Abstract-Terahertz optical thickness and birefringence measurement for thermal barrier coating defect location

 

Andrew J. Waddie, Peter J. Schemmel, Christine Chalk, Luis Isern, John R. Nicholls, and Andrew J. Moore

Schematic of experimental setup. M1 – flat mirror; P1 – linear polarizer; BS – Si beam splitter; M2 – parabolic mirror; L1 – PTFE lens; A1 – linear analyzer; PD – photodiode.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-21-31535

We present a normal incidence terahertz reflectivity technique to determine the optical thickness and birefringence of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). Initial verification of the method was achieved by measurement of a set of fused silica calibration samples with known thicknesses and showed excellent agreement (<1% of refractive index) with the literature. The THz-measured optical thickness and its variation through the depth profile of the YSZ coating are shown to be in good agreement (<4%) with scanning electron microscope cross-sectional thickness measurements. In addition, the position of discontinuities in both the optical thickness and birefringence appear to be correlated to coating failure points observed during accelerated aging trials.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

OT-Abstract-Monitoring stress changes in carbon fiber reinforced polymer composites with GHz radiation

Peter Schemmel and Andrew J. Moore

https://www.osapublishing.org/ao/abstract.cfm?uri=ao-56-22-6405&origin=search

We performed proof of concept experiments to demonstrate that the reflected power of GHz illumination from the surface of carbon fiber reinforced polymer (CFRP) composites is linearly related to the stress in the material. We introduce a stress coefficient to describe the change in normalized power with applied stress, analogous to the stress-optic coefficient, because the effect is attributed to changes in the refractive index of the effective medium comprising the polymer matrix and carbon fibers. Stress coefficients of −0.549±0.134/GPa$-0.549\pm 0.134/\mathrm{GPa}$ and −0.154±0.024/GPa$-0.154\pm 0.024/\mathrm{GPa}$ were measured for two different composite materials, both linear in the measurement range of 40 MPa and 100 Mpa, respectively. This technique opens up the possibility of non-destructive evaluation of stresses in CFRP components for quality assurance in manufacturing and in structural health monitoring of in-service aerospace and automotive parts.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.