QUANTITATIVE MEASUREMENT BY SOFT-X-RAY OF CRACK FORMATION IN WOOD AS A RESULT OF NAILING .2. EFFECTS OF WOOD FACTORS ON THE CRACK SIZE

Crack formations caused by driving nails was studied using the Barium- soaking method.(1)) The objectives of this research were 1) to classif y the crack formation patterns, 2) to investigate the effects of wood sections into which the nail is driven, widths of annual rings. moistu re content, and species. The following conclusions were reached. 1) Th e patterns of the crack were classified into three types, I type, O ty pe and U type (Fig. 1). I type occurred in small density wood, but rad iata pine, Agathis, Japanese beech were always cracked in the shape of I type even if 16d nailes were driven. O type was the most common. U type was induced in dense wood by large large nails such as 16d. Crack formation in the wood cannot be estimated by surface crack characteri stics. 2) The maximum crack length (L-m) and the area of crack (S) had good correlations (Fig. 3). The crack area (S) was equivalent to the area of an ellipse made by the axis of L-m and the drivien length. 3) The difference of L-m between nailing into flat grain and into diagona l grain was very little. The crack size when driving into edge grain a nd the effect of widths of annual rings were evident for the size of c rack (Fig. 4). 4) Air drid wood had larger crack than did green wood b y 1.1-1.2 times. Excessively dried wood had twice the size of cracks a s green wood, and the variation became larger as moisture content less ened (Fig.5). 5) In the same species, L-m was proportional to the wood density, but the slopes of the regression lines were much different a mong wood species (Fig. 6). 6) For the rational design of timber const ruction, three wood groups were employed; group C1: easy to crack such as Douglas-fir, hemlock, and Lawson cypress, Group C3: difficult to c rack such as radiata pine, Agathis, Japanese beech, Group C2: intermed iate between C1 and C3 (Figs. 7 and 8). According to the five percent upper exclusion limit line shown in Fig. 8, C1 group species of greate r specific gravities than 0.5, need more than 15d end distance (d=diam eter of a nail). These wood species groups by crack formations will be useful for rational nail joint design.