Modelling of growth stress generation and timber distortions related to log sawing and forced drying

• Growth stresses can cause fibre collapse in living trees (often in combination with strong wind loading), internal checking resulting in end-splitting of logs, and instantaneous board distortions when the log is split into timber. How much the growth rate and growth stresses affect the final shape stability of solid timber products is not fully understood. For trees with abnormal growth conditions resulting in eccentric growth and generation of reaction wood, it is very complicated to estimate how timber products made of such material will behave during moisture variation. To study this behaviour a finite element analysis in which stress formation during normal and abnormal tree growth was simulated with the aim of better understanding of the growth stress formation.The model for progressive growth stress generation in trees with normal growth conditions is formulated as a one dimensional axisymmetric general plane strain model of the tree stem. The trunk is considered as a very long solid cone with zero shear stresses. In the model, each new (and stress free) annual ring is progressively added to the stem during the analysis. Thereafter the cell maturation is assumed to start, i.e. the crystallization of the cellulose leads to longitudinal shrinkage of the new annual ring whereas the lignification process results in transversal expansion of the fibres. Since the maturing annual ring is attached to the old and already matured rings, a strain constraint develops in the stem. The new annual ring becomes stretched longitudinally and compressed tangentially, whereas the matured rings are exposed to the opposite stress conditions. The material model used is based on the assumption of small strains and the biological maturation strains are used as a driving force for the growth stress evolutions. The aim here is also to take into account viscous effects of the wood material. ......

Subject headings

LANTBRUKSVETENSKAPER  -- Lantbruksvetenskap, skogsbruk och fiske -- Trävetenskap (hsv//swe)

AGRICULTURAL SCIENCES  -- Agriculture, Forestry and Fisheries -- Wood Science (hsv//eng)

Keyword

Growth Stress

Board Distortion

Wood

Numerical Simulation.

Forestry and Wood Technology

Skog och träteknik

Publication and Content Type

ref (subject category)

kon (subject category)