Researchers with the Air Force Research Laboratory's (AFRL) Materials and Manufacturing Directorate at Wright-Patterson Air Force Base in Ohio recently completed a series of tests that enabled the use of a new measurement tool and quality assurance process for engine inlet production of the F-35—a single-seat, single-engine, stealth fighter jet. The tool, a terahertz coating thickness probe, is a non-contact, nondestructive device that allows users to measure coating thickness quickly and easily without risk of damage to coating surfaces.
One disadvantage of traditional, manual thickness measurement tools such as the eddy current method is that they require at least four hours of cure time before any coating thickness measurement can be made, and 48 hours of cure time for a final coating thickness measurement. These methods are also comparatively slow and labor intensive, can potentially damage coating surfaces, and are poor at producing repeatable and reproducible results when used on complex curved surfaces.
The terahertz coating thickness probe uses a high-frequency terahertz signal to penetrate materials. The change in refractive index between two adjacent layers causes some of the energy in the signal to reflect back toward the probe. The time-of-flight and strength of the reflected signal are measured to calculate the material thickness. The signal’s energy that is not absorbed or reflected continues into the next material layer, and the process repeats. Multiple layers generate multiple reflections, and the probe is able to calculate the thickness of each individual material layer in the stack-up. Because this measurement technique is not affected by subsurface features such as gaps and fasteners, it is a faster, more accurate, and more reliable approach than eddy current methods currently used.
The terahertz measurement technique does not require contact with the surface, and measurements can be made on wet coatings as the material is being applied. It produces high-resolution images, and accurately predicts the final, cured coating thickness within material tolerances.
AFRL conducted a thorough gauge reliability and reproducibility study of the terahertz coating thickness probe’s capability of accurately measuring the thickness of robotically sprayed coatings in F-35 inlet ducts. The study was very successful, showing a drastic improvement in reliability and reproducibility over the baseline manual eddy current method.
“These tests ensured that the terahertz coating thickness tool performed as expected, with repeatable and reliable results,” says Juan Calzada, AFRL project engineer. “This was essential in assuring the efficacy of this tool and its subsequent implementation in the manufacturing and quality assurance process.”
The completion of the AFRL testing effort led to a formal Manufacturing Readiness Level 7 assessment. Following the publication of new quality assurance procedures, the terahertz coating thickness measurement capability will be incorporated into the inlet production line.
Source: Holly Jordan, Air Force Research Laboratory, wpafb.af.mil.