## 2 Open-Hole Compression Testing
It is worth noting that 3D DIC measurements were used in this work, despite using only in-plane strain components for inverse characterization in the FEMU approach. One of the most prominent reason for using 3D DIC measurement instead of 2D DIC was verification that the anti-buckling Plexiglas plates were working as intended. Under compression loading, ILT stresses develop at the 12 and 6 o'clock locations around the hole and initiate delamination failure. After initiation, delamination progresses unstably towards the specimen extremities. The OHC coupon is oriented along the vertical loading direction, as shown in Figure 2. Figure 4a shows an example of interlaminar normal strain distribution on the surface of a unidirectional IM7/8552 OHC specimen measured prior to failure using the DIC technique and showing ILT strain concentrations around the hole. Figure 4b shows the delamination failure that initiates upon further loading at the 12 and 6 o'clock locations due to interlaminar tension. Figure 3: An example of speckle pattern created on the surface of an OHC test specimen and region of interest for digital image correlation (DIC) analysis. Figure 5 shows the full-field DIC strain data measured in a generic composite fabric OHC specimen considered in this work, with contour plots for the interlaminar normal \(\varepsilon_{33}\), fiber-direction normal \(\varepsilon_{11}\), and interlaminar shear strain \(\gamma_{13}\) fields, respectively. Due to small specimen thickness compared to the other dimensions and relative out-of-plane compliance of the anti-buckling device, deformation of the OHC coupons can be considered under a plane-stress approximation. Figure 4: An example of (**a**) DIC-measured ILT strains prior to failure and (**b**) ILT delamination failure initiating at the 12 and 6 o’clock locations around the hole in a unidirectional IM7/8552 OHC specimen. Figure 5: Example of full-field DIC strain measurement in a fabric OHC specimen considered in this work with contour plots of (**a**) transverse through-thickness \(\varepsilon_{33}\), (**b**) longitudinal fiber-direction \(\varepsilon_{11}\), and (**c**) interlaminar shear \(\gamma_{13}\) strains, respectively. As shown in Figure 5, the longitudinal and shear strains at the 12 and 6 o'clock locations are several orders of magnitude lower than the maximum through-thickness tensile (ILT) strains, validating the assumption of ILT delamination failure under pure mode I conditions. It is worth noting that a concentration of longitudinal fiber-direction compressive stresses also develops at the three and nine o'clock locations, as well as interlaminar shear stresses. Alternate failure modes, including fiber-direction compressive failure and interlaminar shear failure, could occur depending on the combination of material constitutive anisotropy, material strength properties, and coupon geometry. However, both for the IM7/8552 tape, generic carbon/epoxy tape, and carbon/epoxy fabric materials considered in this work, delamination failure at the 12 and 6 o'clock locations always occurred before any other failure mode was observed (except in presence of drill damage, as discussed later in Section 5).