Large-scale non-contact strain measurement using diffraction of optical grating arrays
Year: 2025
Type: dissertation
Abstract: Strain measurements allow to derive information about the condition of a sample or technical part. Applications include deformation analysis in material development, measuring wear for predictive maintenance in engineering, and structural health monitoring in the construction sector. Noncontact strain gauges are used when wiring is impossible or an inference with the result is experienced. An example is moving parts like blades of wind power plants. However, conventional non-contact strain gauges like video extensometers require time-consuming adjustment and do not allow absolute measurements over long observation periods. This thesis presents a concept for a non-contact strain gauge that inherits the advantages of a standard non-contact strain gauge but is also capable of fast adjustment and long-term use. The concept is based on tilted optical grating arrays, which show a strain-dependent spectral response when illuminated. An optical design of the grating arrays is derived based on requirements from application use cases. For this, a simulation environment is created to predict the behavior and resolution of a specific structure design. The simulation is applied to various configurations, and parameter regions of interest are defined for the tilting angle and the grating pitch. Fabrication concepts for tilted optical grating arrays by two-photon polymerization are presented in this thesis. New fabrication strategies superior to conventional techniques in shape fidelity and fabrication time are investigated. They include customized writing trajectories and dynamic power variation, achieved by developing algorithms for splitting input geometries and trajectory planning. Deviations of the printing process are discussed, and strategies for minimal stitching errors and shrinkage compensations are derived. The transfer via molding of the tilted grating arrays to a stretchable material is presented. A demonstrator is constructed from the three main parts needed for the measurement principle: a light source, a detector, and the cast of the tilted optical grating array. The demonstrator can apply strain to the cast and change the angles of the individual components. Validation measurements are made for the angular and strain spectral responses. The theoretical uncertainty of different configurations and the achieved uncertainty of the demonstrator are presented. The simulation is applied to the configuration and supports the results. A self-calibration structure for angular adjustment is developed and successfully verified. While this thesis successfully verifies the principle of a demonstrator under laboratory conditions and states solutions for questions arising in applications such as directional strain measurement, an outlook is given on the necessary steps for achieving an industrially usable grating strain gauge.
Source: RWTH Publications (RWTH Aachen)
Author: Mathwieser, Andreas
Institution: RWTH Aachen University
Language: English
Subfield: Computer Vision and Pattern Recognition
Field: Computer Science
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7107867754
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