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

聚合物和复合材料内部位移和应变场的测量在技术上非常具有挑战性，但对于改进损伤和失效模型的发展至关重要。三维应变测量的主要技术之一中子衍射通常不适用于这些类型的材料，而且空间分辨率很差（通常为1毫米或更低）。在这个项目中，我们的目标是开发一种称为波长扫描干涉法（WSI）的光学技术，以测量干涉精度（~ 10 nm）和空间分辨率为0.01 mm的体积三维位移场。该技术使用了过去20年来在散斑干涉测量界发展起来的相位测量（包括相位展开）能力，但波长扫描方法提供了体场，而不是传统散斑干涉测量对表面场的限制。在多相机原型仪器的构建和相体积重建和注册软件的开发之后，将通过使用标准加载几何形状的均质聚合物样品以及包含嵌入式光纤应变传感器的更现实的材料（玻璃纤维复合材料）来实现验证。验证后，该系统将应用于测量具有一系列受控损伤状态的复合材料样品的体积位移和应变场。在光学系统开发和验证的同时，一个数值工作程序将侧重于损坏样品的有限元建模（FEM），以及一种称为虚拟场法（VFM）的新型“逆有限元分析”技术的实现。VFM最近已扩展到三维，并允许从全场位移数据直接计算模量分布（这是开发改进的损伤力学模型的关键），而不是FEM所要求的迭代方法。然而，与FEM相比，VFM相对不成熟；因此，使用WSI的实验数据对两种方法进行并排比较对于VFM作为结构工程师的工具的未来发展至关重要。由于这个项目，实验确定的这些材料损伤的高分辨率三维地图将首次可用，以及这种损伤对受损区域承载能力的影响。这些数据对于这些日益重要的结构材料的坚固性，物理上准确的强度和寿命预测的发展至关重要。

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.