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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.

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