Fantin Analucia V, Willemann Daniel P, Viotti Matias R, Albertazzi Armando
Laboratório de Metrologia e Automatização, Universidade Federal de Santa Catarina, Florianópolis, SC, Brazil.
Appl Opt. 2013 Aug 1;52(22):5460-8. doi: 10.1364/AO.52.005460.
Shearography is an optical and nondestructive technique that has been largely used for damage detection in layered composite materials where delaminations and debondings are found to be among the most common flaws. Shearography detects derivative of the displacements. It is a relative measurement in which two images are recorded for different loading conditions of the sample. The applied loading induces some deformations into the sample, generating a displacement field on its surface. Thermal, acoustical, or mechanical loading are typical excitations applied in a static or dynamic way. The absolute difference between two phase maps recorded at two different loading instances produces an interference fringe pattern, which is directly correlated to the displacements produced on the material surface. In some cases, depending on the loading level and mainly on the sample geometry, interference patterns will contain fringes resulting from geometry changes. This will mask those fringes correlated to flaws introduced into the material, resulting in an image misinterpretation. This phenomenon takes place mainly when the sample has curved geometries, as in, for example, pipe or vessel surfaces. This paper presents an algorithm that uses a mathematical process to improve the visualization of flaws in shearographic images. The mathematical process is based on the calculation of the phase variation, and it is used to search for local deformations contained in the image. This algorithm highlights defect regions and eliminates fringes caused by geometry changes, providing an easier interpretation for complex shearographic images. This paper also shows the principle and the algorithm used for the process. Results, advantages, and difficulties of the method are presented and discussed by using simulated fringe maps as well as real ones.
剪切电子散斑干涉术是一种光学无损检测技术，已广泛应用于层状复合材料的损伤检测，其中分层和脱粘是最常见的缺陷。剪切电子散斑干涉术检测位移的导数。它是一种相对测量方法，在样品的不同加载条件下记录两幅图像。施加的载荷会使样品产生一些变形，在其表面产生位移场。热、声或机械载荷是典型的以静态或动态方式施加的激励。在两个不同加载实例下记录的两幅相位图之间的绝对差值会产生干涉条纹图案，该图案与材料表面产生的位移直接相关。在某些情况下，根据加载水平，主要是根据样品的几何形状，干涉图案将包含由几何形状变化产生的条纹。这将掩盖与材料中引入的缺陷相关的条纹，导致图像误判。这种现象主要发生在样品具有弯曲几何形状时，例如管道或容器表面。本文提出了一种算法，该算法使用数学过程来改善剪切电子散斑干涉图像中缺陷的可视化。该数学过程基于相位变化的计算，用于搜索图像中包含的局部变形。该算法突出显示缺陷区域并消除由几何形状变化引起的条纹，为复杂的剪切电子散斑干涉图像提供了更易于解释的结果。本文还展示了该过程所使用的原理和算法。通过使用模拟条纹图和真实条纹图，展示并讨论了该方法的结果、优点和难点。
