Finite element simulation of growth stress formation and related board distortions resulting from sawing and forced drying

Artikel i vetenskaplig tidskrift, 2006

Matching timber quality with end-user requirements is a major research issue and lack of straightness in timber is the most frequent complaint worldwide. The final distortion of timber boards is caused mostly by moisture-related deformations and growth stresses that develop during growth of the tree, but how much the growth rate and growth stresses affect the final shape stability is not fully understood. A finite element analysis in which stress development during tree growth was simulated was performed with the aim of better understanding how growth stresses are generated. The tree growth model was formulated in terms of large strain settings (large changes in volume), whereas the material model for stress development was based on the theory of small strains. An earlier three-dimensional distortion model was developed further for studying the influence of growth stresses on final distortion of the board. The results showed that growth stresses clearly vary during tree growth and they also form a large stress gradient from pith to bark. This itself can result in significant bow and crook deformation when the log is sawed into boards.