Tool for Analysis to Predict Strength and Durability of Curved and Tapered Composite Structures under Multiaxial Loading
Navy SBIR 2019.2 - Topic N192-068
NAVAIR - Ms. Donna Attick -
Opens: May 31, 2019 - Closes: July 1, 2019 (8:00 PM ET)


TITLE: Tool for Analysis to Predict Strength and Durability of Curved and Tapered Composite Structures under Multiaxial Loading


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.

PHASE I: Develop a concept for and demonstrate the technical feasibility of an innovative approach to accurately (threshold 25%/ objective 10% error with respect to test results) model complex composite geometries under multi- axial loading. The Phase I effort should include a plan to be developed under Phase II.


PHASE II: Develop a prototype of the innovative analytical tool and demonstrate the ability to accurately predict the stress/strains produced under multi-axial loading and delamination for complex composite geometries. Provide validation by comparing analysis results to test results of a small scale flexbeam-like subcomponent containing the relevant features mentioned in the description.


PHASE III DUAL USE APPLICATIONS: Validate analysis tool with experimental data from a relevant flexbeam configuration with flight realistic loading. Transition the tool to NAVAIR Structures 4.3.3 and the Fleet Readiness Centers; both will benefit from the tool in acquisition and sustainment respectively.


Flexbeam structures are not unique to military rotorcraft, but are used on civilian rotorcraft as well. Current analysis tools used by major rotorcraft manufacturers have fallen short in accurately modeling flexbeams, especially in fatigue. A successful tool would provide the private sector improved analysis tools, reducing costly and schedule slipping redesign and retesting of flexbeams that fail strength or durability requirements. Any industry that uses helicopters (e.g., tours, transportation) and performs maintenance on helicopters would benefit from this technology development.



1.   Hoos, K., Larve, E., Braginsky, M., & Zhou, E. “Progressive Failure Simulation in Laminated Composites Under Fatigue Loading by Using Discrete Damage Modeling.” 57th AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 2016. under_Fatigue_Loading_by_Using_Discrete_Damage_Modeling


2.   Kant, T. & Swaminathan, K.“Estimation of Transverse/Interlaminar Stresses in Laminated Composites - A Selective Review and Survey of Current Developments.” Composites Structures, Volume 49, Issue 1, pp. 65-75.


3.   Larve, E., Gurvich, M., Mollenhauer, D., & Rose, C. “Mesh-Independent Matrix Cracking and Delamination Modeling in Laminated Composites.” International Journal for Numerical Methods in Engineering, 88(8), 2011, pp. 749-773. independent_matrix_cracking_and_delamination_modeling_in_laminated_composites


4.   Murri, G. B. “Influence of Ply Waviness on Fatigue Life of Tapered Composite Flexbeam Laminates.” 1999 Report/Patent Number: NASA/TM-1999-209830, NAS 1.15:209830, ARL-TR-2110, L-17908.


5.   Murri, G. B., & Schaff, J. R. “Fatigue Life Methodology For Tapered Hybrid Composite Flexbeams.” Composites Science and Technology, Volume 66, Issues 3-4, March 2006, pp. 499-508.


6.   Murri, G. B., Schaff, J. R., & Dobyns, A. L. “Fatigue and Damage Tolerance Analysis of a Hybrid Composite Tapered Flexbeam.” NASA Langley Technical Report Server, 2001.


7.   Weiss, A., Trabelsi, W., Michel, L., & Barrau, J.-J. “Influence of Ply-Drop Location on the Fatigue Behaviour of Tapered Composites Laminates.” Procedia Engineering, 2(1), April 2010. drop_location_on_the_fatigue_behaviour_of_tapered_composites_laminates


8.   Wui Gan, K., Allegri, G., & Hallett, S. “A Simplified Layered Beam Approach for Predicting Ply Drop Delamination in Thick Composite Laminates.” Materials & Design, Volume 108, 15 October 2016, pp. 570-580.


KEYWORDS: Composite Analysis; Strength; Fatigue; Flexbeam; Multiaxial Loading, Ply Drop-offs



Gabriel Murray





Tommy Chen





These Navy Topics are part of the overall DoD 2019.2 SBIR BAA. The DoD issued its 2019.2 BAA SBIR pre-release on May 2, 2019, which opens to receive proposals on May 31, 2019, and closes July 1, 2019 at 8:00 PM ET.

Between May 2, 2019 and May 30, 2019 you may communicate directly with the Topic Authors (TPOC) to ask technical questions about the topics. During these dates, their contact information is listed above. For reasons of competitive fairness, direct communication between proposers and topic authors is not allowed starting May 31, 2019
when DoD begins accepting proposals for this BAA.

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