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Damage of Composites - Helicopter Tail Rotor Spar

A method for incorporating a damage model for composite structures into the ANSYS general-purpose finite element program was developed using MRLife, a damage model developed at Virginia Polytechnic Institute and State University. The damage model was incorporated into a subroutine linked to ANSYS, creating a life prediction system in which entire composite structures can be evaluated for damage using realistic geometry and loading.

This method was used to predict the life of a composite helicopter tail rotor spar. Life prediction results using this new damage-based method compared favorably with experimental and field data on the life of tail rotor spars. Since the tail rotor spar fails by an accumulation of damage in actual operation due to the cyclic loading, the damage-based life prediction method represents a physically realistic approach for life prediction of composite rotorcraft components.

MRLife calculates the residual or remaining strength during the application of cyclic loading and compares this value to the failure function. As cyclic loading is applied, the remaining strength will decrease due to damage of the composite material. The failure function is based on the applied loading and the known strength of the composite. When the remaining strength decreases enough so that it reaches the value of the failure function, failure of the composite occurs. MRLife predicts the residual life at any time in addition to predicting failure.

By incorporating the damage model in MRLife into a finite element code and applying the damage calculations to every point in the finite element model, knowledge of the location of the critical region prior to analysis is not required, since damage will accumulate throughout the structure. Also, when damage occurs, internal loads can re-distribute, more accurately modeling the physical behavior of the composite structure.

Key to the development were the following:

  • The use of the SHELL91 element, which permitted use of the user plasticity subroutine for incorporating the damage module.
  • The use of the user table option in ANSYS for entering an unlimited amount of material input data in the damage problem without having to re-link the code.
  • Availability of a state variable array for storing the previous values of the residual strength, required to perform damage calculations.
  • The ability to store the failure function and residual strength data in element variables that can be listed and plotted as regular ANSYS element result quantities.