Sub-microscale speckle pattern creation on single carbon fibers for scanning electron microscope-digital image correlation (SEM-DIC) experiments
Sub-microscale speckle pattern creation on single carbon fibers for scanning electron microscope-digital image correlation (SEM-DIC) experiments
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DOI：
10.1016/j.compositesa.2022.107331
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发表时间：
2022-11
期刊：
影响因子：
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通讯作者：
Karan Shah;S. Sockalingam;H. O'Brien;G. Yang;Mohammad El Loubani;Dongkyu Lee;M. Sutton
中科院分区：
文献类型：
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作者：
Karan Shah;S. Sockalingam;H. O'Brien;G. Yang;Mohammad El Loubani;Dongkyu Lee;M. Sutton
高性能碳纤维广泛用作航空航天、汽车和国防应用的复合材料系统中的纤维增强材料。这种复合材料系统的纵向拉伸失效是在微尺度上（大约几微米到几百微米）发生的单纤维拉伸失效的聚集的结果。由于在微米尺度下的纤维拉伸强度对复合材料强度具有一阶效应，因此重要的是表征在微米尺度标距长度下的单纤维的强度，这是极其具有挑战性的。基于SEM下单纤维横向加载与DIC相结合的实验技术是一种潜在的方法来访问微尺度标距长度。SEM-DIC技术需要在弯曲的光纤表面上产生均匀、随机和对比的亚微米级散斑图案，以进行精确的应变测量。在本文中，我们研究了通过溅射涂层在标称直径为5.2 μm的单个尺寸的IM 7碳纤维上形成这种亚微米级散斑图案。研究了不同的工艺条件，如工作压力、溅射电流和镀膜时间，以金-钯（Au-Pd）靶在光纤表面形成图案。在不同的涂覆条件下，在光纤表面获得了纳米团簇型亚微米图案。数值平移实验使用所获得的图案来研究图像的相关性，并确定一个合适的模式SEM-DIC实验。在120-140 mTorr的工作压力下，50 mA电流持续10 min获得的图案被发现具有53 nm的平均散斑尺寸和良好的图像相关对比度。使用涂覆有最佳图案化条件的尺寸IM 7碳纤维进行漂移/变形校正的刚体平移SEM实验表明，由于其弯曲表面，需要Stereo-SEM-DIC来准确表征纤维应变场。通过单纤维拉伸试验研究了溅射镀膜对纤维拉伸强度和应变的影响。结果表明，在5%的显著性水平下，未涂覆和涂覆纤维之间的平均拉伸强度和失效应变没有显著差异（由于涂覆，纤维直径的平均增量为1.221 nm）。未涂覆和涂覆的纤维的失效表面的SEM图像也证实了纤维的拉伸失效，如文献中对聚丙烯腈PAN基纤维所观察到的。
High performance carbon fibers are widely used as fiber reinforcements in composite material systems for aerospace, automotive, and defense applications. Longitudinal tensile failure of such composite systems is a result of clustering of single fiber tensile failures occurring at the microscale, on the order of a few microns to a few hundred microns. Since fiber tensile strength at the microscale has a first order effect on composite strength, it is important to characterize the strength of single fibers at microscale gage lengths which is extremely challenging. An experimental technique based on a combination of transverse loading of single fibers under SEM with DIC is a potential approach to access microscale gage lengths. The SEM-DIC technique requires creation of uniform, random, and contrastive sub-microscale speckle pattern on the curved fiber surface for accurate strain measurements. In this paper, we investigate the formation of such sub-microscale speckle patterns on individual sized IM7 carbon fibers of nominal diameter 5.2 µm via sputter coating. Various process conditions such as working pressure, sputtering current, and coating duration are investigated for pattern creation on fiber surface using a gold-palladium (Au-Pd) target. A nanocluster type sub-microscale pattern is obtained on the fiber surface for different coating conditions. Numerical translation experiments are performed using the obtained patterns to study image correlation and identify a suitable pattern for SEM-DIC experiments. The pattern obtained at a working pressure of 120–140 mTorr with 50 mA current for a duration of 10 min is found to have an average speckle size of 53 nm and good contrast for image correlation. Rigid body translation SEM experiments for drift/distortion correction using a sized IM7 carbon fiber coated with the best patterning conditions showed that Stereo-SEM-DIC is needed for accurately characterizing fiber strain fields due to its curved surface. The effect of sputter coating on fiber tensile strength and strain is investigated via single fiber tensile tests. Results showed that there is no significant difference in the mean tensile strength and failure strain between uncoated and coated fibers (average increment in fiber diameter of ∼221 nm due to coating) at 5% significance level. SEM images of failure surfaces for uncoated and coated fibers also confirmed a tensile failure of fibers as observed for polyacrylonitrile PAN-based fibers in literature.