Co-authors​ :

 Wooseok JI (KOREA, REPUBLIC OF), Chaeyoung HONG (KOREA, REPUBLIC OF) 

Synchrotron radiation computed tomography (SRTC)-based in situ testing has emerged as a powerful tool for characterizing complex failure mechanisms occurring inside fiber-reinforced composite materials. Such damage evolution can now be visualized in the form of three-dimensional images. The computed tomography (CT) data are even utilized to compute 3D deformation and strain fields through a digital volume correlation (DVC) technique, which provides unprecedented information about internal material behavior. However, DVC requires extremely high-resolution CT images to track either natural or artificial microscale features inside a sample. This requirement inherently limits the DVC technique to a small scanning area and often disconnects such DVC results from the global response of an entire composite structure. Moreover, because CT scanning should be performed at a specific load level with a loading frame temporarily being paused, the temporal resolution of DVC cannot be as fine as that of a traditional digital image correlation (DIC) technique. This coarse temporal resolution impedes the real-time monitoring of dynamic changes occurring inside a material.
In this study, we performed SRCT-based in situ experiments together with CCD cameras to obtain subsurface and surface deformation data simultaneously. The overall test setup was almost the same as our previous in situ experiments with a slight modification and a few additions. The loading frame cap was newly fabricated with a transparent material in order to take pictures of speckle patterns on the sample surface. Two CCD cameras with 0.5x and 2.0x lenses were placed to capture different ranges of the sample. The two cameras were combined with a special prism device and a blue diffused light was used to prevent possible visible light noise and reflections.
From the test, high-resolution 3D CT images of the sequentially loaded sample were obtained and used for digital volume correlation (DVC) calculations. The DVC-calculated volumetric strain fields were overlayed on the CT-reconstructed microstructure. They were primarily utilized for damage detection based on the deformation behavior of the constituents or localized strain concentration. DVC analysis detected significant deformations in the regions of each fracture mode. The surface images taken by the CCD cameras were utilized to compute strain data of the surface through a conventional DIC technique. The DIC and DVC data were then synchronized to characterize the global and local behavior of the composite because the DIC analysis was performed over a wider region than the DVC area. The synchronization results are expected to provide a new insight into the global and local behavior of the fiber reinforced composite material as well as a basis for the development of reliable and accurate multiscale models.