[Drop Tower] [Plywood Lay-ups] [Hardwood Strips]
3. Strip Lay-ups
Wood/glass weight ratio

One interesting question about lay-ups revolves around the optimum wood thickness.  In just about every introductory text on the “strength of materials”, there is a solution for the ideal web height of a wooden beam capped by steel plates.  The same solution can be applied to a wood/glass beam, but several important cautions apply.   For one, we do not know the mechanical properties of hand laid fiber/epoxy laminates as well as we know the mechanical properties of steel.  For another, the solutions are strictly valid for one-dimensional beams under conditions of small elastic deflection.  With an instrumented drop-tower, it is possible to quantitatively rank the resistance to local loading of plate samples with different wood to fiber/epoxy weight (thickness).

Data for samples made with western red cedar strips (WRC) and 3 oz 4-harness satin E-glass, and Raka 127 epoxy with 606 hardener are shown in Table 3.1.  The WRC thickness was adjusted to target a constant sample weight as layers of glass were increased. Values of energy to peak load as a function of wood/glass ratio are plotted in Figure 3.1.
Table 3.1: Data for samples of WRC and 3 oz 4-harness satin E-glass
Sample
Number




01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
20
Propagation
Energy
(lbf in)



254
293
242
227
233
187
228
234
192
NA
194
237
207
231
207
255
310
160
233
245
Layers
of
Glass
(out,in)


2,1
2,1
2,1
2,1
4,2
4,2
4,2
4,2
1,1
1,1
1,1
1,1
2,2
2,2
2,2
2,2
3,3
3,3
3,3
3,3
Weight
Total
(oz)



3.89
3.88
3.83
3.82
3.69
3.72
3.69
3.71
3.89
3.95
3.88
3.88
3.28
3.27
3.28
3.28
3.83
3.82
3.78
3.77
Weight
Glass
and
Epoxy
(oz)

1.00
1.02
1.00
0.96
1.82
1.83
1.80
1.82
0.75
0.72
0.72
0.69
1.22
1.20
1.25
1.25
1.91
1.90
1.83
1.89
Weight
Ratio
Wood/
Glass


2.89
2.79
2.82
2.98
1.03
1.03
1.06
1.03
4.16
4.52
4.42
4.61
1.69
1.72
1.64
1.63
1.00
1.01
1.07
1.00
Panel
Weight
(oz/ft^2)



9.65
9.63
9.50
9.48
9.15
9.23
9.16
9.21
9.66
9.80
9.63
9.63
8.15
8.10
8.15
8.13
9.51
9.49
9.38
9.37
Average
Wood
Thickness
(in)


0.238
0.239
0.239
0.239
0.156
0.157
0.156
0.155
0.218
0.222
0.222
0.222
0.170
0.170
0.169
0.169
0.123
0.123
0.123
0.123
Energy
to Peak
Load
(lbf in)


192
190
213
214
212
239
223
233
161
NA
173
182
160
161
161
168
212
206
207
203
Weight
Wood
(oz)



2.89
2.86
2.83
2.86
1.87
1.89
1.89
1.89
3.14
3.23
3.16
3.19
2.06
2.06
2.04
2.03
1.92
1.92
1.95
1.89
Figure 3.1.  Energy to peak load plotted as a function of wood/glass weight ratio.  Double glass refers to the samples with two layers on the outside (impact side) for every layer on the inside.  Symmetric glass refers to samples with the same number of layers on each side.
Nick Schade's samples

Nick had some northern white cedar (NWC) samples left over from the
three-point bend work we had done a few years ago, and he sent them out for testing with the drop tower.  Table 3.2 is a summary of the samples, and Figure 3.2 is a plot of the energy to maximum load. It is interesting to note that the use of Kevlar did not provide an advantage over glass.  Another interesting feature of this plot is the peak energies for samples with 6 oz glass.  One would expect that the samples with two layers on the impact side would have higher energies to peak load than the samples with one layer.  A look at the load/displacement curves in Figure 3.3 shows that the result is mostly due to how the energy is calculated.  The displacement at peak load was taken as 0.520 in for the samples with two layers of 6 oz glass on the impact side, while the displacement to peak load was taken as 0.625 in for the samples with one layer of glass on the impact side.  The larger displacement used for the samples with one layer contributes more to the peak energy calculation than the generally higher loads recorded for the samples with two layers of glass on the impact side.  If we compare the trend for loads between samples with two layers and samples with one layer of glass, it is clear that the second layer does provide additional resistance to localized loads, especially in terms of propagation energy, or total energy to puncture.  The plot in Figure 7 is numerically correct, but gives a misleading impression of how a second layer of glass improves the resistance to puncture.
Table 3.2:  Northern white cedar samples
Sample




A1
A2
B1
B2
C1
C2
D1
D2
E1
F1
Wood
Thickness
(in)


.250
.250
.188
.188
.250
.250
.250
.250
.188
.250
Front




4 oz PW E
4 oz PW E
6 oz PW E
6 oz PW E
2 x 6 oz PW E
2 x 6 oz PW E
6 oz PW E
6 oz PW E
4 oz PW E
4 oz PW E
Back




4 oz PW E
4 oz PW E
9 oz AeroFab
9 oz AeroFab
6 oz PW E
6 oz PW E
6 oz PW E
6 oz PW E
4 oz PW E
Kevlar
Sample
Weight
(oz/ft^2)


8.47
8.47
8.82
8.82
10.28
10.28
8.98
8.98
7.21
9.23
Energy
to
Peak Load
(lbf in)

171
178
197
204
263
293
326
347
130
217
Total
Energy
(lbf in)


381
374
714
828
650
659
521
506
306
513
Figure 3.2.  Energy to peak load for northern white cedar strip panels.
Figure 3.3.  Load/displacement data for northern white cedar strip samples.  The data shows the effect of two layers of glass compared to one layer.  Figure 3.2 shows a  lower energy to peak load for the samples with two layers of glass, but the load curves clearly show an advantage to having two layers.
Insight to the effect of fiber orientation on failure caused by localized loading is shown in Figure 3.4, which compares broken samples of 3/16’ NWC.  One sample has 4 oz plain weave E-glass and the other has 6 oz plain weave E-glass on the front and 9 oz AeroFab on the back.  The AeroFab is a stitched axial with fibers oriented at +-45 degrees to the roll axis.  Nick laid the fibers at +-45 degrees to the strips in the samples.  The backside of the AeroFab sample shows failure orientations at +-45 degrees.  What all of the views in Figure 3.4 show is that failure tends to occur perpendicular to the fiber direction.  This has implications for the proposition that given the same glass content, multiple layers of glass oriented at different angles will be “stronger” than a single layer.  In the case of localized loading, multiple layers of balanced cloth laid at different angles will not provide benefit based on fiber orientation alone.
Figure 3.4.  Sample of northern white cedar.  The images in the right-hand side of the figure show the back sides of the samples.  Note the angle of failure in the lower left image showing the AeroFab side of sample B1.
Mike Loriz has been building boats using hardwood strips.  To see how his samples made with Black Locust strips compare to cedar strips, click here, or on the link below.
[Drop Tower] [Plywood Lay-ups] [Hardwood Strips]

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