CompositesWorld
Issue link: https://cw.epubxp.com/i/442058
JANUARY 2015 64 CompositesWorld Michael R. LeGault is a freelance writer and regular CW contributor located in Ann Arbor, Mich., and is the former editor of Canadian Plastics magazine (Toronto, Ontario, Canada). mlegault@compositesworld.com FOCUS ON DESIGN bond. Second, the laminate at the joint interface is constructed of ±45° fabric, which reportedly reduces stress in the laminate at the joint surface. Extensive testing of laminate specimens was conducted in parallel with FEA to ensure that actual mechanical properties (in particular, stifness) correlated with the simulations. After testing validated the computer models, RIBA received approval from CIFA to build the frst commercial boom sections for a concrete-pumping machine. Tey replaced the fnal two articulating sections — roughly half the total length of a 45m mechanical arm. Each section is about 11m in length. Tey do not replace the initial steel sections of the arm with composites because the cost vs. beneft tradeof favors metals. Building the entire arm with composites would be cost-prohibitive. Boom sections are molded in female molds. During layup, overlaps of 25.4 mm to 38 mm are created between adjacent plies of prepreg, per aerospace practice. Te molds are joined at the laminate overlaps, creating a hollow tube. After layup, the molds are bagged internally and externally, and the bag materials are joined at the mold's open ends to form a single bag (see photos, above). During autoclave cure (at 6 bar), the vacuum pulled on the bag combined with the external pressure applied by the autoclave ensures optimal consolidation and strength-to-weight ratio in the laminate, yet avoids pressure-related deformation or alteration of the mold shape. "Te pressure is acting on all the surface of the bag, internally and externally," says Deschi. "In this way we squeeze the plies without afecting the mold." A fnal challenge was curing parts with walls as thick as 30 mm. Epoxy undergoes a robust exothermic reaction — the thicker the laminate, the higher the temperature. Te risk is uneven heating of the mold and laminate layers, resulting in material degradation at the center of the laminate's through-thickness. Using a theo- retical autocatalytic model, engineers predicted cure behavior as a function of temperature and devised a cure cycle with a cure temperature of 130°C and dwell steps that avoided exothermic peaks and resulted in homogeneous heating of the mold and laminate. Cycle duration, including cool down, is ~18 hours. After cure, the demolded boom is CNC-machined on a 5-axis system, bushings are bonded into place and each section is inspected using a phased-array ultrasonic NDT system. Since making its frst 22m boom section set, RIBA's line of booms for concrete-pumping equipment has steadily expanded. Te company now makes 10 diferent carbon booms for concrete- pumpers with arms ranging from 25m to 101m. Widths of booms can vary from 15 to 45 cm. Last year, RIBA produced booms for 110 arms— about 250 indi- vidual booms, and Bedeschi expects a 10% increase in sales during 2015. Tat the booms have quickly made inroads into an inher- ently conservative industry is a testament to their highly functional design and benefts. At 101m, this is the world's longest concrete pumping arm (a certifed Guinness Book of World Records awardee). The fnal 50m of the arm (colored gray) is assembled from three composite booms molded by RIBA Composites. The longest boom, at 14m, weighed ~1000 kg, including 650 kg of carbon fber, and was cured in a single autoclave cycle. Source /RIBA Composites Three types of carbon fber prepreg are layed up in female molds, two tools (lower, pictured here, and a matched upper tool) per boom section. Molds are bagged internally and externally (below) and cured in an autoclave. Source / RIBA Composites Read this article online: short.compositesworld.com/RIBAboom