Yuri Nikishkov

Modeling rotorcraft dynamics with finite element multibody procedures

This paper describes a multibody dynamics approach to the modeling of rotorcraft systems and reviews the key aspects of the simulation procedure. The multibody dynamics analysis is cast within the framework of nonlinear finite element methods, and the element library includes rigid and deformable bodies as well as joint elements. No modal reduction is performed for the modeling of flexible bodies. The structural and joint element library is briefly described. The algorithms used to integrate the resulting equations of motion with maximum efficiency and robustness are discussed. Various solution procedures, static, dynamic, stability, and trim analyses, are presented. Postprocessing and visualization issues are also addressed. Finally, the paper concludes with selected rotorcraft applications.

An Implicit Floquet Analysis for Rotorcraft Stability Evaluation

Floquet theory has been extensively used for assessing the stability characteristics of periodic systems. In classical application of the theory, the transition matrix of the system is explicitly computed flrst, then its eigenvalues are evaluated. Stability of the system depends on the dominant eigenvalue: if this eigenvalue is larger than unity, the system is unstable. The proposed implicit Floquet analysis extracts the dominant eigenvalues of the transition matrix using the Arnoldi algorithm, without the explicit computation of this matrix. As a result, the proposed method yields stability information at a far lower computational cost than that of classical Floquet analysis, and is ideally suited for stability computations of systems involving a large number of degrees of freedom. Examples of application of the proposed methodology are presented that demonstrate its accuracy and computational e‐ciency.

Simulation of damage in composites based on solid finite elements

Solid finite element‐based techniques are attractive for simulation of the matrix-dominated failure modes in composites. This work shows the ability of such techniques to capture the initiation and growth of matrix ply cracks and delaminations in carbon/epoxy laminates. Three-dimensional (3D) finite element models for open-hole tensile test articles are developed. Such models account for the micromechanical damage in the matrix through nonlinear interlaminar stress‐strain relations. Stress-based and fracture mechanics‐based failure criteria are used to predict the matrix-dominated failures. Material stiffness loss consistent with the failure criteria is implemented in the 3D solid finite element material definition procedure forsimulationofthematrix-plycracks.Initialdamageandapredefinedpatharenotrequired.Also,cohesive-zonemodelsare used to capture delamination. Damage initiation and growth simulations for the open-hole tensile articles are accomplished. Damage sequence, surface strains, and failure loads are verified with tests.

Finite element mesh generation for composites with ply waviness based on X-ray computed tomography

A method for automated generation of finite element meshes for unidirectional composites with waviness defects is proposed. Images used as input for mesh generation are recorded with X-ray computed micro-tomography. Quality and contrast of the scanned images is such that fiber directions cannot be detected everywhere. To generate finite elements mesh that follow fiber directions it is suggested to interpolate available fiber slope data using radial basis functions and to create mesh nodes by integrating ordinary differential equations for fiber slopes. Examples demonstrate practical steps of detecting waviness from volume slice images and generation of meshes that model waviness defects with acceptable accuracy.

An Implicit Transition Matrix Approach to Stability Analysis of Flexible Multi-Body Systems

The stability of linear systems defined by ordinarydifferential equations with constant or periodic coefficients can beassessed from the spectral radius of their transition matrix. Inclassical applications of this theory, the transition matrix isexplicitly computed first, then its eigenvalues are evaluated; if thelargest eigenvalue is larger than unity, the system is unstable. Theproposed implicit transition matrix approach extracts the dominanteigenvalues of the transition matrix using the Arnoldi algorithm,without the explicit computation of this matrix. As a result, theproposed implicit method yields stability information at a far lowercomputational cost than that of the classical approach, and is ideallysuited for stability computations of systems involving a large number ofdegrees of freedom. Examples of application of the proposed methodologyto flexible multi-body systems are presented that demonstrate itsaccuracy and computational efficiency.

Structural analysis of composites with porosity defects based on X-ray computed tomography:

Advanced structural analysis methods that account for manufacturing defects in composite parts are needed to enable accurate assessment of their capability and useful life and to enhance current design and maintenance practices. In particular, porosity/voids are typical defects in carbon/epoxy and glass/epoxy composite aircraft flight-critical components. High-fidelity nondestructive evaluation by X-ray computed tomography allows accurate defect measurement and automatic conversion to structural models to assess the effects of defects on structural properties. This study presents a comprehensive structural analysis methodology, which includes nondestructive detection and finite element modeling of the defects in composites. Effects of porosity/voids on interlaminar tensile and shear strength of unidirectional carbon/epoxy composite specimens are investigated. Failure predictions and subsequent test correlations are presented.

Fatigue Life Assessment for Composite Structure

This work presents some of the most recent advances in the technologies which could enable accurate assessment of useful life for composite aircraft fatigue-critical, flight-critical components and structure. Such technology advances include: (1) nondestructive subsurface measurement shift from just detection of defects to three-dimensional measurement of defect location and size; (2) material characterization methods ability to generate 3D material allowables at minimum time and cost; and (3) fatigue structural analysis techniques ability to capture multiple damage modes and their interaction. The authors summarize their recent results in all three subjects.

Characterization of Complex Porous Structures for Reusable Thermal Protection Systems: Porosity Measurements

This work is focused on the nonintrusive characterization of the local and average porosity of a prototype carbon–carbon nose, representative of a reusable thermal protection system based on transpiration cooling. A study based on the x-ray computed tomography scan of the specimen has been carried out with the purpose of defining the most important guidelines for the permeability tests, which are the minimum area to be probed with a hot-film anemometer and the correct distance of the mass flux sensor from the wall. The former has been calculated from the average porosity calculation, whereas the latter has been retrieved from the statistical analysis of the dimensions, and the distribution of the void structures inside the porous network coupled to the theory of fluid flow through perforated plates. Several longitudinal and transversal sectioning planes with respect to the symmetry axis of the carbon mask have been analyzed to calculate the internal porosity from the two-dimensional images, whereas the th...

Methods for assessment of interlaminar tensile strength of composite materials

Among the mechanical properties of polymer-matrix composite materials, the interlaminar tensile strength is among the most difficult to characterize. ASTM Standard D 6415 uses a curved-beam configuration for measuring interlaminar tensile strength. Not only the manufacturing process to produce curved-beam coupons with uniform radius and thickness could be challenging but also the curved-beam strength data typically exhibits large scatter. One question is whether ASTM D 6415 curved-beam interlaminar tensile strength data are coupon-specific, that is the curved-beam strength is not really a coupon-independent material property, suggesting that ASTM D 6415 is not adequate to measure interlaminar tensile strength. The objective of this work is to develop efficient and accurate methods to capture interlaminar tensile strength of composites. The authors expand a recently developed short-beam method coupled with the digital image correlation full-field deformation measurement technique to measuring the interlaminar tensile strength. The interlaminar tensile strength data are presented for IM7/8552 tape composite system. However, average curved-beam strength value is significantly lower compared to the short-beam test results. Micro-focus CT measurements show that porosity in the radius area is the reason for the low average strength value and the large scatter in the curved-beam strength test data. Once the stress concentration effects of porosity are captured through transfer of CT measurements into three-dimensional finite element model, the short-beam and the curved-beam test results agree. The short-beam method, which measures the interlaminar tensile strength for a pristine material, and the refined curved-beam method which accounts for manufacturing defects, represent more complete interlaminar tensile strength assessment methodology for composite structural designs.

Structures perspective for strength and fatigue prognosis in composites with manufacturing irregularities

Recent advances in understanding deformation and failure mechanisms of polymermatrix composites used in rotor structures enable accurate and efficient measurement of material stiffness, strength, and fatigue characteristics based on testing small unidirectional laminate specimens. Successful failure predictions increased our confidence in the development of virtual test methods replacing some of the standard tests of multi-directional laminated composite materials with three-dimensional models accurately predicting deformation, damage topography, strength, and cycles to failure. However, the remaining key questions are related to the ability of transitioning the material-scale virtual test information to larger composite structures. This work presents results of the feasibility assessment targeting the scaling of knowledge and methods acquired at the material scale, to larger structural elements.