Non-Destructive Testing
Meaning ~ Inspection techniques that evaluate the internal structure of manufactured materials without causing damage represent a formal verification category.
Liquid Penetrant Testing
Meaning
Surface inspection techniques utilize a colored or fluorescent liquid to reveal flaws that are otherwise invisible to the naked eye. This process known as liquid penetrant testing works on non-porous materials like aluminum, copper, stainless steel, and ceramics. It depends on the ability of a fluid to enter a crack and then be drawn back out by a developer.
The method is widely used in the aerospace and automotive industries for its simplicity and low cost.
Surface Condition
Cleanliness of the part is the most influential factor in obtaining an accurate result. In liquid penetrant testing any oil, grease, or paint will block the fluid from entering the discontinuities. Mechanical cleaning like grinding or shot blasting can smear the metal and hide the cracks.
Technicians must use chemical solvents to ensure the surface is completely free of contaminants.
Dye Selection
Choice of the penetrant type depends on the environment and the required sensitivity. Fluorescent liquid penetrant testing requires a darkened area and a black light to see the indications. Visible red dye is often used for field inspections because it does not require special lighting.
Each system has a different cost and a different level of cleanup difficulty.
Detection Capability
Limitations of the method include the inability to find flaws that do not break the surface. While liquid penetrant testing is excellent for finding cracks, it cannot measure the depth of the flaw. Results are recorded as indications until they are evaluated by a senior inspector.
Misinterpreting a bit of lint as a crack is a common error that leads to unnecessary rework.