For tiny cracks, pores or other surface glitches in castings and forgings that are hard to detect by the human eye, fluorescent-dye-penetrant testing is a well-established and widely used inspection technique for both ferrous and nonferrous materials.
"Mixed-light inspection" is the term given when trying to detect the very small levels of light emitted from fluorescent-penetrant dyes against a background of strong daylight or other brightly lit environment. However, the effectiveness of these checks currently requires that the ambient-lighting conditions are acceptably low (e.g., as specified in ASTM E709 and ASTM E1209), which often limits its deployment to being used in darkened conditions indoors or under specially constructed covered awnings and tents while working outdoors in daylight conditions or direct sunlight.
This limits certain nondestructive testing (NDT) inspection operations and makes others impossible altogether. Furthermore, for inline inspection in a continuous production environment, the extra time and resources necessary for operating under low-light conditions can cause serious production bottlenecks and are expensive in terms of manpower.
A U.K. company, Inspection Technologies Ltd., has developed a mixed-light system that eliminates these production bottlenecks and significantly increases throughput rates at inline inspection points. Furthermore, the system comes with software that completely automates the inspection process, which removes the need for expensive, highly qualified inspectors. After the automated inspection process is complete, visual photographic records of all inspections – pass or fail – are permanently stored for compliance and 100% quality audit purposes.
The system (Patent application #GB1610988.6) is shown in Fig. 1. It is comprised of a specialized camera, patented driver electronics and a self-contained UV light source.
The system allows for the effective detection of fluorescence, even in the presence of high levels of background illumination. It avoids the need for items to be moved indoors or covered with tents or awnings when performing NDT techniques using fluorescent dyes and completely avoids the need for dimly lit environments. Indeed, performance trials, which are the initial commercial embodiments of this system, have detected fluorescent radiation of 200 lux or lower in the presence of background illumination of over 100,000 lux.
For most common NDT inspections via fluorescent techniques, the excitation source is an ultraviolet (UV) light-emitting diodes (LEDs) radiation source. When, for example, the fluorescent material of interest is a fluorophore such as the commercially available Magnaflux® 14-HF (or similar) or Magnaflux® ZL-60C as for use in fluorescent magnetic-particle inspection (FMPI) and penetrant inspection (PI), then a suitable range of wavelengths for excitation radiation for such materials is between 350 nm and 450 nm.
In addition to optical techniques, the system also employs digital signal processing-assisted noise reduction, filtering and contrast-enhancement methods in order to provide a clearer-difference image in which any detected fluorescence can be more easily seen.
The final image, which eliminates all background light and shows only the part, is shown in real-time, thus allowing the user to visually identify an area in which the fluorescent material is present as a continuous-streaming video feed. Such an arrangement is advantageous in an NDT setting where the fluorescence corresponding to a crack or other defect can be detected as part of a dynamic or moving inspection process, such as walking while inspecting or a static in-line inspection of objects on a moving conveyor belt in a production setting.
The user interface for the system also contains automatic defect-detection software that identifies and sizes areas of interest and alerts the user. Optionally, as desired, the software can provide streaming real-time visual overlays of fluorescent regions of fluorescence and identify defects on top of ordinary visual images of the object under inspection. Moreover, all captured images, regardless of whether they contain detected defects or other regions of interest, are permanently stored. This has several advantages in terms of compliance and insurance issues in that it effectively provides a truly 100% quality audit.
Such an approach also makes it possible for technician-grade staff to be used in lieu of a more fully trained inspector. Also, the productivity of a qualified inspector could be increased dramatically if they were able to inspect remotely via telemetry by supervising or reviewing the work of technicians on the ground in multiple geographically separated sites.
Several industry-standard samples and test pieces were provided by Magnaflux® EMEAR. These were of a type that are normally used for quality-assurance purposes on fluorescent magnetic-particle dyes (used on ferrous metals) and fluorescent penetrant dyes (commonly used for the detection of defects associated with nonferrous metals). Figures 2, 3 and 4 show the performance of the mixed-light apparatus on these samples under laboratory conditions.
Figure 5 shows one configuration of the mixed-light system in which the scanning head is attached to a robot arm, which then performs either spot inspections at preset positions or a continuous sweep of the surface. Visual records of all inspections are permanently stored for compliance and quality-assurance purposes.
As the precise positions of the defects are recorded, then the software is also useful for process control should, for example, faults occur more frequently in some locations as opposed to others.
For more information: Contact Dr. Geoff Diamond of Inspection Technologies Ltd., 26 Adelaide Road, Royal Leamington Spa, Warwickshire CV31 3PL, United Kingdom; e-mail: firstname.lastname@example.org; web: www.inspectiontechnologies.co.uk