Embedded Optical Waveguide Strain Sensor for Through-Thickness Measurement
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Solution Overview
Problem
Current strain measurement techniques in laminated materials are limited to in-plane directions, unable to measure strain along the orthogonal or out-of-plane direction, which is crucial for detecting structural defects like delamination and understanding the structural behavior of composite materials.
Innovation Solution
A method using two planar optical waveguides with Bragg gratings embedded between layers of laminated materials, interrogated with TE and TM polarized light to extract through-thickness strain components, enabling triaxial strain sensing along three perpendicular directions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If existing electronic or optical strain measurement techniques are used, then in-plane strain measurement is achieved, but out-of-plane strain measurement capability is lost
Solution Approach 1:
The patent transitions from planar in-plane strain measurement to three-dimensional strain measurement by embedding waveguides between laminate layers. This dimensional change enables measurement of out-of-plane strain components (εz) that were previously inaccessible, achieving triaxial strain sensing capability while maintaining optical measurement advantages.
Solution Approach 2:
The patent embeds the optical waveguide sensor structure within the laminated composite material itself, nesting the measurement device inside the structure being measured. This allows the sensor to experience the same strain field as the material, enabling accurate internal strain measurement without external attachment.
2Ease of operation
If optical fibers are embedded within laminated material for strain sensing, then remote monitoring and distributed strain sensing are enabled, but out-of-plane strain measurement remains unattainable
Solution Approach 1:
The patent reorients the waveguide structure and Bragg grating alignment to be sensitive to out-of-plane strain components. By configuring the waveguide plane parallel to the laminate layers and the Bragg grating vector perpendicular to the waveguide propagation direction, the system achieves sensitivity to εz while maintaining optical interrogation capabilities for remote monitoring.
Solution Approach 2:
The patent utilizes polarization-dependent Bragg wavelength shifts in the waveguide grating to differentiate between in-plane and out-of-plane strain components. By measuring wavelength shifts for both TE and TM polarizations and analyzing their differential response, the system extracts out-of-plane strain information while maintaining remote monitoring functionality.
3Measurement precision
If metal foil gauge sensors are placed on the surface, then discrete location strain measurement is achieved, but embedded measurement and delamination detection are compromised
Solution Approach 1:
The patent embeds the optical waveguide sensor between laminate layers, nesting it within the structure rather than placing it on the surface. This embedded position allows the sensor to detect strain changes at the laminate interfaces, enabling reliable delamination detection while maintaining precise strain measurement capability at specific locations.
Solution Approach 2:
The patent replaces mechanical contact-based strain gauges with optical waveguide-based sensing. This substitution eliminates the need for surface attachment and electrical connections, allowing true embedded measurement that can detect both strain and delamination events through optical parameter changes in the waveguide.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach allows for accurate measurement of strain in all three orthogonal directions, improving the detection of structural defects and overall understanding of composite material behavior under loading conditions, reducing over-engineering and enabling lighter, more efficient components.
Implementation Method 1
a Bragg grating in the waveguiding core, the optical propagation direction of the first planar optical waveguide being non-parallel to the optical propagation direction of the second planar waveguide
Implementation Method 2
interrogating the Bragg grating of the first planar optical waveguide with transverse electric (TE) polarized light and with transverse magnetic (TM) polarized light to obtain a TE spectral response of the Bragg grating for the TE polarized light and a TM spectral response of the Bragg grating for the TM polarized light
Implementation Method 3
the strain sensor comprises a first planar optical waveguide and a second planar optical waveguide, each of the first planar optical waveguide and the second planar optical waveguide having a waveguiding core defining an optical propagation direction parallel to the plane of the laminated material
Data Source
AI summary
A method of measuring strain includes providing laminated material having ply layers, and a thickness along a direction orthogonal to the ply layers, and a strain sensor embedded between adjacent ply layers, wherein: the strain sensor includes first and second planar optical waveguide, each of the waveguides having a waveguiding core defining an optical propagation direction parallel to the laminated material and a Bragg grating in the waveguiding core, the optical propagation directions of the optical waveguides being non-parallel; interrogating the first optical waveguide Bragg grating with transverse electric (TE) and transverse magnetic (TM) polarized light, to obtain a TE spectral response and a TM spectral response; interrogating the second optical waveguide Bragg grating with TE and TM polarized light to obtain a TE spectral response and a TM spectral response; and processing the TE spectral responses and the TM spectral responses to extract a through-thickness component of strain.