Evaluation strategy of softwood drying stresses during conventional drying: a “mechano-sorptive creep gradient” concept

Author

• Jian-feng ZhanEmail author
• Stavros Avramidis

1. 1.
2. 2.

Original

First Online: 

03 July 2017

Abstract

The detection and analysis of drying-induced stresses in wood are of fundamental importance for quality evaluation and grading of kiln-dried lumber, and thus, various such procedures have been developed commercially. In this paper, a softwood drying-induced stress evaluation concept was proposed that is based on the drying rheology and wood mechano-sorptive mechanism. The evaluation variables for the drying-induced stresses included moisture content gradient (MCG) and mechano-sorptive creep strain gradient (MSCG), both of which are calculated through the lumber thickness. The softwood species needle fir (Abies nephrolepis) was processed into flat-sawn lumber pieces of 40 mm × 120 mm in cross section and was further kiln-dried in conventional laboratory dryers. Width deformation changes along the thickness of lumbers were measured by a slicing method. Shrinkage and elastic and viscoelastic creep strains in the tangential direction were measured quantitatively. Based on the dynamic free shrinkage functions for this softwood species, determined according to small specimen tests, the mechano-sorptive creep strain variables were calculated theoretically. By comparing the mechano-sorptive creep strain differentials between wood surface and its center section, a conspicuous corresponding trend could be revealed between this difference and those of the shrinkage strain differences. A combined variable set, which includes the moisture content differences and mechano-sorptive creep differences between the wood surface and its core section, was proposed based on this research test. The mechano-sorptive creep gradient concept was defined to formulate the drying stress and strain development during conventional drying. After some further mathematical approximations, these newly proposed variables were recommended to estimate the magnitude of drying stress during the mid- and final stages of softwood drying. The effectiveness of this theoretical inference was further verified according to the experimental results of the needle fir drying test.

List of symbols

A

Material constant, determined based on the physical properties of the wood species

B

Material constant, determined based on the physical properties of the wood species

c

Wood center layer

D

Measured dimension (mm) of the shrinkage specimen

cη

Viscous coefficient (dimensionless)

E

Modulus of elasticity (MOE, MPa)

L

Width of the specimen (mm)

M

Wood moisture content (%)

s

Wood surface layer

T

Temperature (°C)

t

Times (min)

x

Coordinate along thickness (mm)

Greek letters

α

Shrinkage coefficient (1/100%)

αve

Correction coefficient (dimensionless)

ε

Drying strain component

η

Material viscoelastic coefficients (MPa ⋅$\cdot$ min)

σ

Drying stress inside the wood (MPa)

Subscripts

e

Elasticity

fs

Free shrinkage

FSP

Fiber saturation point

ms

Mechano-sorptive

n = 0–3

Deformation dimensions of the slice specimens

o

Green state

s

Kiln-drying state

N

Shrinkage

ve

Viscoelasticity

Superscripts

s−˙c$s\dot{-}c$

Differences between the surface and center of wood

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