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位移和应变场的测量在技术上非常具有挑战性，但对于开发改进的损伤和失效模型至关重要。用于3-D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，而且具有差的空间分辨率（通常为1mm或更差）。在这个项目中，我们的目标是开发一种光学技术，称为波长扫描干涉术（WSI），测量体积三维位移场的干涉精度（~10 nm）和空间分辨率为0.01 mm量级。（包括相位展开）但是波长扫描方法提供了体积场，而不是对表面的限制传统的散斑干涉测量法所施加的场。多相机原型仪器的建设和相体积重建和注册软件的开发，验证将通过使用均匀的聚合物样品在标准的加载几何形状，并与更现实的材料（玻璃纤维复合材料）包含嵌入式光纤应变传感器。经过验证后，该系统将被用于测量复合材料样品的体积位移和应变场，这些样品是在一系列受控的损伤状态下制备的。在开发和验证光学系统的同时，一个数值工作程序将集中在损伤样品的有限元建模（FEM）上，并实施了一种新的“逆有限元分析”技术，称为虚拟场法（VFM）。VFM最近已扩展到三维，并允许分布的模量-关键是开发改进的损伤力学模型-直接从全场位移数据计算，而不是有限元所需的迭代方法。然而，VFM是相对不成熟的相比，有限元法，并排比较两种方法使用的实验数据从WSI因此是必不可少的未来发展的VFM作为一种工具的结构工程师。由于这个项目，实验确定的高分辨率三维地图，在这些材料中的损害，连同这种损害的影响，对受损害地区的承载能力将是第一次。这些数据对于为这些日益重要的结构材料开发可靠的、物理上准确的强度和寿命预测至关重要。

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.