Problems
Most of the problems encountered during the course of contract NObs-2799 have been described elsewhere in the text. However, those problems and their solutions related to material behavior and production procedures will be briefly enumerated here, in much the order in which these problems presented themselves.
1. The problem of adapting the Marco method to the production of large single skin vessels was never solved and this effort was abandoned in connection with contract NObs-2799.
2. The problems of resin drainage and short pot life as applied to open mold hand layup procedures was solved tentatively through adoption of preimpregnated cloths as the laminating medium and ultimately by the introduction and adoption of thixotropic room temperature curing resins.
3. The tendency in the female mold method for the selvage of each ply of the outer skin shingle layup to curl away from the laminate was solved through cutting into the selvage at intervals. This tendency did not occur during the male mold process.
4. The problem of getting consistent satisfactory bonding between honeycomb and face laminates was solved through using one ply of impregnated mat next to the honeycomb and using a vacuum bag to assure intimate contact until cure was complete. Laminates were bonded directly to C.C.A. cures in the keel and internal reinforcements of the hull shell.
5. The impregnation of the bonding mat was subject to much experimentation. The problems were:
(a) Only enough resin should be used to thoroughly wet the mat so that added weight would be kept to a minimum;
(b) The method for applying the resin should give uniform application and not tear the mat or displace too many of the mat fibers;
(c) A means for handling the impregnated mat should be established which would minimize tearing and distortion.
For early flat panels such as bulkheads and girder webs, the resin would be spread out over the desired area and the dry mat set in place in the resin. A length of wire screen was placed over mat and resin, and the mat rolled into the resin using a length of 2" pipe above the screen. Mat intended for upper bonding layers was bench impregnated and carried to the layup on sheets of cellophane. While this method allowed most uniform distribution of resin and allowed minimum amounts of resin to be used, it compacted the mat more than appeared desirable considering the use intended, and was of course not applicable to use in the hull proper.
It was found that the most satisfactory method for all purposes was to apply most of the resin first, press the mat firmly into the resin, and trowel additional resin into the mat from the outside using plenty of pressure to complete impregnation with a minimum of trowelling. The appearance of the mat gave good indication of when impregnation was complete. Outer bonding layers were similarly done as bench operations except that mat was impregnated on one ply of cloth, which served to prevent excessive distortion or tearing of the highly fragile impregnated mat during handling.
6. Poor vacuum bag performance in the first boat was improved through incorporation of air stops across the honeycomb in place, to which the bag could be sealed. Snug-fitting dowels of C.C.A. were inserted with filled resin in adjoining honeycomb cells across the area in question. For the second boat, gaps were left between areas of honeycomb, so that vacuum bags could be sealed directly to the inner skin laminate of the hull or the bottom skin of the deck.
7. One problem in connection with mat bonding layers was the periodic failure of small areas to cure. This behavior was observed at intervals throughout the work on both vessels, and was never completely eliminated. A large percentage of all repair work was the result of this behavior. Certain possible contributing factors were recognized and possibly all occurrences are attributable to one or more of these factors.
Suspected causes were:
(a) not enough bulk in the layup
(b) ambient temperatures too low
(c) inhibiting agents in the honeycomb
(d) entrapped air in the mat
(e) inadequate dispersion of activators in resin
(f) moisture in the mat
(g) physical characteristics of molding setup resulting in excessive dispersion of exothermic heat.
8. During early practice layups, it was noted that flanged angle layups tended to close up upon curing. Therefore, before beginning fabrication of girder sections, a series of tests was made to determine the amount and uniformity of this distortion. As a result of these tests, it was possible to correct the angle of the flange mold so that upon curing, each flange angle would close to the desired 90 degrees.
9. During application of the vermiculite-resin mix to the internal reinforcement of the hull core, considerable cracking occurred and excessive exothermic heat was evident. This behavior was the result of excessive bulk and was corrected on subsequent work through application of vermiculite-resin fillers in three or more steps.
10. Another case where excessive bulk and high exothermic heat were encountered was the 30-ply bottom portion of the keel layup. Excessive heat was evidenced by the characteristic yellowish brown color of the laminate. While the effects of such heat on laminate properties other than appearance were not known, layup operations were suspended on several occasions in order to allow the laminate to cool.
Other solutions could have been either to alter the layup sequence to cut down bulk accumulation or to adjust activator concentration. The latter would probably not have been desirable because of the low-bulk character of the tapering flange to the hull, which was a part of the same layup.
11. The highly abrasive nature of cured glass-reinforced laminates posed the problem of determining suitable equipment for cutting and drilling these laminates. Ordinary woodworking tools were generally unsuitable except for the bandsaw, which was adequate for cutting relatively thin laminates, but at the expense of rapid blade wear. The tools found to be most suitable for various operations are listed below:
(a) For long straight cuts or cuts of relatively large radius in simple laminates and in sandwich panels, the electric handsaw equipped with an abrasive wheel was most used. Carbide-tipped saw blades are available which are capable of doing excellent work, but are quite expensive. Those blades perform very well when used on table and radial saws with similar materials and it is conceivable, considering abrasive blade breakage frequency, that use of a good carbide-tipped blade might be economical, but no information was available on the ability of these blades to take the binding strains likely to be encountered in handsaw use.
(b) For cuts of smaller radii such as lightening holes in floors and certain bulkhead and deck openings, a pneumatic reciprocating saber saw proved satisfactory. There was some tendency toward delamination when using this tool.
(c) Hole saws of high-speed steel performed well in cutting round openings in bulkheads, deck, and hull for stuffing tubes and thru-hull fittings.
(d) Ordinary high-speed steel drills were used in drilling for bolts and self-tapping screws, but required care in effecting final penetration so that excessive pressure would not result in serious delamination around the hole.
(e) Surface irregularities were best handled with electric disc sanders using fairly coarse grits. These were also used in areas where secondary bonding was to be effected, in making sure that all residual parting film was removed and in roughening otherwise smooth surfaces as an aid to bonding.
12. The problem of providing material between the faces of sandwich panels to resist compressive stresses of through-bolts was solved in two ways:
(a) For bolts the locations of which could not be known in advance, slugs of filled resin were injected between the skins after the bolt hole had been drilled and the honeycomb routed back around the hole. A mixture of 30% Edgar ASP filler and 70% Marcothix resin proved satisfactory.
(b) In known through-bolt areas, solid laminate was provided (MSB-23); or 20%-80% vermiculite-resin (ExMSB-23).