Quasi-Static Cyclic Response and Failure Modes of CNC-Machined Pinus radiata D. Don Plywood TIE Joints in the WikiHouse Skylark System: An Exploratory Study

Integral CNC-machined timber connections provide an efficient fastening-free solution for digitally fabricated modular construction systems. Their response under repeated loading is governed by the interaction between joint geometry, plywood architecture, contact, and progressive material damage. This study experimentally investigates the quasi-static cyclic response of full-scale TIE joints used in the WikiHouse Skylark system and manufactured from 18 mm Pinus radiata D. Don plywood. Three plywood configurations were evaluated: standard structural plywood (STD), weather-resistant plywood (HR), and fire-resistant plywood (FR), with the central TIE element oriented either parallel (//) or perpendicular (⊥) to the applied load. A displacement-controlled cyclic protocol adapted from ISO 16670 was applied. The exploratory cyclic program used one full-scale specimen per tested configuration (n=1), comprising six tensile configurations and three shear configurations. Tensile joints were subjected to one-sided loading–unloading cycles, whereas shear joints were subjected to reversed cyclic displacement. Under cyclic tension, maximum forces ranged from 6.082 to 9.794 kN at displacements between 6.6 and 10.3 mm, with the highest measured force obtained for the HR ⊥ configuration. Reversed cyclic shear tests produced maximum forces between 8.212 and 8.976 kN at displacements between 4.7 and 7.0 mm. Across all tested configurations, the apparent initial stiffness (Ke) ranged from 1.202 to 1.641 kN/mm, while the cumulative dissipated energy (Ed) ranged from 105.02 to 425.81 J. Damage consistently localized within or adjacent to the reduced central TIE region and involved combinations of tensile cracking, shear fracture, local crushing, fiber separation, and fiber rearrangement. The results show that the integral TIE geometry maintains a well-defined load-transfer and failure region under progressively increasing cyclic displacement. The experimental evidence provides a component-level basis for subsequent replicated testing and the development of nonlinear models of digitally fabricated plywood connections.