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HIGH-RESOLUTION SURFACE DEFECT MEASUREMENT FOR SHOP FLOOR ENVIRONMENTS
By Mike Zecchino
Production damage, defects, service wear and corrosion can compromise performance and potentially lead to catastrophic failure. Determining their severity is critical to ensuring component performance and safety. A pit or scratch tens of microns deep may be sufficient to cause early part failure, particularly in high-speed machinery.
Similarly, the depth of fine features such as dot peening or laser marking must also be well controlled. Markings that are too shallow will wear off and lose part serialization. Markings that are too deep can compromise part longevity.
However, inspectors are limited in measurement options when quantifying defects. The most common inspection methods are to visually compare a defect to features of known quality on a coupon, or to use a scribe gauge to attempt to determine depth. These methods are neither precise nor repeatable, and are subject to interpretation. They are of limited use for assessing large areas, and for measuring difficult areas under flanges, inside of bores and between blades.
Higher resolution metrology systems, such as optical and stylus profilometers, are also widely used for dimensional and roughness analysis. These systems, though, can be expensive, slow and too susceptible to environmental factors for use on the shop floor. They are typically mounted in workstations that limit their ability to measure large components or hard-to-reach areas.
Polarized Structured Light
Polarized Structured Light technology is based on interferometry, in which a light beam is split and directed to a test surface and to a high-quality reference surface. When recombined, the beams form a pattern that indicates the differences that exist between two surfaces.
One surface is then shifted relative to the other, and multiple images are obtained. From this series of measurement frames, the test surface’s shape is determined with nanometer level resolution. But the method is susceptible to vibration and is only suitable for use in a controlled metrology laboratory.
Dynamic Interferometry enables measurement despite vibration. In a dynamic interferometer, the light beam is split into phases, such that all measurement data can be acquired simultaneously. Acquisition takes tens of microseconds, enabling the instrument to measure accurately in noisy environments.
Polarized Structured Light (PSL) combines the vibration-immunity of dynamic interferometry with a vertical range of several millimeters. The non-contact technique provides multiple benefits for measuring defects and fine features. A handheld, PSL gauge can inspect difficult to reach areas. This approach can be used for measuring pits, scratches, nicks, dents and bumps on flat or curved surfaces (Figure 1).
Figure 1: Measuring a pit on the edge of a turbine blade. The black-tipped shaft is an alignment aid that sets the gauge at the correct measurement distance
In addition to defects, fine surface features can also be precisely characterized. A PSL gauge can measure the depth of laser markings or dot peen strikes to ensure compliance with specification. Rivet depth and shape, the radius of small corners, and other precision features can also be assessed.
While the PSL technology allows for instantaneous surface measurements, for the information to be useful the instrument must extract relevant information for the operator. The analysis software has automatically removed the overall shape of the surface, enabling accurate measurement of pits and peaks. The operator can set thresholds such that only pits below a certain depth (or peaks above a given height) are analyzed, as shown on the right. The table shows the maximum depth, area, volume, aspect ratio and other key metrics for each found feature. Software, such as this, provides an inspector with repeatable height values, without estimation based on manually manipulated data and 2- dimensional cursors.
Mike Zecchino is Manager of Technical Communications for 4D Technology Corporation. For more information, contact email@example.com or visit www.4dinspec.com