[Drop Tower] [Plywood Lay-ups] [Cedar Strip Lay-ups]
Hardwood strips

Mike Loriz has been building boats using hardwoods, including Black Locust.  One of the interesting trends with wood is that as the density increases, the mechanical properties generally increase.  For example, Table 3.3 lists some relevant properties for Black Locust and Western Red Cedar (The Wood Handbook).  To estimate how these two wood species behave on a basis of equivalent weight, we can divide the mechanical properties by the specific gravity or density.  Table 3.4 shows the properties listed in Table 3.3  divided by the specific gravity of the wood at 12% moisture (Wood Handbook).  These can be called weight specific properties.  Mike understood that the weight specific properties of hardwoods meant he could use thin strips (5/32 in) to keep the finished weight low, and still have the wood contribute as much or more to the structure than if he had used the common softwoods.  To test his use of Black Locust, Mike made some sample lay-ups and we tested them with the drop tower.  The samples are described in Table 3.5.
Table 3.5: Properties of Black Locust and Western Red Cedar at 12% moisture
(Wood Handbook)
Wood





Black Locust

Western Red Cedar
Specific Gravity




0.69

0.32
Modulus of Rupture (lbf/in^2)



19400

7500
Work to Maximum Load
(in lbf/in^3)

18.4

5.8
Impact Bending
(in)




57

17
Compression Strength Parallel to Grain(lbf/in^2)


10180

4560
StiffnessParallel to Grain
(lbf/in x10^6)



2.05

1.11
Table 3.6:  Weight Specific Properties of Black Locust and Western Red Cedar
Wood





Black Locust

Western Red Cedar
Modulus of Rupture (lbf/in^2)



28100

23400
Work to Maximum Load
(in lbf/in^3)

26.7

18.1
StiffnessParallel to Grain
(lbf/in x10^6)



2.97

3.46
Impact Bending
(in)




83

53
Compression Strength Parallel to Grain(lbf/in^2)


14800

14200
Table 3.7:  Data for samples made with Black Locust strips
Sample




ML01
ML02
ML03
ML04
ML05
ML06
ML07
ML08
ML09
ML10
ML11
ML12
ML13
ML14
ML15
ML16
Outside lay-up




2x4 oz PW E
2x4 oz PW E
2x4 oz PW E
2x4 oz PW E
2x5 oz Raka
2x5 oz Raka
2x5 oz Raka
2x5 oz Raka
2x3.25 4-HS E
2x3.25 4-HS E
2x3.25 4-HS E
2x3.25 4-HS E
2x3.25 4-HS E
2x3.25 4-HS E
2x6 oz PW E
2x6 oz PW E
Inside lay-up




4 oz PW E
4 oz PW E
4 oz PW E
4 oz PW E
5 oz Raka
5 oz Raka
5 oz Raka
5 oz Raka
1.8 oz Kevlar
1.8 oz Kevlar
1.8 oz Kevlar
1.8 oz Kevlar
2x1.8 oz Kevlar
2x1.8 oz Kevlar
6 oz PW E
6 oz PW E
Sample Weight
oz/sqft


9.93
10.12
9.50
9.56
11.06
10.88
10.93
10.29
10.06
10.18
10.81
10.29
11.48
10.77
12.15
11.84
Energy to Peak Load
lbf in

313.00
288.00
240.00
247.00
1,056.00
1,272.00
no data
1,165.00
452.00
346.00
428.00
425.00
428.00
418.00
556.00
647.00
Total Energy
lbf in


1,240.00
1,276.00
1,305.00
1,211.00
2,536.00
2,290.00
no data
2,438.00
1,199.00
1,496.00
1,721.00
1,450.00
1,370.00
1,216.00
1,650.00
1,892.00
Weight
oz



4.04
4.12
3.83
3.83
4.39
4.27
4.36
4.23
4.20
4.19
4.20
4.28
4.50
4.38
4.92
4.82
Figure 3.5 is a plot of energy to peak load and sample weight for the Black Locust samples.  For comparison to the other strip lay-ups, see Figure 3.9 at the bottom of this page.  As was the case with the Okoume plywood samples, the most striking feature of this plot is the effect of 5 oz Raka (direct sized, tight woven, plain weave E glass) on the energy to peak load.  Another interesting feature is the apparent low return on adding another layer of 1.8 oz Kevlar to the inside.  A check of the total thickness of samples with one and two layers of Kevlar showed no differences that could be attributed to the wood thickness, so we can not say that the samples with two layers contained less wood than the samples with one layer.  A better explanation might be that another layer of 1.8 oz Kevlar has such a marginal effect on the energy to peak load that we would need a larger data set to see the difference.
Figure 3.5.  Plot of energy to peak load vs lay-up weight in oz/square foot for samples made with Black Locust strips..
One of the more interesting aspects of the Locust sample behavior can be seen in the load displacement plot of Figure 3.6.  The first peaks in the plot indicate failure of the fiber/epoxy skins while the second peaks indicate continued load bearing from the wood, although there may also be a contribution from progressive failure (tearing) of the fiber/epoxy.  The load displacement plots and the total energy values listed in Table 3.7. show that the Black Locust provides a significant degree of load carrying capacity after the fiber/epoxy skins have initially failed.  To know the extent, if any, of damage to the Locust at the point of peak load we would need to do drop tests in which the energy of the impact head was varied to match the energy at peak load.

For the cedar strip samples tested so far, the drop tower has been setup with the rubber stop block too close to the samples to determine if a load peak from the Cedar appears in the same way as the wood peak from the Locust.  However, we can expect that the Locust deformed more than the Cedar prior to failure, so any load carried by the Cedar should appear at smaller displacements than for the Locust. To illustrate, data for the Cedar sample 01 listed in Table 3.1 is compared to the locust sample ML01 in Figure 3.7.  The very large secondary rise in load for sample 01 is due to the impact head contacting the rubber stop block.  Load from the Cedar is probably located close to the first peak. For the ML01 test, the stop block had been put in its lowest possible position beneath the sample, so the wood peak was not masked by the stop load.  Also shown in Figure 3.7 is a load plot for an Okoume plywood sample P01, listed in the
plywood lay-up section Table 2.1.  For this sample, a load signal from the wood is located between the first peak and the stop block load.  Comparison of the Locust wood peak and the Okoume wood peak shows that the locust provided more secondary resistance to complete failure than the Okoume.  It is reasonable to expect that the Locust also provided more secondary resistance to complete failure than the Cedar.
Figure 3.6.  Representative load vs displacement plots for Black Locust lay-ups.  Sample details are listed in Table 3.7.
Figure 3.7.  Comparison of load vs. displacement curves showing the effect of  the stop block position.  The stop block was set lowest for the the Locust sample ML01.
Another interesting aspect of the Locust samples was the appearance of the samples after testing.  The Cedar samples tended to be punched down below the sample support and show significant fragmentation of the wood.  In contrast, the Locust samples tended to be ejected from the sample support in two pieces, and show relatively clean breaks, as shown in Figure 3.8.
Figure 3.8.  Images of two Black Locust lay-ups showing the relatively straight fracture.
Figure 3.9.  Energy to peak load for all strip samples.
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