You are here

Novel Multi-Axial Fatigue Analysis Tool for Dynamic Components using Frequency Domain Method



TECHNOLOGY AREA(S): Air Platform, Space Platforms

ACQUISITION PROGRAM: PMA-276, H-1 USMC Light/Attack Helicopters

OBJECTIVE: Develop a novel repair assessment and remaining useful life analysis tool for rotorcraft dynamic components using a frequency domain fatigue analysis method which takes into account the effects of multi-axial, local plasticity, and damage state of the component.

DESCRIPTION: During service operations, aircraft structures sustain damage. That damage is routinely discovered via inspection and subsequently repaired. This damage is often caused by fatigue, corrosion, accidents, and mishaps. Repairs are performed to restore the integrity of the part to the original part strength and durability. However, some repair operations involve improper blending/grind-outs, or over-stiffening that may move the fatigue critical locations to another point in the structure causing cracks to start in new locations. There is currently no standard repair procedure that applies to all cases. The problem of improper repair is even more acute on rotorcraft dynamic components because of their constant exposure to damaging environments and increased frequency of incidents.

Analysis of each repair for strength, durability, and damage tolerance is an involved process as it requires evaluation of static and fatigue margins and impact on adjacent structures. In some cases, there are load redistributions because of local changes to stiffness and geometry that need to be analyzed by finite-element (FE) analysis. The complexity of repair assessment further increases if the structure or component is subjected to multi-axial loadings.

Fatigue evaluation in spectral methods is typically simplified by substituting spatial tri-axial stress state to the equivalent uniaxial one with suitable failure criteria. Appropriate probabilistic characteristics are then applied for calculations of fatigue life under the uniaxial random loading. Components under random multi-axial loading need multi-axial fatigue analysis at numerous critical points which requires significantly higher computational effort. In addition, the local plasticity affects at stress concentrators needs to be incorporated in the solution. An alternative novel formulation of multi-axial fatigue analysis under random loading is needed.

Assessment of repair work done in service on dynamic components requires a quick and reliable fatigue damage evaluation method which takes into account the effects of static and dynamic response of the component for a given loading exposure and application of the right fatigue methods to investigate the impact on fatigue life. The recent advances in structural analysis methods and fatigue damage evaluation using frequency domain methods offer the potential to quickly evaluate and assess the repair work of components subjected to complicated service loads.[3, 4]

Develop an analytical tool to assess repair and evaluate remaining useful life of dynamic components in service. The tool must consider local repair geometry, load redistributions, and static and dynamic response to quickly assess strength, durability and damage. The tool must also be general enough to address simple to complex repair geometries and loading situations. This analysis tool should address both static and dynamic analysis needs, damage evolution within the component, and different responses to dynamic excitation due to the presence of damage. In addition, the tool should be able to efficiently calculate accumulated damage starting from input service histories and include quasi-static and dynamic events in complex loading sequences and superposition effects.

Though not required, coordination with original equipment manufacturers (OEM) is recommended throughout the effort.

PHASE I: Develop an innovative, analytical tool using frequency domain methods to assess strength and durability of repaired dynamic components subjected to quasi-static and dynamic excitations. Demonstrate proof of concept and efficiency of solution.

PHASE II: Further mature the approach developed under Phase I to include the effects from a variable multi-axial stress state, local plasticity, and the resulting changes in component dynamic and static response to accumulated damage. Demonstrate the accuracy of the numerical solutions for repair assessment using experimental data of varying degrees of complexity and type of loading. Integrate the methodology within a user interface environment to enable the analysis of components starting from its geometry, applied loads, and boundary conditions.

PHASE III DUAL USE APPLICATIONS: Commercialize and transition the developed repair assessment and remaining life prediction tool as an analysis package. A detailed verification and validation effort will be performed along with a demonstration of application capability in a production-type and widely used tool. To further the technology transition, the developed repair could be installed and flight tested on a fleet representative airframe in with cooperation with the interested PMA(s) and OEM. Private Sector Commercial Potential: Methods and techniques developed can be included in a commercial software package for broad use in a wide variety of industrial applications in order to estimate the life of safety critical structures and components.


  • Bishop, N. W. M. & Sherratt, F., Fatigue life prediction from power spectral density data. II: Recent developments. Environmental engineering (1988) 2.2 (1989): 11-15.
  • Potoiset, X. & Preumont, A., (1998). Tools for a multiaxial fatigue analysis of structures submitted to random vibrations. Proceedings European Conference on Spacecraft Structures Materials and Mechanical Testing, Germany.
  • Braccesi, C., Cianetti, F., Lori, G., & Pioli, D., (2015). Random multiaxial fatigue: A comparative analysis among selected frequency and time domain fatigue evaluation methods. International Journal of Fatigue, Volume 74.
  • Benasciutti, Denis, Frank Sherratt, and Alessandro Cristofori. Basic Principles of Spectral Multi-axial Fatigue Analysis. Procedia Engineering 101 (2015): 34-42.

KEYWORDS: Durability; Multi-Axial Fatigue; Remaining Useful Life; Frequency Domain Fatigue Analysis Method; Multi-Axial Loads; Repair Assessment

US Flag An Official Website of the United States Government