Penetrant Inspection

What is it?

Fluorescent dye penetrant inspection (FPI) is one of the oldest NDT methods in existence. It is an excellent means of revealing surface-breaking cracks and voids as small as a few microns in length. However, the traditional visual FPI method is typically labor-intensive and time-consuming, restricted to applications that allow operators easy access to large parts, subjective, operator-dependent and not well suited to automation.

LTC has developed an innovative means of obtaining high-resolution images of features such as surface-breaking cracks on critical part surfaces using an automated fluorescent penetrant inspection method. LTC’s patented Laser Scanned Penetrant Inspection (LSPI™) probes, supported by our Laser MicroMap™ system, can be used to generate digital images of features on critical part surfaces, including tubing, flat plates, or complex surfaces. Using the LSPI™ technology NDT professionals can now remotely and automatically locate and display surface-breaking cracks that are less than 0.039 inch (1 mm) in length, in near real-time.

Laser scanned penetrant inspection is one of the technologies used in by LTC's Laser MicroMap™ system, supported by our LaserViewer™ software.

Advantages of LSPI™ include:

Applicable to small diameter tubes, bolt-holes, and internally threaded parts

Automated

Real-time displays

Complete digital record for comparison between inspections

How does it work?

As in typical fluorescent penetrant inspection (FPI) techniques, LSPI™ first requires that a liquid fluorescent penetrant be applied to a clean part surface. After allowing the penetrant to adsorb into features such as cracks and laps, the excess penetrant is rinsed from the surface. Next, the surface is exposed to UV light, causing any trapped penetrant to fluoresce. Traditional FPI methods use a divergent light source, such as a hand-held UV lamp, to excite the penetrant. Observation of the fluorescent indications is conducted either visually or, more recently, with a CCD camera. LTC’s laser-scanning FPI probes employ a focused violet laser beam and a single-element photodetector in a confocal arrangement. This configuration concentrates a large amount of the laser energy on small features such as intergranular stress corrosion cracking.

By translating the focused laser spot over a treated surface, either in a helical path for a tube scan or with a raster motion for a flat plate scan, the LSPI™ sensor generates a high-resolution image of penetrant-holding features such as cracks and laps. The resolution of the image is a function of the size of the focused laser spot, the speed at which the sensor is moved, and the data sampling rate. With a confocal configuration, it is possible to produce spot sizes as small as 0.001 inch (0.025 mm), enabling operators to achieve high-resolution imaging of small cracks in part surfaces that are otherwise very difficult to inspect using conventional visual methods.

Applications

Crack detection in aircraft

Crack detection in tubes

For more information

Also see our Technologies page for other laser-based NDT and QC methods.