## 5 Discussion  
As mentioned previously, hole quality had an impact on failure mode and accuracy of the ILT strength and fatigue measurement, especially in the carbon/epoxy tape specimens tested. It was observed that if the hole is free of drilling damage, OHC specimens exhibit a consistent failure mode that is a crack at 6 and 12 o'clock locations. For example, Figure 18a shows a cross-sectional CT slice of a specimen with acceptable failure mode obtained from non-destructive CT inspection prior to testing. As illustrated, a clean hole free of significant drilling damage is documented. In such specimens, a symmetrical ILT strain distribution and a consistent ILT failure was observed as shown in Figure 18b,c.  
Figure 16: Fabric ILT fatigue. R = 0.1, 4 Hz.  
Figure 17: Unidirectional tape and fabric ILT fatigue; R = 0.1, 4 Hz.  
On the other hand, CT data in Figure 19a show a specimen with significant drilling damage detected prior to testing. Figure 19c shows that delamination failure in this specimen initiated at the location of the initial flaw, instead of initiating at the 12 and 6 o'clock location for pristine specimens. In some specimens, surface strain distribution was visibly affected by the drilling damage, as suggested by the asymmetry of ILT strain field shown in Figure 19b.  
The ILT failure mode in the fabric specimens seemed to be less affected by the hole quality, which might be attributed to the micro-structure of the fabric material. ILT failure mode occurred in all fabric OHC coupons despite the drilling damage. Prior to testing, all OHC ILT samples were CT scanned to identify as-manufactured state. Specimens showing significant drilling damage, and inconstant failure modes were discarded. It is worth noting that drilling process can be much improved after this preliminary testing to allow for clean holes.  
As indicated in the results listed in Tables 2 and 3, scatter in the extracted ILTS was higher for the fabric material compared to ILTS results for the unidirectional tape material, despite constituent failure mode. In particular, an 18.2% CV in average ILT strength was obtained for the fabric, compared to 7.6% CV tape material. It is worth noting that the sample size for the fabric specimens was four coupons versus five coupons for the tape material, which might contribute to the higher coefficient of variation. The larger scatter might also be attributed to the out-of-plane irregularity inherent to the woven  
Figure 19: (**a**) Cross-sectional CT data prior to testing, (**b**) DIC-measured ILT strain distribution and (**c**) failure mode in a unidirectional tape specimen with non-acceptable failure mode due to the effects of drilling damage.  
Figure 18: (**a**) cross-sectional CT data prior to testing, (**b**) DIC-measured ILT strain distribution and (**c**) failure mode in a unidirectional tape specimen with acceptable failure mode.  
prepreg fabric material. For example, Figure 20 shows a cross-section image of a fabric OHC specimen after delamination failure under interlaminar tension. As shown in Figure 16, delamination failure did not initiate exactly at the 6 o'clock and 12 o'clock location and the locus for failure initiation and delamination path seem to be correlated with the out-of-plane waviness of the fabric material. The FE model used in the FEMU procedure assumes a perfectly homogenized orthotropic material for the fabric material. Material heterogeneity and small variations of failure locus due to random out-of-plane irregularities are therefore likely to be the source of larger scatter in ILTS for the fabric material.