Composites improve boat- building

(Published on October-November 2005 – JEC Magazine #20)

BY BOB SCHARTOW,
DIRECTOR, SPECIALTY PRODUCTS,
3TEX, INC.

At one time, 3D woven preforms were considered experimental and exotic. In the early 1990s, a novel process of manufacturing thick, integral three-dimensional (3D) orthogonal woven fabrics was invented at NCSU College of Textiles. Using this process, such textile preforms have been intensively commercialized for composite reinforcement.

By designing and building industrial scale 3D weaving machines, 3TEX, under the trademark 3WEAVE™, is weaving relatively thick (up to 1 inch) fabrics from virtually any kind of fibre. This has opened numerous commercial opportunities in the composites industry. These materials are being implemented in the marine, armour, automotive, aerospace, wind energy, and recreational markets at an increasing pace.

In the marine industry, there are processing advantages that 3D woven fabrics provide. The effects of high conformability, labour cost reduction from using thicker fabrics and fewer plies, and high p e rmeability providing ease of wet lay-up and vacuum infusion, are of special interest. In addition to process enhancements, composite stru c t u res and products reinforced with 3D woven preforms are not susceptible to delamination, obtain improved damage tolerance and increased impact resistance. 3D woven hybrid preforms combining E-glass, carbon, S2-Glass, Kevlar® and metallic can also easily be produced during the weaving process.

Weight and cost savings

While 3WEAVE™ preforms provide several important advantages in composite fabrication, the most obvious advantage is in lowering fabrication costs by reducing labour and cycle times. Users are replacing multiple layers of 2D fabric by a fewer number of thick 3WEAVETM plies to obtain an equivalent composite structure. Weight savings in the finished composite can also be achieved, due to higher fibre content resulting in less resin consumption. The preforms have been used to increase fibre volume fraction without adding weight to the final laminate, or maintain fibre volume fraction and reduce final laminate weight. 68-72% glass by weight is commonly achieved using 3WEAVETM and the vacuum infusion process.

The architecture of 3WEAVETM fabrics provides for ease of wet-out by hand or vacuum infusion. Other thick fabrics sacrifice fibre volume fraction to obtain the necessary permeability for wet-out. The fibre architecture of 3WEAVETM ( straight fibres), including its Z- yarn placement, explains its high permeability. The Z-yarn s act as capillary channels that transfer resin into the preform interior from the outer surface. The increased permeability translates directly into reduced cycle time, due to faster and easier wet-out. This permeable and thick fabric with high f i b re content allows the builder to reach the desired laminate thickness or weight in fewer plies and cut days from the production schedule. The resulting productivity improvement translates into greater plant capacity.

Mirage Yacht of Miami (Florida) produces sports fishing yachts from 40 to 61 feet long. 77oz/yd2 3WEAVE™ is hand laid with mat between layers. On a 40’ yacht hull, 2 days of labour were saved. The material, with its higher fibre volume fraction, remains relatively dense after hand wet-out resulting in a composite structure with greater glass content. While the glass content increased, the resin content decreased which resulted in resin savings of about 55 gallons.

Thicker materials must also remain highly conformable. 3WEAVETM fabrics are woven in such a way that they can be shaped into complex corners and features, reducing darting that is required of conventional materials. The absence of interlacing between warp and filling yarns allows the fabric to be bent and internally shifted rather easily. Less conformable fabrics would require extensive cutting and darting to avoid wrinkles and/ or buckling in the laminate.

For First Derivative of Stuart (Florida), the combination of converting from a solid, hand-laid laminate to vacuum infusion, balsa cores and 3 W E AVETM resulted in dramatic improvements to their business and product. The cost, weight and maintenance savings improved their product’s market position. The conversion also increased plant capacity, and met future MACT (clean air) regulations.

The bending stiffness challenge

Boat builders take advantage of the improved properties of 3WEAVETM composites to achieve improved performance and lighter weight products. Tensile, compressive and flexural stiffness, and strength are overall better in laminates made from non-crimp 3WEAVETM preforms, than in those made from comparable 2D woven or knitted fabrics.

Of primary importance to boat builders is flexural rigidity, or bending stiffness of laminate structures such as hulls and decks. The challenge when increasing fibre volume by using these higher performance materials, such as when converting to vacuum infusion, is maintaining an acceptable level of bending stiffness. The reason being that this property is proportional to the cubic thickness. A more dense fabric will be somewhat thinner than a less dense fabric of the same weight. However, the effect of thickness is partially overcome by the higher overall mechanical properties resulting from the greater glass content and straight fibres in composites reinforced with 3 W E AVETM. While the warp (X) flexural stiffness in composites made from 3WEAVETM preforms is comparable to common 2D fabrics, the flexural stiffness in the fill (Y) direction is typically much greater.

With this in mind, users have found that a 5-15% reduction in laminate thickness can take place using 3 W E AVETM. In thicker laminates, some users have used a layer of continuous strand mat between layers of 3WEAVETM as an inexpensive way to boost the thickness of the laminate. While this somewhat reduces the overall fibre content, it moves the denser, higher performance material toward the surfaces where they provide maximum effect on bending stiffness.

The unique architecture of 3WEAVETM can result in higher bending stiffness in the Y as compared to the X direction when integrated into composites. This can be ideal for smaller boats. For example, in marine hull applications this higher bending stiffness acts as a beam across the stringers. Boat builders are also using versions of the material configured with 60% fibre in the X direction for larger hulls where the material is applied at 90 degrees to the stringers, and for such items as hardtops and radar arches where additional unidirectional fibre is desirable.

Dramatically improved impact damage tolerance

Perhaps the most compelling performance advantage for marine applications is the dramatic improvement in composite impact damage tolerance. In drop tower impact testing, 3 W E AVETM laminates have shown reduced delamination area, and require an increased number of energy blows to penetrate the panel as compared to laminates made from multiple layers of conventional 2D reinforcement. Even very moderate through-thickness fibre content increases apparent short beam shear strength by 10-30% over 2D textile laminates. Studies show that delamination resistance, defined via the critical strain energy release rate characteristic in 3 W E AVETM composites with 1% to 8% fibre volume fraction in Z direction, is up to 20 times higher. Boat builders using a single layer of 3 W E AVETM optimize this effect.

The form and architecture of the fibrous reinforcement is perhaps the single most important component determining both the performance and cost in a textile composite material. This is evidenced by the trend of glass reinforcements from random mat/fibre, followed by woven roving, and more recently stitch bonded fabrics. This evolution continues with the non-crimp 3WEAVETM material. The ability to take complexity and labour out of composites fabrication through the automation of engineered 3D preforms is improving marine manufacturing. It is apparent that these materials will continue to gain acceptance as more companies recognize the value they offer.