Modeling matrix cracking in composite rotor blades within VABS framework

Abstract

This paper deals with modeling of the first damage mode (i.e. matrix micro-cracking) in helicopter rotor or wind turbine blades and how this effects the overall cross-sectional stiffness. The helicopter rotor or wind turbine rotor system operates in a highly dynamic and unsteady environment leading to severe vibratory loads present in the system. Repeated exposure to this loading condition can induce damage in the composite rotor blades. These helicopter rotor or wind turbine blades are generally made of fiber-reinforced laminated composites and exhibit various competing modes of damage such as matrix micro-cracking, delamination, and fiber breakage. There is a need to study the behavior of the composite rotor system under various key damage modes in composite materials for developing Structural Health Monitoring (SHM) system. Each blade is modeled as a beam based on geometrically non-linear 3-D elasticity theory. Each blade thus splits into 2-D analyzes of cross-sections and non-linear 1-D analyzes along the beam reference lines. Two different tools are used here for complete 3-D analysis: VABS for 2-D cross-sectional analysis and GEBT for 1-D beam analysis. The physically-based failure models for matrix in compression and tension loading are used in the present work. We detect the matrix cracking using two failure criterion: Matrix Failure in Compression and Matrix Failure in Tension which are based on the recovered field. We set the strain variable which drives the damage variable for matrix cracking and this damage variable is used to estimate the reduced cross-sectional stiffness. The procedure presented in this paper is implemented in VABS as matrix micro-cracking modeling module. To investigate the matrix failure model, three examples are presented which illustrate the effect of matrix cracking on cross-sectional stiffness by varying the applied cyclic load. Finally, the stiffness degradation of composite cross-section of rotor blade due to matrix micro-cracking is correlated to the life of composite rotor blade using damage accumulation model. At the end, an empirical equation is given for the Stress Intensity Factor (SIF) to simulate the matrix micro-crack growth in future.