# Flexural Performance of Novel Nail-Cross-Laminated Timber Composite Panels
## 4 Conclusions  
To mitigate shortcomings of conventional cross-laminated timber (CTL) composites, this study introduces nail cross-laminated timber (NCTL) panel composites. The behavior of NCLT panels under four-point bending load was studied experimentally in this study. Test parameters included the nail penetration angle, nail type, number of nails, and number of nailed layers. In addition, a modified formula for calculating the bending moment of NCLT panels was proposed. From this study, the following conclusions can be drawn:  
1. NCLT panels are characterized by tensile failure. The stress-carrying process of NCLT panels is divided into elastic phase and elastic-plastic phase. The elastic phase typically extends longer compared to the elastic-plastic phase.
2. The failure mode of NCLT panels depends on the angle, type, and quantity of nails. When the nailing angle was 30\({}^{\circ}\) or larger (less than 45), cracks mainly developed along the transverse direction; when the nail type was threaded nails and screws, cracks mainly develop along the direction of the grain; when the number of nails was larger, cracks developed primarily along the horizontal direction.
3. Comparing the theoretically calculated bending capacity of NCLT panels to corresponding experimentally measured values, the test values for three-layer NCLT panels were larger than the theoretical values. Hence, the bending moment calculation formula for CLT panels can be corrected with a nail connection influence coefficient \(\alpha\) = 1.33.
4. The effective bending stiffness of NCLT panels was positively correlated with the force between layers. Strong interactions between panel layers improved the bending stiffness, and the interaction between the layers was positively related to the nail pullout force. The greater the nail pullout force, the greater was the force between the layers.
5. From the nailing angle analysis, 30\({}^{\circ}\) nailing angle increased inter-panel force, and the ductility of NCLT panels also increased by 2.27%. The 30\({}^{\circ}\) nailing mitigated both the cooking of the wood slabs during damaged and the development of rupture cracks along the direction of the grain.
6. In terms of the type of nailing, the damage was ranked from worst to least in the order: polished rod NCLT panels, threaded nail NCLT panels, and screw NCLT panels. The ultimate bearing capacity was 47.96 kN, 58.46 kN, and 45.46 kN for the polished rod nail NCLT panels, threaded nail NCLT panels, and screw NCLT panels, respectively.
7. The increase in number of nails seemed to cause stress concentration in weak parts of the NCLT panels but also enhanced the interlaminar connection force, which was beneficial to improving the effective bending stiffness of the NCLT panels.
8. The effective bending stiffness increased with the number of nailed layers, and the deformation rate of the wood at the bottom tensile zone was greatly decreased. Under the same displacement, the force performance and the damage form were both greatly increased with increased number of nailed layers.
9. NCLT panels are more sustainable and more resilient compared to conventional glued CLT panels. Hence, NCLT panels can be a strong contender for diverse structural applications. This work should blaze the trail for further research towards the development of pertinent design provisions for NCLT panels and their wide implementation in full-scale construction.  
Y.Z.: Conceptualization, methodology, validation, formal analysis, resources, data curation, writing original draft. M.L.N.: Conceptualization, methodology, formal analysis, validation, resources, writing review and editing. X.G.: Data curation, writing original draft, writing review and editing. L.V.Z.: data curation, writing review and editing. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.