## 3 Methodology  
### Measurement of ILT Strength  
A relationship between the peak load measured at failure using the testing machine load cell and the maximum ILT stress in the specimen is needed for assessment of the material ILT strength. In standard testing, the common design constraint of uniform stress distribution over the specimen cross-section allows using convenient closed-form relations for recovery of the maximum stress at failure.  
It is worth noting that the maximum stress concentration factor \(K_{t}^{inf}\) for a circular hole in an infinite anisotropic plate subjected to normal far-field compressive stress can be derived analytically as [31]:  
\[K_{t}^{inf}=\sqrt{\frac{E_{33}}{E_{11}}} \tag{1}\]  
where \(E_{11}\) and \(E_{33}\) are the composite material Young's moduli in the fiber and thickness direction, respectively.  
As shown in Equation (1), the maximum stress concentration factor \(K_{t}^{inf}\) for a circular hole in an infinite anisotropic plate depends on material properties.  
In the OHC unidirectional specimens considered in this work, the width-to-hole-diameter ratio is equal to two (R = 2) and the effect of finite specimen's width, in addition to the effects of material anisotropy, must be included for evaluation of the stress concentration factor at the hole. To the best of the author's knowledge, there is no evident analytical closed-form solution for such a problem. However, when material constitutive stress-strain properties are known, the maximum ILT stress at failure can be easily determined from a simple 2D plane stress FE analysis. The stress concentration around the hole occurs far from the test fixture tabs, and it was verified that it was relatively insensitive to specimen boundary conditions for the test coupon geometry considered.  
An illustration of the 2D FE mesh used in this work for stress analysis and FEMU inverse characterization is shown in Figure 6. The FE model is generated for Abaqus FEM analysis and includes about 5000 four-node bilinear 2D plane stress elements CPS4 from Abaqus element library. The total number of nodes is about 5300 and the total number of degrees of freedom is approximately 11,000. A linear elastic homogenized orthotropic material model is used for implicit static stress/displacement analysis using Abaqus standard solver. Mesh refinements are assigned to ensure proper convergence of the stress field around the hole. A mesh sensitivity study showing convergence of the ILT stress field around the hole with mesh refinement is provided in Appendix A.  
Figure 7 shows FE contour plots of the normalized interlaminar normal stress component in the carbon/epoxy tape (Figure 7a) and carbon/epoxy fabric (Figure 7b) OHC coupons used in this work. The interlaminar stress is normalized by the far-field longitudinal normal compression stress, which is equal to the applied force divided by the specimen rectangular cross-section area. The two materials have different homogenized orthotropic properties, which result in different values for \(K_{t}^{inf}\) calculated using Equation (1), as shown in Figure 7.  
OHC coupons ILT stress concentration factors \(K_{t}^{max}\) differ from \(K_{t}^{inf}\) for an infinite plate since they account for the effect of finite width, for which there is no obvious closed-form solution for anisotropic materials as previously mentioned. In these examples, a maximum ILT stress concentration factor \(K_{t}^{max}\) of 0.365 is obtained for the tape material, versus 0.595 for the fabric material. These values can be compared to \(K_{t}^{inf}=0.24\) and \(K_{t}^{inf}=0.37\) for an infinite plate with a circular hole for the tape material and fabric material, respectively.