Evaluation of the drying stress in industrial kiln-dried boards using a force-based technique

Abstract

The introduction of a restoring force technique has allowed the internal stress within kiln-dried lumber to be assessed, without requiring a modulus. However, an analytical model based on the elastic beam theory was only valid within a flexural range and only internal stress in the direction of the width was assumed to exist. Within this work, the model was extended to lumber, which could also contain some other internal stress components, by testing 30-mm-thick and 100-mm- to 130-mm-wide flat-sawn rubberwood boards. The improved model successfully separated the effects of other internal stress components in terms of a remnant force. Also, a finite element analysis was employed to validate the internal stress behavior. With little impact from other internal stress components, the finite element model, which used the released strain and Young’s modulus in the tangential direction, successfully simulated the restoring force profile for an entire half-split length, including the relatively short half-split length outside the flexural range. But the finite element model failed to perfectly capture the restoring force behavior in the presence of other internal stress components. Future work is required to fully investigate the internal stress in all three main orthogonal directions.

Assessment of Asymmetrical Stress Profile within Wood Using Restoring Force Technique

“…According to Leelatanon et al [9], typical plots P against the term 4bl 2 /Sd still behave in flexural response for all the restoring force profiles but a positive y-intercept of the remnant force Po is observed. Consequently, Eq.…”

“…The maximum stress is assumed to be equal at the outer surface and the inner core. This assumption has been proved by both experimental and numerical results proposed in the previous works [8][9][10][11]. The magnitude of maximum stress (m) can be calculated by a deflection of the half-split specimen as expressed by:…”

“…The modulus of elasticity in the tangential direction (T) was fixed to a constant of 500 MPa, and all other moduli were calculated as ratios of the modulus 582 in tangential direction (EL : ER : ET = 20 : 1.6 : 1, GLR : GLT : GRT = 10 : 9.4 : 1 and EL : GLR = 14 : 1). Three Poisson's ratios were estimated to be RL = 0.02, LT = 0.02 and RT = 0.35 based on values of tropical hardwood species [8][9][10][11][15][16][17][18]. Only a half of the specimen was generated to reduce the computational time because the specimen had a plane of symmetry in the RT-plane as shown in Fig.…”

“…( 3) and ( 5) without knowledge of the modulus of wood. The restoring force should be proportional to the term 4bl 2 /Sd with zero intercept (Po = 0) at a relatively low value of 4bl 2 /Sd or at a relatively long half-split length in the flexural range [8,9]. However, this type of kiln-dried lumber was relatively rare.…”

“…The restoring force technique has been employed to examine the kiln-dried lumbers [8]. It appeared that a lot of measured restoring force profiles could not entirely be explained by the analytical model derived from an elastic beam theory [9]. Without a clear explanation, an additional remnant force obtained by extrapolation of the cutting length to infinity must be incorporated into the analytical model to completely describe the restoring force profile.…”