Abstract:
Eighteen Ponderosa Pine (Pinus ponderosa Dougl.ex.laws) veneer flitches
were randomly selected and crosscut into paired flitches. Serpentine-end-matched
(SEM) joints were machined in the flitches, joint amplitude equal to the width in
one half of the paired flitch and joint amplitude equal to 2/3 width in the other half
of the paired flitches. A cross-linking polyvinyl acetate emulsion (PVA) was used.
The glued flitches were conditioned, sliced and dried using typical industry
methods. Three thicknesses of veneer were cut from each veneer flitch, 1/16 in
(1.59mm), 1/12 in (2.17mm) and 1/8 in (3.18mm).
Of 468 pieces of veneer produced, only 20 pieces exhibited some bond
failure during manufacturing. The failed bonds occurred in veneer from 7 of the 36
flitches sliced. Subsamples of paired flitches were chosen and the related veneers
tested in tension. The strength of each veneer and failure mode was recorded. Of 155 pieces of veneer tested, 55% failed with some wood failure. The average
percentage of wood failure was 32.5% with most failure occurring along the entire
glue line except at the joint apex.
Finite element modeling was applied to examine the stress distribution
through an SEM joint in the veneer. To address the thin nature of the adhesive
layer, the submodeling procedure was conducted. The effects of adherend stiffness
dissimilarity, glue line discontinuity, adhesive stiffness, adhesive layer thickness
and geometrical shape were evaluated.
The adherend stiffness dissimilarity was found to have a significant effect
on the stress distribution. When the convex adherend was chosen as the stiffer of
the two adherends, a favorable stress distribution was achieved. This choice also
optimized the impact of stress magnification due to glue line discontinuity. An
adhesive with stiffness that is about the same as the stiffness of the adherend tends
to reduce stress magnification. Stresses in the adherends and adhesive were not
sensitive to the thickness of the adhesive layer and the geometric shapes of the
SEM joints.
