Effect of the Drying Method of Pine and Beech Wood on Fracture Toughness and Shear Yield Stress

Abstract

The modern wood converting processes consists of several stages and material drying belongs to the most influencing future performances of products. The procedure of drying wood is usually realized between subsequent sawing operations, affecting significantly cutting conditions and general properties of material. An alternative methodology for determination of mechanical properties (fracture toughness and shear yield stress) based on cutting process analysis is presented here. Two wood species (pine and beech) representing soft and hard woods were investigated with respect to four diverse drying methods used in industry. Fracture toughness and shear yield stress were determined directly from the cutting power signal that was recorded while frame sawing. An original procedure for compensation of the wood density variation is proposed to generalize mechanical properties of wood and allow direct comparison between species and drying methods. Noticeable differences of fracture toughness and shear yield stress values were found among all drying techniques and for both species, but only for beech wood the differences were statistically significant. These observations provide a new highlight on the understanding of the effect of thermo-hydro modification of wood on mechanical performance of structures. It can be also highly useful to optimize woodworking machines by properly adjusting cutting power requirements.

Keywords: beech wood; cutting power; cutting process; drying process; fracture toughness; pine wood; sawing process; shear yield stress.

Figure 1

Schematic of schedules for dying experimental beech samples with four studied processes. Note: air p—air pressure, air RH—air relative humidity, air T—air temperature, FSP—fiber saturation point.

Figure 2

Cutting speed directions when splitting orthotropic materials; axial || cutting along fibers (cutting direction 90°–0°), perpedicular ⊥ cutting across fibers (cutting direction 90°–90°), and cross-cutting ≠ direction (cutting direction 0°–90°) [38,52,53].

Figure 3

Relation between cutting power per one saw blade and uncut chip thickness when sawing pine wood dried with different methods: PA—air drying, PC—conventional kiln drying, PS—warm heated-steam experimental drying, PV—vacuum kiln drying.

Figure 4

Relation between cutting power per one saw blade and uncut chip thickness when sawing beech wood dried with different methods: BA—air drying, BS—warm heated-steam experimental drying, BC—conventional kiln drying, BV—vacuum kiln drying.

Figure 5

Relation between normalized (by density) cutting power per one saw blade and uncut chip thickness when sawing pine wood dried with different methods: PA—air drying, PC—conventional kiln drying, PS—warm heated-steam experimental drying, PV—vacuum kiln drying.

Figure 6

Relation between normalized (by density) cutting power per one saw blade and uncut chip thickness when sawing beech wood dried with different methods: BA—air drying, BC—conventional kiln drying, BS—warm heated-steam experimental drying, BV—vacuum kiln drying.