19
specimen, the MOE of each individual strand needed to be measured prior to gluing them
together.
We first prepared MOE specimens parallel to the grain by cutting 6 mm x 150
mm pieces out from individual strand. We then nondestructively measured modulus of
elasticity (MOE) for each hybrid poplar specimen in tension parallel
to the grain using
Instron model 5445 with a clip gage. More than 1000 specimens were loaded in tension
parallel to the grain for MOE results.
Figure 2.2. An asymmetric double lap shear specimen with strands of thickness t, bond
lines of length l and ends tabs of length L. The specimen width is w, elastic modulus E,
shear modulus G, P is load and F factor accounts for unsymmetric load.
The specimens with known MOE were then used
to prepare DLS specimens by
gluing them together using PF or PVA resin. The analysis is simplified if the DLS
specimens are symmetric. To insure symmetry, we cut one strand with known MOE in
FP
(1-F)P
L
G
1
G
2
E
2
E
1
L
2
L
1
l
t
2
t
3
t
1
G
3
E
3
P
20
half and used these two halves for the two outside strands in the DLS specimen. The
central strand was from a different strand.
The strands were coated with resin droplet arrays using a modified flexographic
printing technique that was described by G. Smith (2003).
The flexographic printing
process is commonly used to produce newspapers and other printed materials. The
surface of the printing-plate is composed of a multitude of raised dots; the diameter and
local density of the dots determines the image that is printed. The printing-plates used in
this work were composed of square arrays of circular dots and the distance between the
centers of adjacent dots (the dot-pitch) was 25 lines per inch (lpi). The grey-scale of the
plate was controlled by varying the diameter of the dots at the center of the pattern. Wire-
wound rods were used to precisely control the thickness of coating adhesive on
the base
plate. The wire-wound rod consisted of a 1/2-inch-diameter stainless steel rod wound
with the wire that had a diameter of 1mm. As shown by Smith (2003) a rod of 1/2-inch-
diameter with the wire produced uniform adhesive layers of 1 µm.
A range of dot sizes was used to vary the adhesive surface coverage. Three
different adhesive surface coverage area levels were applied for both PF and PVC and
they were 1%, 25% and fully glued (100%) coverage. In the fully glued (100%)
specimens, the adhesive was manually spread onto the strands. A total of over 500 DLS
specimens were printed for the normal strands and viscoelastic thermal compression
(VTC) strands (Kamke and Kutnar 2007).
After each DLS specimen was glued, they were then cured in a hot press at 100
psi and 180
o
C for 5 minutes. The DLS specimens were then left in
the control room for
several days to get MC to 12%. Finally, the DLS specimens were loaded in tension using
the Instron with an extensometer spanning the entire bond line section of the specimen.
The global stiffness was measured from the load vs. displacement results.
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