OCT 2018


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NEWS 53 and composites are complex, in general, we can say that metals deform in a linear fashion and pass the energy wave through to the next structure, causing serial failure (breakage). Compos- ites, on the other hand, deform in a nonlinear fashion." Advanta- geously, they can be designed to deform predictably and then fail in a specific mode, which enables the designer to tailor the energy absorption via selective use of the matrix. Lowsdale suggests one way to look at this is to compare force vs. deflection curves for metals and composites. With metals, there is a straight line (linear curve) until the material's ultimate tensile strength or yield strength is reached. en there is a sharp drop off as the material fails and energy is released. is curve is roughly the same at both high and low strain rates. "With composites," he notes, "the material's curve continues to rise well above that for metals and then gradually tapers off as failure begins and energy is released." e area under each curve is the amount of energy that can be absorbed in a crash. And if one is laid over the other, the compos- ite's curve is much larger, indicating that the composite can absorb much more energy in a crash and that it can do this at much higher strains and at much faster rates. "e ability to absorb more energy at a given force and over a given distance," Lownsdale concludes, "is a big advantage of composites." Manufactured by low-emission SCRIMP To establish material and process parameters, the design team conducted extensive finite element analysis (FEA), using ANSYS (Canonsburg, PA, US) simulation software for the body structure, in addition to sample validation testing. E-glass fibers in stitched noncrimp fabric (NCF) and unidirectional (UD) carbon fiber NCF materials, in a vinyl ester resin closed infusion molding process, were selected to best meet mechanical requirements and environ- mental conditions, such as weather and ultraviolet (UV) light. TPI manufactures the bus bodies using the Seeman Compos- ites Resin Infusion Molding Process (SCRIMP), a customized upgrade of vacuum-assisted resin infusion developed for making high-quality, repeatable composite parts with nearly zero VOC emissions. TPI engineering manager Brian Lucchesi says TPI has been producing large structural composite components with this method for more than 25 years. He notes that tests have verified its workplace safety, noting that "VOC emissions of 0.2 to 0.6% from TPI's SCRIMP process have been observed and confirmed by the Rhode Island Department of Environmental Management." TPI also manufactures all its own tooling, including multi-piece composite molds supported by a structural steel framework. e bus is built in four basic sections using the same mate- rials and manufacturing process, in a mix of solid laminates and sandwich structures with a balsa wood core. e bus body consists of the upper body from the roof down to the lower edge of the windows, which includes the entire windshield and rear window geometries; and the lower body from the lower window line down to the floor, which includes all structural chassis geometries for vehicle subsystem attachments. (Fig. 2) e roof consists of one upper fiberglass structural sandwich molding down to the window line, and another for the lower (ceiling) geometry. Manufacture generally proceeds as follows: First down in the mold is a gel-coat layer to produce a smooth, appealing exterior. Dry fabrics are then manually arranged in the mold: Triaxial (0°, ±45°) fiberglass reinforcements provide longitudinal and torsional stiffness for the main global plies. Unidirectional (UD) carbon fiber NCF strengthens window frames and the A-pillars that attach the The buses are lightweight enough to offer operators a nominal range of 350 miles on a single charge. FIG. 2 Infusion of bus body, upper and lower Upper bus body layup (top) and lower body layup (bottom), ready for infusion. Source | TPI Technology for Zero-Emissions Buses

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