Theres little debate over the adhesive quality and toughness of epoxy resin-just look at where its being used. We hear about its presence in crucial structures such as aircraft wings, race car bodies and high-end custom racing yachts. But it takes a little familiarity with engineering lingo to help us understand why epoxy trumps its ester relatives.
Photo by Ralph Naranjo
Tormenting composites is a key part of material science and tracking the resulting stress and strain relationship tells us a lot about any given structure. Cured resin, for example may be part of an FRP laminate, or, in the case of epoxy fillers and fairing compounds, a putty made up of powder/filaments/hollow spheres that increases the viscosity (thickness) of the resin.
The reason why engineers measure tensile, flexural and compressive strength is to better understand how specific materials will behave under load. To gather the data, they clamp specific sized samples in powerful machines that press together, pull apart or add side-load to each test coupon. Tensile strength tests tug on each sample, measuring the load all the way to the breaking point. The elongation (stretch) that occurs as the load increases is called strain and graphing how stress (the load) and strain (elongation) interact tells us a lot about the material.
A tall, steep stress vs. strain curve signifies lots of strength and little stretch, while a lower peaked, lesser sloping curve that runs almost level for a good distance across the strain axis, signifies a material that allows lots of stretch prior to fracture. Both of these characteristics can be of value in a structure, and the area under the curve is technically referred as material toughness.
Naval architects and sailboat designers carefully calculate how much stiffness and strength they need to build into each part of a boat and how much core material and units or layers of FRP laminate will deliver whats needed. They may opt for higher modulus material (stiffer under load) and esoteric reinforcement such as S-glass or carbon fiber. But the most important step they make in improving an FRP laminate’s strength to weight ratio is switching to epoxy resin.
Theres more to composite engineering than the concern over sailing loads. Accounting for fatigue linked to the life span of the vessel is a big deal and the longevity hedge is made by building in a larger safety margin, especially in highly loaded parts of the hull and deck.
Making sailboats immune to bad navigation results in added material and weight-performance-robbing attributes to some, extra insurance to others. Thicker laminates, longer, wider keel to hull junctions, lower aspect rudders, and higher structural safety factors are easier to incorporate in a cruising boat.
However, there are a few downsides to epoxy resin and the first is conveyed via the price tag. Quart per quart, the stuff costs more than Mount Gay Rum. But unless youre planning to laminate an entire hull with the stuff, the volume to be used is usually not too excessive. Even so, it makes sense to mix modest size batches and learn to contour the surface with the skill of lath and plaster journeyman. Less excess material, means less sanding, less kicked-off-in-the-pot throwaway and less material wasted.
