To obtain the loading rate in a brittle fracture process, a small metallic cylinder was embedded into the central hole during 3D-printing. Given the large size of the sample and no requirement for a specific scattering geometry, the technology is applicable for the microstructure and elasto-brittle crack observation and investigation of the crack initiation process, even with a complex geometry. In addition, the weak interface between the substrate and the optically transparent sample prevents the influence of residual stress in the sample.
The current technique does not require any modification to sample to obtain the strain patterns, and hence is suitable for fatigue testing at the tip of the crack. The strain-optical patterns of the pre-crack and post-crack configurations are acquired using identical excitation condition to compare the stress fields before and after the crack propagation. In principle, the stress at the tip of the crack can be visualized using the conventional strain-optical method. The post- and pre-crack configurations are first matched by maximizing the surface-to-surface distance through an iterative process using the phase retardation maps. The near-tip stress is also quantified by acquiring the post-crack phase retardation and calculating the equivalent stress at the tip of the crack. In order to acquire the crack tip position, the transparent sample is sliced along the cross-sectional axis and its thickness profile is obtained by measuring the phase retardation with a CCD camera.
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