Given the complexity of modern boat construction, particularly in composites, the number of potential technical topics to cover in Professional BoatBuilder is, if not inexhaustible, then certainly enormous. One testament to that complexity: in 26 years’ worth of bimonthly issues, the magazine’s editorial content has rarely had occasion to repeat itself. Even if you were to add in the dozens of technical seminars on composite construction presented at now 25 major editions of IBEX—the acronym for the marine-industry trade show founded by and co-produced by the magazine—there’s been little duplication of content between the seminar program and the magazine’s lengthy list of feature articles.
Sometimes, though, PBB’s editors intentionally move a topic between the two venues, by selecting an article to become a seminar, or vice versa: adapting a seminar into an article, which occurs in the latest issue (No. 158, October/November 2015) of the magazine. The feature on page 34 titled “Making and Testing Laminate Samples,” originally an IBEX seminar a while back, contains valuable information never covered in detail in the magazine. The so-called deck for this article tells the reader what to expect, in the briefest of terms. It states: “Two veteran marine-composites engineers [Richard Downs-Honey of Gurit and Derek Novak of American Bureau of Shipping] offer practical guidance on how to prepare suitable test specimens. And just what to test.”
However, space limitations during magazine production prevented us from running a sidebar to accompany the main text. The information in that sidebar was not part of the IBEX seminar; it simply enhanced the print adaptation. Here it is, along with artwork from Eric Greene’s book, discussed below.
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Naval architect Eric Greene (Annapolis, Maryland) is credited with helping Tim Hodgdon, owner-operator of Hodgdon Yachts (East Boothbay, Maine), make the tricky transition from large cold-molded motoryachts and sailing yachts to advanced-composite pre-preg construction (see the cover story of Professional BoatBuilder No. 153). Greene also helped naval architect Robert Scott create an updated revision of his popular book Fiberglass Boat Design and Construction, published in 1996 by the Society of Naval Architects and Marine Engineers (see PBB’s review in issue No. 47). That same year, Greene submitted the final report of the Design Guide for Marine Applications of Composites, a 278-page document prepared for the U.S. Department of Transportation’s Ship Structure Committee; Greene was principal author. Out of that report came his book Marine Composites, the second edition of which was published in 1999 and remains a definitive technical reference work in the industry.
In the chapter on structural design, in Marine Composites (Second Edition), Greene devotes a lengthy section to laminate testing. He covers all the American Society for Testing and Materials (ASTM) tests, including test-method variants formulated by the International Organization for Standardization (ISO) and Suppliers of Advanced Composite Materials Association (SACMA). Greene’s written descriptions are concise, and illustrated with simple line drawings. This first section deals with micromechanics; a later section, with structures. At the end of the first section Greene concludes: “The marine industry has yet to develop a set of tests which yield the right type of data for the marine designer.…Until these tests are developed, there is still a need for some common testing.” At a minimum he recommends a “comprehensive laminate test program” that includes the ASTM D3039 tensile test, the SACMA compressive test, ASTM D790 flexural test, and a sandwich panel test, which—unlike traditional tests—can realistically model edge conditions. (In addition to appearing in Greene’s book, those ASTM tests are explained in the Professional BoatBuilder article.)
Regarding sandwich panel testing, Greene writes: “Traditional test methods involve testing narrow strips, using ASTM standards….These tests assume that hull panels can be accurately modeled as a beam, thus ignoring the membrane effect, which is particularly important in sandwich panels. The traditional tests also cause much higher stresses in the core, thus leading to premature failure….A student project at the Florida Institute of Technology investigated three-point bending failure stress levels for sandwich panels of various laminates and span-to-width ratios. The results were fairly consistent for biaxial (0/90°) laminates, but considerable variation in deflection and failure stress for double bias (±45°) laminates was observed as the aspect ratio was changed. Thus, while the traditional tests yield consistent results for biaxial laminates, the test properties may be significantly lower than actual properties for double bias and triaxial laminates.”
Greene describes a specially designed test apparatus “consisting of a table, a water bladder for pressurizing the panel, a frame to constrain the sides of the water bladder, and framing to restrain the test panel.” It’s an apparatus similar to one shown in a PBB article titled “Two Labs” (issue No. 148), where test panels were pressure-loaded during research conducted for the Hodgdon yard’s initial foray into advanced composite construction: an 80’ (24.4m) epoxy-infused high-performance prototype for the U.S. special operations command.
Marine Composites (Second Edition) is published by Eric Greene Associates Inc., based in Annapolis, Maryland. The volume is available as Adobe Acrobat digital files either in its entirety or as individual chapters; for download links see the website www.ericgreeneassociates.com.