## 6 Conclusions  
A new methodology for measurement of interlaminar stress-strain, ILT strength, and ILT fatigue properties of composites based on a unidirectional open-hole compression specimen and DIC-based inverse characterization was proposed and investigated. The method was successfully used for measurement of elastic properties, ILT strength and fatigue, ILT \(S\)-\(N\) curves of a carbon/epoxy unidirectional tape material, and a carbon/epoxy fabric material. It was found that consistent ILT properties can be measured using the OHC-ILT method although significant effect of the hole drilling damage has been found in tape specimens. ILT failure mode occurred in all fabric OHC coupons despite the drilling damage.  
In the authors' experience, controlling hole quality in unidirectional OHC specimens is easier than controlling porosity in ASTM D 6415 curved-beam specimens or early tab failure in ASTM D7291 testing for ILT properties that typically result in large scatter and underestimated ILT strength properties. Therefore, the OHC-ILT method is a viable alternative methodology for accurate measurement of ILT properties.  
The OHC-ILT test does require FE analysis and prior knowledge of material for calculation of the maximum ILT stress at failure, since a closed-form solution is not available and the stress concentration factor depends on material properties. This work showed that the OHC-ILT test can be combined with DIC measurement of surface deformation and finite element model updating (FEMU) for simultaneous assessment of the elastic material constitutive properties and ILT strength properties. Such self-sufficient methodology was validated using independent measurements of the elastic constitutive properties obtained through other existing test methods. Simultaneous measurement of multiple properties could significantly accelerate material characterization, which currently suffers from the lack of efficient and cost-effective qualification.  
Another advantage from simultaneous measurement of strength and elastic properties is that potential variability in constitutive properties between individual coupons is taken into account in an \"average\" sense and decoupled from other sources of variability in stress measurement, which might  
Figure 20: Cross-section image of an OHC ILT fabric specimen showing out-of-plane irregularities and correlation with the location of delamination failure.  
lead to more accurate assessment of strength properties. Furthermore, there is a practical interest in using an open-hole test specimen for material characterization, since open-hole laminate coupons are typically used for verification of failure criteria and failure models used in progressive damage and failure analysis. Constitutive properties of composites measured using standard tests might be coupon or test configuration dependent. Such dependency is oftentimes overlooked, which might delay the development and implementation of most advanced failure models for composites that rely on accurate material characterization.  
Conceptualization, A.M.; methodology, A.M. and G.S.; software, G.S.; writing--original draft preparation, G.S.; writing--review and editing, A.M., J.D.S., B.J.; supervision, A.M.; project administration, A.M., J.D.S., B.J.  
This work was part of an effort funded by the Boeing Company as part of the \"CALE Project 4: Assessing the Durability and Damage Tolerance of Advanced Composite Structural Features\", which was performed under Contract # FA8650-17-C-2700 to the Air Force Research Laboratory (AFRL).  
The authors are grateful to [PERSON] (AFRL) for his guidance and advice during the course of the work. The authors also thank [PERSON] and [PERSON] at the University of Texas at Arlington for their assistance with specimen testing.  
The authors declare no conflict of interest. The views and conclusions contained in this article should not be interpreted as representing the official policies, either expressed or implied, of the U.S. Government.