One of efficient methods of detecting through and surface discontinuities is capillary control. It uses tracing fluids with a high penetration ability. They are also called penetrants. As a result of using these substances, the faulty site acquires an enhanced light and color contrast. This makes detection of material damage much simpler.
The capillary control method is very efficient for registering surfaced faults:
· lacks of fusion;
· intergranular attack;
· other discontinuities.
In order for the penetrant to infiltrate and form an indication pattern, the discontinuity and its opening point should be cleaned from contaminations. The controlled object should be thoroughly treated with penetrant. Another mandatory requirement is that the width of a discontinuity should not exceed its depth.
Capillary control is preceded by pre-cleaning of the surface followed by thorough drying. The applied penetrant infiltrates into the discontinuity. After a certain time it should be removed from the controlled area so that the indication fluid remains only in the fault cavities.
After that, the object should be treated with developer. These manipulations will result in visualized indication patterns. On the background of the intact surface, the faults look brighter and more contrast.
Requirements for capillary NDT inspection
The technology is regulated by the national standard. Liquid penetrant diagnostics should precede other methods of fault detection. Besides that, it is important to keep in mind other requirements for the conditions and procedure of the control:
· Temperature range: −40 to +40˚ С.
· Relative humidity: within 90%.
· Defect templates or rejected areas (minimum 3) are used in the tests. The results are registered in a protocol.
The method is designed for detecting faults of a 0.2–0.5 µm width. Loss of sensitivity is most often caused by the following factors:
· bad preparation of the inspected surface; grease film and other contaminations;
· excessive roughness of the surface;
· change in the penetrant composition: if it has been stored incorrectly or for too long, efficiency of capillary control will inevitably deteriorate;
· use of substandard fault detection materials;
· ungroundedly prolonged waiting time before removal of penetrant excess;
· violation of temperature mode, humidity and other conditions.