Effect of Pilot Hole Application on Screw Withdrawal Resistance and Full-Field Strain Distribution in Beech and Oak Wood Using Two-Dimensional Digital Image Correlation

Abstract

Effects of pilot-hole application and screw insertion surface were studied relative to the maximum screw withdrawal force and full-field strain distributions in beech (Fagus orientalis) and oak (Quercus petraea) wood using two-dimensional Digital Image Correlation (2D DIC). A total of 80 specimens were evaluated in a 2 × 2 × 2 factorial experimental design, including two wood species, two anatomical surfaces (transverse and tangential), and two pilot-hole conditions (with and without pilot holes). Pilot-hole application increased the mean withdrawal force on transverse surfaces by approximately 30.0% in beech and 7.0% in oak. Conversely, it decreased the mean withdrawal force on tangential surfaces by approximately 15.7% in beech and 30.4% in oak. The factorial ANOVA indicated significant effects of anatomical surface and pilot-hole application, together with significant two-way interactions among the experimental factors. The DIC strain maps revealed spatial variations in normal and shear strain distributions around the screw withdrawal region. However, comparisons between experimental groups were interpreted qualitatively because the strain maps were obtained at different maximum load levels. The findings demonstrate that the influence of pilot-hole application depends on the anatomical surface and wood species, highlighting the importance of considering their interactions when evaluating screw withdrawal performance.