Abstract:
Due to its low density, optical transparency, and ability to withstand large plastic deformations without failure, polycarbonate is being increasingly used as a structural material for light weight, impact resistant helicopter windshields. However, polycarbonate can exhibit brittle failure with degraded impact resistance when exposed to some types of high triaxial loads or high strain-rate loading conditions. This paper presents the state-of-the art simulation techniques developed at Bell Helicopter Textron Inc. (BHTI) to support the design of bird impact resistant windshields. Linear Elastic Fracture Mechanics (LEFM) methods were used to investigate mechanisms that can trigger the polycarbonate to exhibit brittle failure. Finite element simulations were conducted to explore the threshold conditions under which brittle failure can occur, and to correlate the material constitutive model against test data under a wide range of loading conditions and strain rates. The failure model of the polycarbonate material model was validated to reflect the specific loading conditions. The validated analytical tool provides the capability to design for equivalent or greater levels of impact protection using thinner and lighter polycarbonate and has been successfully used to guide the development of bird impact resistant windshields.