Lower tolerance limits for screw withdrawal in wood

Abstract

In this study, the lower tolerance limits (LTLs) for screw withdrawal strength in wood were investigated. For this purpose, specimens were prepared from white oak and red oak wood (22.2 × 63.5 × 305 mm), a material widely used in furniture industry. Screw withdrawal tests were performed from transverse, radial, and tangential sections of wood specimens. Sample sizes for this study were determined by using modified Faulkenberry-Week methods. After considering normality, randomness, and homogeneity assumptions for tolerance analysis, LTLs were obtained from data sets in each sample group. According to screw withdrawal tests, ultimate tensile strength was 15.04 MPa, 17.93 MPa, and 16.77 MPa for red oak specimens; from each section, respectively. Likewise, those of white oak specimens were 16.36 MPa, 19.67 MPa, and 17.21 MPa, respectively. Results of LTLs for 0.99/0.99 confidence/proportional level were 8.69 MPa, 11.96 MPa, and 10.30 MPa for red oak specimens and 9.67 MPa, 11.14 MPa, and 11.58 MPa for white oak specimens from transverse, radial, and tangential sections of wood in screw withdrawal test, respectively. The study provides a systematic procedure to estimate design values for screws joints.