AFRL demonstrates improved measurement capabilities for aircraft engine inlets

WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- AFRL Materials and Manufacturing Directorate researchers recently completed a series of tests that are enabling the use of a new measurement tool and quality assurance process for F-35 inlet production.

Now users can measure for proper thickness of inlet material coatings quickly and easily without risk of damage to coating surfaces.

The Terahertz Coating Thickness probe is a non-contact, nondestructive approach that uses a high-frequency terahertz signal to penetrate materials and allow the measurement of material thickness. The change in refractive index between two adjacent layers causes some of the energy in the signal to reflect back toward the probe. Users can measure the time-of-flight and strength of the reflected signal to calculate the material thickness. The energy of the signal that is not absorbed by the medium and is not reflected by the boundary continues into the next material layer, and the process repeats.  Multiple layers generate multiple reflections across the received signal, allowing the user to calculate the thickness of each individual material layer in the stack-up.

This process can be automated using a simple, easy-to-use machine/human interface to provide quick and easily interpretable results in real time.  Additionally, 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 the currently-used eddy current Fischerscope tool. 

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.

Conversely, 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.

To achieve this testing effort, AFRL conducted a thorough gauge reliability and reproducibility study of the Terahertz Coating Thickness probe capability to accurately measure 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 Fischerscope method. 

“These tests ensured that the terahertz coating thickness tool performed as expected, with repeatable and reliable results,” said 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 the achievement of 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.