The measured penetration forces amalgamate numerous sources
The measured penetration forces amalgamate numerous sources, the V5 Tag ones being the Coulomb friction and the force associated with the wood deformation–fracture. The following experimental procedure allows the separation of the dominant Fr components. First, the nail is driven into the sample to evaluate Fr, and secondly, the nail is withdrawn, and then driven back into the hole generated during the initial penetration, and the resistance force is recorded. The initial penetration Fr includes the Coulomb friction and the deformation–fracture components. Following the nail extraction, the remaining resistance force appearing during the penetration is assumed to be entirely generated by the Coulomb friction. Therefore, subtracting Fr measured during the second penetration from the first measurements isolates the wood deformation–fracture force.
Since the penetration proceeds without any rotation, the experiment is easily carried out for smooth and annularly threaded nails, with their penetration remaining unchanged for the second penetration movement. In contrast, helically threaded nails rotate while penetrating, and depending on the interaction at the wood–nail interface, the rotation appears to be irregular and hardly predictable. Therefore, precise withdrawal involving no alteration of the interface is virtually impossible. For this reason, the experiment only includes the smooth and annularly threaded nails.