). The uniform application of magnetic particles assists the operator in more accurately pinpointing the location of cracks or defects.

 
- Observation of Defects
The observation process is a critical step in magnetic particle testing. Using the naked eye or a magnifying glass, the operator examines the distribution pattern of the magnetic particles. If cracks or other surface defects are present, the magnetic particles will accumulate along the edges of these defects, forming clear indications. In cases where fluorescent magnetic particles are used, the operator employs an ultraviolet lamp; the particles at the defect sites will fluoresce, significantly enhancing the sensitivity and accuracy of the inspection—particularly in low-light conditions.

 
- Evaluation and Assessment of Defects
Once areas of magnetic particle accumulation have been observed, the operator analyzes the distribution pattern to evaluate the size, morphology, and spatial arrangement of the defects. Typically, cracks exhibit a linear or irregular morphology, while the length, width, and depth of a defect also influence the distribution of the magnetic particles. Based on the severity of the defects, the operator determines whether further testing or repair procedures are required.

 
- Cleaning of the Workpiece
Upon completion of the inspection, the magnetic particles on the surface of the workpiece must be thoroughly cleaned off. Magnetic particles are generally easy to remove; once cleaned, the workpiece is ready for subsequent processing, such as repair or re-inspection. Cleaning the workpiece also helps restore its original surface condition and prevents residual magnetic particles from interfering with its subsequent use.

III. Advantages of Detecting Metal Cracks
Magnetic particle testing equipment offers numerous significant advantages when detecting cracks on metal surfaces:
High Sensitivity
Magnetic particle testing is highly effective at identifying minute cracks—particularly tiny defects present on or just beneath the metal surface. Compared to other inspection methods, magnetic particle testing boasts superior sensitivity, enabling the timely detection of potential hazards.
High Visual Clarity
Since magnetic particles accumulate at defect sites to form visible indications, operators can visually pinpoint the exact location and morphology of cracks, thereby facilitating assessment and subsequent remedial actions.
Non-Destructive Nature
Magnetic particle testing is a non-destructive inspection method; it causes absolutely no damage to the workpiece being tested. Workpieces can be returned to service immediately after inspection, and the method itself can be applied repeatedly.
Broad Applicability
Magnetic particle testing is suitable not only for common metallic materials—such as cast iron and alloy steels—but is also highly effective when applied to workpieces of varying shapes, including complex structures such as large-scale machinery components and welded joints.

IV. Summary
By applying a magnetic field to a metal surface and leveraging the principle that magnetic particles will accumulate in areas containing cracks or other defects, magnetic particle testing equipment effectively detects surface cracks in metals. Through a process involving surface cleaning, magnetic field application, magnetic particle application, and observation of particle distribution, the equipment assists operators in accurately identifying potential cracks on metal surfaces. As a highly efficient, sensitive, and non-destructive inspection method, magnetic particle testing is widely utilized across diverse sectors—including aerospace, automotive manufacturing, and mechanical processing—thereby ensuring the quality and safety of metal components.