Improved methodology for the design of damage tolerant helicopter structures in composite materials

Damage tolerance is the ‘natural’ approach to address static and fatigue strength of composites structures, mainly due to the additional variability in manufacturing parts compared to traditional helicopter metallic components. This requires additional efforts dedicated to design, manufacturing and testing of coupons, elements and subcomponents aimed at reducing time and costs of full scale tests by means of a ‘test pyramid approach’. AgustaWestland, together with the Departments of Aerospace Engineering of Università di Pisa and Politecnico di Milano, is gaining a relevant experience, focused on rotor blade damage tolerance. The rationale of such a multi-centre and multi-disciplinary activity consists in investigating typical complex composites helicopter structures and identifying the critical issues related to fatigue behaviour and damage tolerance (nature of materials, type of joints, geometry details, damage modes). A wide test campaign, carried out both in RTD and environmental aged conditions, has been performed on various types of specimens (ILSS, DCB, ENF, MMB) made of different material systems, in order to collect basic data for the development of intralaminar and interlaminar damage progress models. The numerical method is based on explicit FE computations, used to perform quasi-static analyses. This allows modelling and identification of stable and unstable propagations with no convergence issues; moreover, contact interactions can be easily handled. Details are provided in the paper, where a significant structural detail, prone to develop multiple delaminations, was selected for validating the methodology.