## 1 Introduction
[PERSON] and [PERSON] [14] used the same approach and presented explicit formulas for laminates with general stacking sequence utilizing the FSDT and considered also the extension-bending coupling of the upper and lower sub-beams. This formulation for the contact force, although not specified in the texts, was based on the rigid joint model as well, therefore considering a rigid interface on the crack tip with no relative rotations and translations. An important outcome of the contact force is the presence of friction between the two sub-beams [21]. Friction can cause extra resistance to the crack development, which can increase the critical load and the critical ERR. These variations can also affect the predictions of ERR and multiple studies have been presented where these issues are considered. For instance, [PERSON] et al. [22] discussed the effects of various test conditions, such as specimen size, loading conditions, and friction between delamination surfaces on the mode II interlaminar fracture toughness at the initiation and during delamination growth, through experimental and theoretical methods. A few years later, [PERSON] and [PERSON] [23] used numerical simulations to study the effects of friction, test geometry, and fixture compliance on the perceived mode II interlaminar fracture toughness of laminated composites. A newly developed \"direct energy balance approach\" was employed to obtain the true energy release rate for a range of specimens, test geometries, and coefficient of friction. Additionally, a compliance calibration technique was utilized and was combined with experimental results from fixture compliance tests to obtain the perceived toughness. By varying the different parameters, the individual and combined effects of friction, test geometry, and fixture compliance on the ratio of perceived to true toughness were obtained. A study with a similar aim was performed by [PERSON] and [PERSON] [24]. This research utilized nonlinear finite element analyses to study the impact of friction and geometric nonlinearities on the ERR in three- and four-point ENF tests. The study used the direct energy balance approach to determine the ERR and found that the effects of nonlinearities on the ERR were greater in the four-point test than in the three-point test. [PERSON] and [PERSON] (2018) examined the impact of transverse normal stress on mode II fracture toughness of unidirectional fiber-reinforced composites. The study combined experimental testing and finite element analyses to study the S2/8552 glass/epoxy composite using off-axis specimens with a through-the-width crack. The authors concluded that transverse normal compressive stress had a significant effect on the mode II fracture toughness of the composite. [PERSON] et al. (2018) presented an experimental and numerical study of the effect of friction on mode II delamination in composite laminates. The study highlighted that frictional stresses between the crack edges could absorb and dissipate significant energy during delamination. Their research included experiments on a set of unidirectional ENF specimens. A numerical analysis that employed a cohesive-frictional constitutive model with only frictional strength was performed to aid the understanding of the experiments and evaluate the frictional constitutive parameters. Finally, [PERSON] et al. (2018) evaluated the impact of friction on four-point ENF tests for carbon fiber-epoxy composite materials. The authors examined the hysteresis loop in the experimentally obtained load versus displacement curve due to an unloading-loading cycle. The effects of friction were considered from external (pins) and internal (the interface between the two arms) sources, and numerical simulations were performed aiming to better understand the experimental results. Considering the above, it is evident that analytical and numerical methods can aid the understanding of mode II delamination tests.