## 3 Testing Program
### Digital Image Correlation  
Digital image correlation (DIC) is a full-field displacement measuring approach that tracks the physical points of a speckle pattern on the surface of a specimen under deformation. Developed by [PERSON] et al. [(20)] and [PERSON] et al. [(21)], it is widely applied to experimental stress analysis [(22; 23; 24)]. For each subset region of a sample, the corresponding deformed position is found by searching in the vicinity that renders the correlation coefficient with the maximum likelihood or minimum cross-correlation function [(23; 24)]. Commercial software VIC 3D-7 was used for measurement of crack density, spacing, and damage evolution [(21; 25; 26; 4)]. Formation of a network of cracks and local strain fields are shown in Figures 9 and 10. The relative displacements of two points, as well as crack width and spacing parametersmeasured using the DIC, was compared with the LVDTs in Figure 9a,b. The DIC absolute and relative displacements along two horizontal segments were obtained at 10 s intervals and compared with the mean LVDT responses. The correlationwas close, as shown in Figure 9b, which validates the non-contacting DIC method since it provides a full-range response. The width of each crack was tracked from initiation to development to saturation stages and represented in Figure 10 showing the sequential formation of nine distributed cracks propagating throughout the width and observed as a function of time of a representative specimen. This data was post-processed to generate the crack width and spacing response up to the failure, as shown in Figure 10a,b representing the contour of longitudinal V displacement versus Y location for a MAC 4% (replicate 1) when all the cracks were formed. Each crack was marked as a discontinuity in displacement field, V(x), along with the Y location of the sample. The displacement discontinuity was measured as the crack width, as shown, and the crack spacing was marked as the distance along coordinate Y between any two cracks, as shown in Figure 10c. Experimental stress versus time was compared with the crack formation, propagation, and widening, as shown in Figure 11a, using the individual crack openings measured using DIC post-processing. Results indicated that not all cracks were active at any given time during the loading history and the definition of strain may be significantly dependent on the gage length and the specific region of the specimen. Note that some of the cracks formed and then remained dormant before they opened further during subsequent loading stages. Figure 9: (**a**) Sample with LVDT mounted on sides. (**b**) DIC vs. LVDT displacement correlation. Figure 11: (**a**) Sequence of formation and individual stress–crack width response for MAC 4%, and (**b**) tensile stress–strain and crack-spacing response. Figure 10: (**a**) Distribution of longitudinal strain is reported at distinct time steps, (**b**) DIC V displacement contour showing multiple crack formation at saturation stage, and (**c**) crack width and crack spacing estimation. As shown in Figure 11a, Cracks 1, 5, and 3 developed early on within the first 100 secs. of the test and widened within the time range of 250 to 400 secs. toreach a maximum 0.4 mm opening at the ultimate strength. However, Cracks 6 and 9 openedwhile the sample had reached its maximum stress capacity and the remaining cracks developed at a saturation crack width. The development of new cracks when the sample approachedmaximum stress indicates that multiple cracking stages of the overall composite was ending and the sample response was approaching the saturation stage. After the saturation stage, the majority of the cracks opened uniformly, indicating that the fiber phase was the primary load-carrying component. A stable crack spacing at this point and increased strains resulted in crack widening during the last stage offailure by fiber pullout. The crack spacing was measured as the distance between two cracks, as marked on the contour.