Imaging surface deformation of a coupon specimen in micro-tensile testing with an optical microscope presents challenges due to the narrow depth of field (DoF) of optical microscopes. Materials being heterogeneous at microscopic length scale, the sample surface deforms into a complex 3D surface texture, evolving continuously as the loading increases. Because of the narrow DoF, the region that is in focus within the field of view (FoV) decreases substantially in size with the increasing out-of-plane heterogeneous deformation. To address this challenge, a method based on image blending and stabilization of the captured image frames is proposed. Image blending combines the partial regions that are in focus from a set of successive image frames captured at different working distances from the object surface plane to construct a single image that has a large part of the FoV in focus. The blended images are then obtained at different levels of macroscopic strains, i.e. the global homogeneous strain, in order to characterize the evolution of the heterogeneous deformation. The image stabilization removes any misalignments of the blended images by spatially realigning them choosing a common feature as a reference point. The validation of the proposed method with conventionally and additively manufactured stainless steel 316L (SS 316L) specimens demonstrates excellent improvement in image quality. Almost 100% of the FoV is maintained in focus regardless of the amount of out-of-plane heterogeneous deformation caused during tensile testing, which is quite remarkable for optical microscopy imaging. Consequently, the blended and stabilized images enhanced the accuracy of digital image correlation (DIC). Time-lapse videos of the deformation generated using these images captured the evolution of the slip bands and their transmission through twinning boundaries in the stainless steel microstructure. Overall, this study demonstrates the feasibility of using image-processing techniques to advance optical microscopy to image complex 3D surfaces evolving with time. This article is protected by copyright. All rights reserved.

中文翻译：

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用于表征微拉伸试验中微观结构变形的高分辨率光学显微镜

由于光学显微镜的景深 (DoF) 较窄，因此使用光学显微镜对试样在微拉伸测试中的表面变形成像提出了挑战。材料在微观长度尺度上是异质的，样品表面变形为复杂的 3D 表面纹理，随着载荷的增加不断演变。由于狭窄的 DoF，视野 (FoV) 内聚焦的区域的尺寸随着平面外异质变形的增加而显着减小。为了应对这一挑战，提出了一种基于图像混合和稳定捕获图像帧的方法。图像混合将在距物体表面平面不同工作距离处捕获的一组连续图像帧中聚焦的部分区域组合在一起，以构建具有大部分 FoV 聚焦的单个图像。然后在不同水平的宏观应变（即全局均匀应变）下获得混合图像，以表征异质变形的演变。图像稳定通过选择一个共同特征作为参考点在空间上重新对齐混合图像来消除混合图像的任何错位。使用常规和增材制造的不锈钢 316L (SS 316L) 试样验证所提出的方法表明图像质量得到了极大的改善。无论拉伸测试期间引起的平面外异质变形量如何，几乎 100% 的 FoV 都保持在焦点上，这对于光学显微镜成像来说非常显着。因此，混合和稳定的图像增强了数字图像相关（DIC）的准确性。使用这些图像生成的变形的延时视频捕获了滑带的演变及其通过不锈钢微观结构中孪晶边界的传输。总体而言，这项研究证明了使用图像处理技术推进光学显微镜对随时间演变的复杂 3D 表面进行成像的可行性。本文受版权保护。版权所有。混合和稳定的图像增强了数字图像相关（DIC）的准确性。使用这些图像生成的变形的延时视频捕获了滑带的演变及其通过不锈钢微观结构中孪晶边界的传输。总体而言，这项研究证明了使用图像处理技术推进光学显微镜对随时间演变的复杂 3D 表面进行成像的可行性。本文受版权保护。版权所有。混合和稳定的图像增强了数字图像相关（DIC）的准确性。使用这些图像生成的变形的延时视频捕获了滑带的演变及其通过不锈钢微观结构中孪晶边界的传输。总体而言，这项研究证明了使用图像处理技术推进光学显微镜对随时间演变的复杂 3D 表面进行成像的可行性。本文受版权保护。版权所有。这项研究证明了使用图像处理技术推进光学显微镜对随时间演变的复杂 3D 表面进行成像的可行性。本文受版权保护。版权所有。这项研究证明了使用图像处理技术推进光学显微镜对随时间演变的复杂 3D 表面进行成像的可行性。本文受版权保护。版权所有。