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

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.