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The Aeronautical Sciences Project under NASA’s Fundamental Aeronautics Program is interested in the development of
novel measurement technologies, such as optical surface measurements for the in situ health monitoring of critical
constituents of the internal flow path. In situ health monitoring has the potential to detect flaws, i.e. cracks in key
components, such as engine turbine disks, before the flaws lead to catastrophic failure. The present study, aims to
further validate and develop an optical strain measurement technique to measure the radial growth and strain field of an
already cracked disk, mimicking the geometry of a sub-scale turbine engine disk, under loaded conditions in the NASA
Glenn Research Center’s High Precision Rotordynamics Laboratory. The technique offers potential fault detection by
imaging an applied high-contrast random speckle pattern under unloaded and loaded conditions with a CCD camera.
Spinning the cracked disk at high speeds (loaded conditions) induces an external load, resulting in a radial growth of the
disk of approximately 50.0-μm in the flawed region and hence, a localized strain field. When imaging the cracked disk
under static conditions, the disk will be undistorted; however, during rotation the cracked region will grow radially, thus
causing the applied particle pattern to be ‘shifted’. The resulting particle displacements between the two images is
measured using the two-dimensional cross-correlation algorithms implemented in standard Particle Image Velocimetry
(PIV) software to track the disk growth, which facilitates calculation of the localized strain field. A random particle
distribution is adhered onto the surface of the cracked disk and two bench top experiments are carried out to evaluate the
technique’s ability to measure the induced particle displacements. The disk is shifted manually using a translation stage
equipped with a fine micrometer and a hotplate is used to induce thermal growth of the disk, causing the particles to
become shifted. For both experiments, reference and test images are acquired before and after the induced shifts,
respectively, and then processed using PIV software. The controlled manual translation of the disk resulted in detection
of the particle displacements accurate to ~1.75% of full scale and the thermal expansion experiment resulted in
successful detection of the disk’s thermal growth as compared to the calculated thermal expansion results. After
validation of the technique through the induced shift experiments, the technique is implemented in the Rotordynamics
Lab for preliminary assessment in a simulated engine environment. The discussion of the findings and plans for future
work to improve upon the results are addressed in the paper.
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