Modeling the Effect of Out-of-Plane Fiber Orientation in Lumber Specimens


  • Douglas C. Stahl
  • Steven M. Cramer
  • Kent McDonald


Fiber orientation, lumber tensile strength, fracture, finite elements


A method is presented to account for the effect of three-dimensional fiber orientations near knots in a two-dimensional lumber tensile strength prediction model. Data we have collected show that grain angles dive from 15 to 90 degrees out of the wide face plane of flat-grained lumber within a region of about one knot radius from the visual edge of a knot. The diving nature of the grain angles is accounted for in a two-dimensional model, called GASPP+, by transforming a three-dimensional material compliance matrix, and extracting the appropriate coefficients for use in a two-dimensional compliance matrix. Failure criteria are modified to reflect the decreased strength associated with nonzero dive angles. These modifications led to accurate tensile behavior predictions, as evidenced by load-displacement plots and ultimate load measurements of lumber specimens. It is shown that consideration of dive angles is important in predicting the tensile strength and failure mode of thin lumber specimens. Lumber specimen thickness and the manner of loading influence the magnitude of the dive effect on strength.


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