In the early 1970s when fiberglass boats began to hit their stride, we heard some pretty extravagant claims for the new material. It was maintenance free, lasted indefinitely, didn't rot and didn't corrode.

But fiberglass, with its high strength-to-weight ratio, good mechanical properties, reduced maintenance and improved corrosion resistance, didn't turn out to be the "magical" material it was first thought to be. Fiberglass, indeed all composite boats--like people--age.

In the case of fiberglass boats, we're not talking about extraordinarily rough seas, storms or failures that result from putting a boat on the rocks of running into something. Aging comes from structural deterioration due to the wear and tear in the normal use of the boat.

Fatigue is really the name of this breakdown. It is the accumulation of stress-induced microcracks that gradually increase in size until they are large enough to cause fiber-resin separation, or pullout, and eventually fractures. Think of a paper clip that is repeatedly bent back and forth until it breaks. When failure occurs, it depends on the magnitude and the number of stress cycles. The fatigue limit is the number of times the material can be stressed without suffering any loss of stiffness or strength.

Boats are normally exposed to two types of fatigue. Dynamic fatigue is caused by waves slapping or slamming against the hull, machinery vibration, and cavitation. Static fatigue results from resisting steady loads for long periods of time. Examples are the pressure of water, boat stands of cradles against the hull. The greatest source of fatigue is waves hitting the hull. Fishing boats, when they are hauling, setting and towing gear receive a combination of dynamic and static stresses.

All materials fail, or fracture, when forces acting on them--tensile, compressive and fatigue stresses--exceed the materials ultimate strength. Given a particular material, a designer tries to size the material so that the forces it's subject to are some fraction of their ultimate strength. These are the "design rules" or "safety factors" that, hopefully, ensure adequate life and performance.

For example, for a component that has to withstand a 1,000-pound load, the designer will call for dimensions giving an ultimate strength of 2,000 pounds, or a safety factor of 2. In actual practice, safety factors range from 1.5 to 4 or more, depending on the component. Safety factors such as these have been carried over from metal and wood fatigue tests; thus, for fiberglass composites, they may be either too conservative or not" conservative enough. The problem is that there is not a lot of data available on the durability of fiberglass laminates over time.

Fiberglass fatigue

One report that is available is a six-year study of fiberglass laminate fatigue on hulls carried out under the direction of Paul Miller, assistant professor of Naval Architecture and Ocean Engineering at the U.S. Naval Academy. The academy and the University of California at Berkeley jointly did the work and were supported by the American Bureau of Shipping, the fiberglass boatbuilder TPI Composites (formerly Tillotson-Pearson) and Maricomp (a California marine structural analysis company).

The study depended on locating one or more fiberglass boats that had been in service for a significant period of time and whose service history could be documented in detail for the hours on the water, and wind, weather and sea conditions the hull was subject to. From these figures, the number and magnitude of strain cycles or wave impacts could be estimated with reasonable accuracy.

Then the boats' manufacturer had to be able to supply test samples and panels fabricated to the original specifications to represent the laminate's original strength. Calibrated strain gauges went on the boats to measure the effects of fatigue on stiffness and strength.

The boats that met the criteria were two J/24 one-design racing sailboats, which had been built by Tillotson-Pearson (now TPI Composites) of polyester resin and E-glass. Both boats were used at a sailing instruction and charter company in the San Francisco Bay area and detailed records of usage were available. Typical sailing conditions were 1-foot seas, 10- to 12-knot winds and boat speeds in the 5- to 6-knot range.

One boat, the "high-mileage" J/24, entered the charter fleet in 1984 and had 11,300 hours of sailing, and 10.2-million wave encounters. The "low-mileage" boat began service in 1999 and had 740 hours of sailing and 600,000 wave encounters. The stiffness, or strength, loss in the high-mileage boat was 18 percent, and that for the low-mileage boat was 4 percent.

Bending tests showed, as might be expected, that hull stiffness declined with wave impact loads, though the stress damage was cumulative. For instance, with wave impacts at 12.5 percent of the laminate's ultimate strength there was no loss of stiffness, even out to one million stress cycles or wave impacts but after that point, damage did take place. And at 25 percent stress there was no appreciable stiffness loss below 200,000 wave impacts.

Microcrack fatigue growth is the reason the hull loses its stiffness. You can't see that damage, but it is there. Eventually, stress cracks will begin to appear, first on the hull or deck exterior but also on the interior. If the impacts are severe enough, and they continue long enough they will weaken the hull and cause structural failure. Stress also increases permeability and moisture penetration into the laminate, which introduces a whole new set of problems.

Moisture

In the normal course of events--no breaches in the gel coat, no cracks, crazing or collisions--a boat that stays in the water absorbs moisture. The moisture slowly migrates through the gel coat and into the voids, resin-starved pockets and microcracks in the laminate. It is only the submerged part of the hull that takes on significant moisture and then only to the extent of about 1.8 percent of the hull's weight below the waterline.

The process begins when the boat is put over and reaches equilibrium...

The news.come from www.bossgoo.com