TECHNOLOGY AREA(S): Air Platform, Materials/Processes ACQUISITION PROGRAM: PMA276 H-1 USMC Light/Attack Helicopters

OBJECTIVE: Design and develop an analysis tool to predict the strength and fatigue life of curved and tapered composite rotorcraft flexbeams with emphasis on accurately modeling transverse shear and ply drop-offs.

DESCRIPTION: The Navy currently has a need to accurately measure the durability of rotorcraft flexbeams. NAVAIR policy for durability determined by analysis typically requires the analysis to show 4 times the service life required. However, for flexbeams, the reality is that testing shows actual life well below required service life and what was analytically predicted. This discrepancy between predicted life and tested life has cost both time and money in redesign, with efforts spanning years and costing millions of dollars. Attempts to address these shortcomings have used changes in the ply layup as well as the locations of ply drops with respect to the neutral axis to improve life. However, a current lack of physical understanding of the physics involved in flexbeam fatigue failure prevents the redesign from being based on a more accurate analysis method or understanding than originally used to cleared the failed part. Instead, the same analysis used to show the failed part had sufficient life, is reused on the newly designed part�historically with little success. The analysis used is inadequate because these are complicated composite structures with hundreds of plies, often hybrid materials, and twisted and tapered geometry. Additionally, the loading environment, while understood, is equally complex with axial, bending, and torsion loads. This loading leads to multiaxial stress that, combined with the geometry of flexbeams, makes determining stresses/strains at the ply level of first importance, but is often ignored.

Existing analysis tools contain several areas of weakness. One area is the inability to accurately resolve the out-of- plane shear stresses/strains necessary to predict delamination. Even if accurate stress/strain values are obtained, due to the complex loading environment multi-axial failure criteria may be required. For example, using maximum strain failure criteria would be inappropriate if analysis shows that the ply strains are highly multiaxial as it does not account for multiaxial strain interactions (e.g., hydrostatic strain condition), which cause different failure mechanisms in a material (e.g., yielding vs cavitation). The existence of ply drop-offs (or defects) results in stress concentrations that need to be considered, as they can be a source of matrix cracks or delaminations. Currently the impact of ply drop-offs on the local stresses within the flexbeam are poorly understood and not modelled in analysis. Ply drop-offs and the dimensions of the ply drop-offs used in analysis need to be addressed. Accurate modeling of thick laminates typically requires at least one element per thickness or more, negatively impacting the size of the final model and the run time for solution.

Recent advances in composite damage assessment have allowed for the consideration and tracking of matrix cracks and delaminations. This SBIR topic seeks to extend these methods to include modeling the complex geometry and loadings of rotorcraft flexbeams and similar structures. Extending these methods to fatigue, the inclusion of ply drop-offs, and accurate interlaminar stress estimations will require innovative work. Models will be optimized to reduce the number of elements needed to accurately predict stress/strain. Success would allow not only analysis of plan-built configurations, but also damaged flexbeams and the effects of defects. The ability to obtain accurate stress/strain values with fewer elements is sought. Current practice within academia utilizes at least one element per ply to resolve interlaminar stress/strain. Commercial analysis practice typically doesn�t meet that threshold, making interlaminar stress/strain values inaccurate. An automated process to create the appropriate number of elements per ply is critical for actual tool usage. This should include the number of elements per ply being determined by a convergence of the critical stresses/strains governing the durability of the flexbeam, such as interlaminar stress or the stress around the ply drop-off.

Although not required, it is highly recommended to work in coordination with the original equipment manufacturer (OEM) to ensure proper design and to facilitate transition of the final technology.