Mechanism of Crook Development in Lumber During Drying

Authors

  • William T. Simpson
  • Terry D. Gerhardt

Keywords:

Drying, warp, crook, bow, oak, drying stresses, longitudinal shrinkage

Abstract

Crook can cause yield and grade loss in lumber. In this study the mechanism of crook was studied so that a better understanding might lead to methods for minimizing its development. Crook was observed to begin at an average moisture content of about 50%, and then to increase linearly as moisture content decreases. Differential longitudinal shrinkage on opposite edges of boards is shown to be the cause of crook. A model was developed to predict crook from differential longitudinal shrinkage, and experimental results agree with model predictions. The model was extended so that estimates can be made of the restraining force necessary to prevent crook.

References

Gerhards, C. C. 1982. Effect of moisture content and temperature on the mechanical properties of wood: An analysis of immediate effects. Wood Fiber14(1):4-36.nHsu, N. N., and R. C. Tang. 1974. Internal stresses of wood logs due to anisotropic shrinkage. Wood Sci.7(1):43-51.nHsu, N. N., and R. C. Tang. 1975. Distortion and internal stresses in lumber due to anisotropic shrinkage. Wood Sci.7(4):299-307.nSimpson, W. T. 1982. Warp reduction in kiln-drying hardwood dimension. For. Prod. J.32(5):29-32.nStamm, A. J. 1964. Wood and cellulose science. Ronald Press, New York.nThomas, H. R., R. W. Lewis, and K. Morgan. 1980. An application of the finite element method to the drying of timber. Wood Fiber11(4):237-243.nTimoshenko, S. P., and J. N. Goodier. 1970. Theory of elasticity. Third Edition. McGraw-Hill, New York.nU.S. Department of Agriculture. 1974. Wood handbook. USDA Agric. Handb. No. 72.n

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Published

2007-06-27

Issue

Number 4 / October 1984

Section

Research Contributions

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