3-D strain field mapping of scattering media using Wavelength Scanning Interferometry with application to damaged composites

Lead Research Organisation: Loughborough University
Department Name: Sch of Mechanical and Manufacturing Eng


The measurement of displacement and strain fields within polymers and composites is technically very challenging, yet is vital for the development of improved damage and failure models. One of the main techniques for 3-D strain measurement, neutron diffraction, is not generally applicable to these types of material and furthermore has poor spatial resolution (typically 1 mm or worse). In this project we aim to develop an optical technique, called wavelength scanning interferometry (WSI), to measure volume 3-D displacement fields to interferometric precision (~ 10 nm) and with spatial resolution of order 0.01 mm. The technique uses the phase measuring (including phase unwrapping) capabilities developed in the speckle interferometry community over the past 20 years but the wavelength scanning approach provides volume fields as opposed to the restriction to surface fields imposed by traditional speckle interferometry. Following construction of the multi-camera prototype instrument and development of phase volume reconstruction and registration software, validation will be achieved through the use of homogeneous polymeric samples in standard loading geometries, and with more realistic materials (glass fibre composites) containing embedded optical fibre strain sensors. After validation, the system will be applied to the measurement of volume displacement and strain fields within composite samples prepared with a range of controlled damaged states.In parallel to the optical system development and validation, a numerical program of work will focus on finite element modelling (FEM) of the damaged samples, and implementation of a novel 'inverse finite element analysis' technique called the virtual fields method (VFM). The VFM has recently been extended to three dimensions and allows distributions of modulus - the key to developing improved damage mechanics models - to be calculated directly from full-field displacement data, as opposed to the iterative approach required by FEM. The VFM is however relatively immature compared to FEM; a side by side comparison of the two approaches using experimental data from WSI is therefore essential to the future development of the VFM as a tool for structural engineers. As a result of this project, experimentally-determined high-resolution 3-D maps of the damage in these materials will be available for the first time, together with the effect of this damage on the load bearing capability of the damaged area. This data is essential for the development of robust, physically accurate strength and lifetime prediction for these increasingly important structural materials.
Description A commercially-available Ti:Sapphire laser with > 100 nm scan range has been customized to allow high speed scans of several tens of thousands of frames at rates of up to 30 frames s-1, with variable exposure time to compensate for wavelength variation of laser power output and camera sensitivity. The complete opto-mechanical structure to implement the proposed six-axis interferometer was also developed, including custom loading stage at the centre of the measurement volume.

The laser, which was selected for its wide tuning range in the near infra-red range (thus allowing low-cost CCD and CMOS image sensors to be used), has one drawback which is the mode hops that it undergoes during a wavelength scan. A method was therefore developed based on recording interferograms of multiple wedges to provide simultaneously high wavenumber resolution and immunity to the ambiguities caused by large wavenumber jumps. All the data required to compute a wavenumber shift are provided in a single image, thereby allowing dynamic wavenumber monitoring. In addition, loss of coherence of the laser light is detected automatically. With this approach, the depth-resolution was improved by a factor of more than 100 times, and was found to approach the theoretical limit for scan ranges of up to 37 nm.

In parallel with the optical system development, a numerical technique based on the virtual fields method (VFM) was developed for the calculation of spatial distributions of material properties from experimentally determined displacement fields. The novel feature here is the proposal for a Fourier-series-based extension to the VFM (the F-VFM), in which the unknown stiffness distribution is parameterised in the spatial frequency domain rather than in the spatial domain as used in the classical VFM. An efficient numerical algorithm based on the 2-D and 3-D Fast Fourier Transform (FFT) has been developed, which reduces the computation time by three to four orders of magnitude compared with a direct implementation of the F-VFM for typical experimental dataset sizes.
Exploitation Route Although it was not possible to achieve the anticipated depth-resolved displacement measurements in composites, this is still seen as the main potential application for the technology developed on the project. People developing absolute distance measuring systems may also find application for the results and techniques developed. The F-VFM algorithm is a particularly efficient one which people acquiring data in different fields and with different experimental techniques (e.g. medical with MRI or CT) may benefit from.
Sectors Aerospace, Defence and Marine,Healthcare,Manufacturing, including Industrial Biotechology

Description Loughborough University
Amount £34,830 (GBP)
Organisation Loughborough University 
Sector Academic/University
Country United Kingdom
Start 10/2011 
End 09/2013