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2. Okoume plywood lay-ups

Table 2.1 contains data for samples made with 4 mm Okoume plywood and five different styles of glass that are of interest to kayak builders.  All samples were made with Raka 127 epoxy and 606 hardener.  Each panel received a seal coat prior to glassing.  Two layers of the respective glass were laid on the impact side (outside) with one layer on the inside.  One fill coat was applied to the plain weave lay-ups.  Representative load-displacement curves for the 4 mm Okoume samples are shown in Figure 2.1.  A plot of peak energy-sample weight is shown in Figure 2.2.
Table 2.1: Data for samples made with 4 mm Okoume plywood and different styles of glass
Total
Panel
Weight
(oz/ft^2)



9.90
9.81
9.82
9.81
10.15
9.85
9.95
9.99
10.43
10.64
10.64
10.40
9.87
9.99
9.90
9.85
10.88
10.91
11.08
10.99
Sample
Number





P01
P02
P03
P04
P05
P06
P07
P08
P09
P10
P11
P12
P13
P14
P15
P16
P17
P18
P19
P20
Glass Style






4 oz PW E
4 oz PW E
4 oz PW E
4 oz PW E
4 oz PW S
4 oz PW S
4 oz PW S
4 oz PW S
6 oz HS S
6 oz HS S
6 oz HS S
6 oz HS S
5 oz DS E
5 oz DS E
5 oz DS E
5 oz DS E
6 oz PW E
6 oz PW E
6 oz PW E
6 oz PW E
Weight
Wood
(oz)




3.24
3.17
3.20
3.23
3.29
3.19
3.23
3.26
3.13
3.26
3.22
3.04
2.99
3.20
3.03
3.13
3.15
3.14
3.14
3.15
Weight
Total
(oz)




4.39
4.37
4.40
4.35
4.50
4.37
4.43
4.45
4.65
4.69
4.70
4.48
4.25
4.48
4.24
4.41
4.85
4.86
4.89
4.85
Weight
Glass
and
Epoxy
(oz)


1.15
1.20
1.20
1.12
1.21
1.19
1.21
1.20
1.52
1.43
1.47
1.44
1.25
1.28
1.21
1.28
1.70
1.72
1.75
1.70
Glass
and
Epoxy
per
Layer
(oz/ft^2)

0.87
0.90
0.89
0.84
0.91
0.89
0.90
0.89
1.14
1.08
1.11
1.11
0.97
0.95
0.94
0.95
1.27
1.29
1.32
1.28
Energy
to
Peak
Load
(lbf in)


304
308
301
NA
521
512
581
553
759
809
776
762
1108
1188
1352
1166
514
453
Propa-gation
Energy
(lbf in)



227
297
321
NA
211
292
235
240
182
243
234
196
61
60
71
68
299
317
All samples made with Raka 127 epoxy and 606 hardener
4 oz PW E: BGF style 1522 - 24x22 plain weave made with a 2-ply twisted yarn purchased from J.R. Sweet
4 oz PW S: BGF style 6522 - 24x22 plain weave made with a 2-ply twisted yarn purchased from J.R. Sweet
6 oz 8-HS S: BGF style 6580 - 73x70 6-oz 8-harness satin weave made with a 2-ply twisted yarn purchased from   J.R. Sweet
5 oz DS E:  tight plain weave made with direct sized E glass purchased from Raka
Figure 2.1.  Representative load-displacement data for the 4 mm Okoume samples
Figure 2.2.  Energy to peak load for 4 mm Okoume samples.
The surprising result from this series of tests was the energy to peak load for samples made with 5 oz Raka “tight weave”.  Because this is E-glass with an actual weight of around 4.6 oz/yd^2, I was surprised to find that the Raka claims of “stronger than 6 oz’ were not only justified in terms of energy to peak load, but that it out performed 6 oz 8-harness satin S-glass.  More work needs to be done to understand this result, but one of the obvious differences between the failed Raka samples and all the others was a large extent of strain whitened glass/epoxy, as shown in Figure 2.3.  Another difference was the low propagation energy in comparison to the other samples.
a b
Figure 2.3.  Tested samples of 4 mm Okoume with 5 oz Raka E-glass and 6 oz 8-HS S-glass.  The impact side of the samples is shown in (a) and the underside of the samples is shown in (b).  In each image the 5 oz is on the left and the 6 oz is on the right.  (note:  the 6 oz sample in (b) is labeled incorrectly - it should read 1 x 8-HS)
Carbon fiber

Ted Henry laid-up some carbon fiber (CF) and 6 oz glass on 4 mm Okoume plywood.  There were two objectives for this series of samples.  One objective was to evaluate the benefit of using CF.  The second objective was to evaluate the effect of resin on sample strength.  Because one would not expect large differences in impact strength originating from the properties of the epoxy, we decided maximize the effect by choosing a lay-up with relatively high resin content and relatively low strength.  To minimize other variables, Ted used 6 oz plain weave from the same roll as was used for the samples described in Table 2.1, and plywood from the same sheet as for samples described in Table 2.1.  However, he used MAS epoxy rather than the Raka that was used in the other samples.  These samples had two layers of 6 oz on the impact side and one layer of 6 oz on the backside.  The impact sides received a fill coat, as did the other 6 oz samples.  For the CF samples, Ted used one layer of  5.8 oz 2x2 twill CF on the inside, and one layer of 6 oz plain weave E-glass on the impact side, with one fill coat. 

Table 2.2 summarizes the sample details and test results.  The 6 oz plain weave E-glass reference samples are the same ones described previously in Table 2.1 and Figure 2.1.  Note that Ted's all-glass samples were heavier than samples with the equivalent lay-up listed in Table2.1. The  weight difference indicates that the samples had a higher resin content attributed to cool shop temperature causing the epoxy to be thicker and harder to control..  Also note that Ted's CF samples were not much lighter his all-glass samples, despite having only one layer of glass on the impact side.  Because CF is less dense than glass, it has a greater fiber volume than the equivalent weight of glass.  Therefore, for an equivalent weight fabric and fiber/matrix volume fraction, CF will use more resin than glass would use, resulting in more weight per layer.
Table 2.2 Ted's glass and carbon fiber smples
Sample





TH01
TH02
TH03
TH04
TH05
TH06
TH07
TH08
Lay-up





6 oz PWE
6 oz PWE
6 oz PWE
6 oz PWE
6 oz PWE/CF
6 oz PWE/CF
6 oz PWE/CF
6 oz PWE/CF
Weight Wood
(oz)



2.89
2.91
2.85
2.90
2.90
2.84
2.89
2.91
Weight
Total
(oz)



4.46
4.54
4.54
4.45
4.29
4.20
4.24
4.28
Total
Panel
Weight
(oz/ft^2)


11.13
11.35
11.34
11.13
10.73
10.51
10.60
10.69
Energy
to
Peak
Load
(lbf in)

563
547
595
515
390
373
354
401
Total
Energy
(lbf in)



777
836
951
812
530
552
494
556
Figure 2.4 is a plot of energy to peak load and sample weight for three sets of samples.  Ted’s 6 oz plain weave E-glass samples, made with MAS epoxy, were slightly stronger than the reference samples made with Raka epoxy.  It is interesting to note that not all the reference samples were tested at one time.  Two of the reference samples were tested nearly three months prior to testing Ted’s samples, and two of them were tested at the same time as Ted’s samples.  But Figure 2.3 shows that results for the reference samples group together nicely, as do the results for Ted’s samples.  This adds confidence that the difference can be attributed to the epoxy.  However, based on these results alone it is not possible to separate the contribution from epoxy properties from the contribution of having more epoxy in the lay-up.  The CF samples had significantly lower energy to peak load values than did the 6 oz all-glass samples.  It is reasonable to propose that the CF samples had lower values of energy to peak load because this is a general characteristic of CF compared to glass and because the greater stiffness of CF would tend to concentrate stresses in that layer.  Additional experiments, such as lower energy impacts, could provide insight to how CF influences the impact resistance of wood/epoxy kayak lay-ups.
Figure 2.4.  A plot of energy to peak load and sample weight for samples made with 6 oz plain weave E-glass and samples made with 5.8 oz carbon fiber (CF) on the inside.  The samples made with CF had lower energy to peak load, which signals the onset of complete failure.
(a)
(b)
Figure 2.5.  Images of Ted's  (a) all 6 oz glass samples and (b) 6 oz glass and 5.8 oz carbon fiber after testing.
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