What is nondestructive testing?
Nondestructive testing is a general term that refers to all technical means used to detect defects or nonuniformity in an object being tested, by utilizing the characteristics of sound, light, magnetism, and electricity. This process provides information on the size, location, nature, and quantity of defects, which allows for the determination of the technical state of the object being tested (such as its qualification, residual life, etc.). Importantly, nondestructive testing achieves this without damaging or affecting the service performance of the tested object.
4 Nondestructive testing methods
Common nondestructive testing methods: ultrasonic testing (UT), magnetic particle testing (MT), liquid penetrant testing (PT) and X-ray testing (RT).
Ultrasonic testing (UT) is a nondestructive testing method widely used in various industries.
When an ultrasonic wave enters an object and encounters a defect, a portion of the wave gets reflected.
By analyzing the reflected wave using a transmitter and receiver, the defect can be accurately measured. The location and size of the internal defect can be displayed, and the thickness of the material can be determined.
Advantages of ultrasonic testing:
- It has a high penetration ability; for instance, it can effectively detect steel up to a depth of more than 1 meter.
- It has high sensitivity for detecting planar defects like cracks and interlayers, and it can measure the depth and relative size of defects.
- The equipment is portable, safe to operate, and easy to use for automatic inspection.
Inspecting a workpiece with a complex shape is challenging, particularly when the surface being inspected requires a certain degree of finish. In order to ensure full acoustic coupling, a coupling agent must be used to fill the gap between the probe and the surface being inspected.
Magnetic particle inspection
To begin with, let us grasp the principle behind magnetic particle testing.
When ferromagnetic materials and workpieces are magnetized, the presence of discontinuity causes the magnetic lines of force on and near the surface of the workpiece to become locally distorted, creating a magnetic leakage field. This field attracts the magnetic particles that are applied to the surface of the workpiece, resulting in visible magnetic marks that reveal the position, shape, and size of any discontinuity when viewed under appropriate lighting.
The applicability and limitations of magnetic particle testing are:
- Magnetic particle flaw detection is applicable for detecting discontinuities on the surface and near surface of ferromagnetic materials, including small sizes and extremely narrow gaps that may be difficult to see visually.
- Magnetic particle testing can be used to detect various types of parts and components under different conditions.
- Defects such as cracks, inclusions, hairlines, white spots, folds, cold shuts, and looseness can be identified through magnetic particle testing.
- Magnetic particle testing cannot detect austenitic stainless steel materials or welds that are welded with austenitic stainless steel electrodes. Additionally, non-magnetic materials such as copper, aluminum, magnesium, and titanium cannot be detected with this method.
It may be challenging to identify shallow scratches on the surface, deep holes that are buried, and delaminations and folds with an angle less than 20° to the workpiece surface.
Liquid penetrant testing
The basic principle of liquid penetrant testing is that the part surface is coated with fluorescent or colored dyes. Then, the penetrant can penetrate into surface opening defects under the action of capillary for a specific period.
After that, the excess penetrant on the surface of the parts is removed, and the developer is applied to the surface of the parts. Similarly, under the capillary action, the developer will attract the penetrant retained in the defect, causing the penetrant to seep back into the developer.
Finally, when illuminated by a specific light source (ultraviolet or white light), the trace of penetrant at the defect will be displayed (yellow-green fluorescence or bright red), allowing for the detection of the morphology and distribution of defects.
The advantages of penetrant testing are:
1. It can detect all kinds of materials;
2. High sensitivity;
3. The display is intuitive, the operation is convenient, and the detection cost is low.
The disadvantages of penetrant testing are:
- This method is not suitable for checking workpieces made of porous, loose materials, or with rough surfaces.
- Penetrant testing can only detect surface defects, making it difficult to determine the actual depth of defects and to make a quantitative evaluation of them. Additionally, the results of this method can be greatly influenced by the skill level of the operator.
Radiographic testing is used because X-rays are absorbed differently by different substances and thicknesses, resulting in varying intensities of X-rays passing through the irradiated object.
When the negative film is placed on the opposite side of the irradiated object, corresponding graphics are generated due to the differing X-ray intensities.
Based on the resulting images, the film evaluator can determine whether there are defects inside the object, and the nature of those defects.
Applicability and limitations of radiographic testing:
- It is sensitive in detecting volumetric defects and facilitates defect characterization.
- Radiographs are easy to retain and trace.
- They provide visual display of the shape and type of defects.
- However, there are limitations to this method as it cannot determine the buried depth of defects and has a limited detection thickness. Additionally, negatives require special washing, which is harmful to the human body and can be expensive.
In summary, ultrasonic and X-ray flaw detection are effective for detecting internal defects. Ultrasonic waves are suitable for parts with regular shapes above 5mm. However, X-ray cannot locate the buried depth of defects and involves radiation.
Magnetic particle and penetrant testing are useful for detecting surface defects in tested parts. Magnetic particle inspection is limited to the detection of magnetic materials, while penetrant inspection is only suitable for detecting surface-opening defects.