Journal of Aerospace Engineering

This paper describes the experimental and analytical evaluation of an externally deployable composite honeycomb structure that is designed to attenuate impact energy during helicopter crashes. The concept, designated the deployable energy absorber (DEA), uses an expandable Kevlar honeycomb to dissipate kinetic energy through crushing. The DEA incorporates a unique flexible-hinge design that enables the honeycomb to be packaged and stowed efficiently until needed for deployment. Experimental evaluation of the DEA included dynamic crush tests of multicell components and vertical drop tests of a composite fuselage section retrofitted with DEA blocks onto multiterrain. Finite-element models of the test articles were developed and simulations were performed using the transient dynamic code LS-DYNA. In each simulation, the DEA was represented using shell elements assigned two different material properties: Mat 24, an isotropic piecewise linear plasticity model, and Mat 58, a continuum damage mechanics model used to represent laminated composite fabrics. DEA model development and test analysis comparisons are presented.

The National Aeronautics and Space Administration (NASA) has sponsored research to evaluate an externally deployable composite honeycomb designed to attenuate loads in the event of a helicopter crash. The concept, designated the deployable energy absorber (DEA), is an expandable Kevlar honeycomb. The DEA incorporates a flexible hinge that allows the honeycomb to be stowed collapsed until needed during an emergency. Evaluation of the DEA began with material characterization of the Kevlar-129 fabric/epoxy and ended with a full-scale crash test of a retrofitted MD-500 helicopter. During each evaluation phase, finite-element (FE) models of the test articles were developed, and simulations were performed using the dynamic FE code LS-DYNA. This paper focuses on simulations of two full-scale impact tests involving the DEA: a mass simulator and a DEA-retrofitted MD-500 helicopter. Isotropic (Mat 24) and composite (Mat 58) material models that were assigned to DEA shell elements were compared. Based on simulation results, the Mat 58 model showed better agreement with the test results.

Although the Orion Multi-Purpose Crew Vehicle (MPCV) is being designed primarily for water landings, the possibility exists of an onshore landing at Kennedy Space Center (KSC) following a launch abort. To obtain data to evaluate the adequacy of sand material models used in beach landing simulations, sand impact tests were conducted at National Aeronautics and Space Administration (NASA) Langley Research Center (LRC). Both vertical drop tests and swing tests with combined vertical and horizontal velocity were performed onto beds of common construction-grade sand using a geometrically scaled crew module test article. The tests were simulated using the explicit, nonlinear, transient dynamic finite-element code LS-DYNA. The shape of the test article was similar to a scaled crew module, but the structure was much heavier and was sufficiently robust to survive multiple impacts without permanent deformation. The focus was the sand model, not the structural response of the test article. The sand material models utilized in the simulations were based on tests of sand specimens. Although the LS-DYNA models provided reasonable predictions for peak accelerations, they were not always able to track the response through the duration of the impact. Further improvements to the material model used for the sand were identified based on results from the sand specimen tests.

Ballistic impact testing was conducted on aluminum (Al) 2024 and titanium (Ti) 6Al-4Vanadium (V) sheet and plate samples of different thicknesses. The tests were conducted on flat panels with two different areal dimensions: $61\times 61\hspace{0.17em}\text{cm}$ ($24\times 24\hspace{0.17em}\text{in.}$) (large panel) and $38\times 38\hspace{0.17em}\text{cm}$ ($15\times 15\hspace{0.17em}\text{in.}$) (small panel), with projectile velocities between $150\hspace{0.17em}\text{and}\hspace{0.17em}300\hspace{0.17em}\text{m/s}$ ($500\hspace{0.17em}\text{and}\hspace{0.17em}\mathrm{1,000}\hspace{0.17em}\text{ft/s}$). The smaller panels were impacted in a normal direction with cylindrical projectiles ranging in diameter from 1.27 cm to 1.91 cm (0.5 to 0.75 in.). The larger panels involved a more complex projectile, with some features representing those of real turbine engine fan blades, impacting the panel in an oblique orientation. For the large panels, data are reported on the impact velocity and whether the projectile penetrated the panel; for the small panels, data are reported on the residual velocities. For thick panels, friction between the projectile and the panel as the projectile translated through the panel appear to play a large role in penetration. Projectile hardness affects the penetration velocity. The data presented here provide a useful tool for developing and validating impact models.

A method has been developed to analyze the impact response of triaxially braided carbon fiber composites, including barely visible damage. In the analytical model, the triaxial braid architecture is simulated using four parallel shell elements, each of which is modeled as a laminated composite. Because of the construction of the composite material, the resin regions between layers are weak areas in the material structure and can initiate damage by delaminating. To simulate these regions, each shell element is modeled as a homogeneous material with a zero-thickness cohesive zone (simulated with LS-DYNA’s tie-break contact) between the layers of braid. To accurately define the tie breaks used to model delamination, the resin materials’ cohesive zone must be defined. The length of the cohesive zone is calculated internally from parameters such as convergence scale factor, elastic modulus, peak traction, and strain energy release rate. To reliably model a cohesive zone, the zone should encompass three finite elements. This can lead to a reduced element length, which is impractical for many types of macroscale simulations. The convergence scale factor is used to efficiently model larger elements by applying a parametric reduction factor to effectively lengthen the cohesive zone while maintaining equivalent fracture energy.

This paper is concerned with elastic behavior of a triaxial braided composite by using a three-dimensional analytical model and mesoscale finite-element (FE) analysis, in conjunction with experimental observations. The analytical method and FEM take into account the actual fabric structure by considering the fiber undulation and actual architecture parameters. A representative unit cell model of the triaxial braided architecture is first identified based on fiber volume ratio, specimen thickness, and microscopic image analysis. Detailed geometric parameters for axial and bias fiber bundles are obtained, which provide precise information to enable the development of analytical and FE models. A general three-dimensional analytical model based on realistic architecture is developed with consideration of axial and bias fiber undulation. A typical study on the effect of axial fiber undulation is presented through the analytical model and axial tensile test. The prediction of effective elastic constants of the composite are presented and are compared with the experimental data. Edge damage, which is identified through FE simulation and experiments, is found to occur periodically along the free edge and to result in reduced transverse properties. The edge effect on the elastic modulus is studied through FE simulation with various numbers of unit cells. The axial fiber undulation and edge effect explains the discrepancy between the analytical and computational predictions and the experimental results.

The focus of this paper is on improving the modeling of dry fabric-based systems widely used in propulsion engine containment systems. Simulations of fabrics under impact using finite elements (FEs) provide valuable insight into the mechanics of fabric behavior and its energy absorption characteristics. In this model, the fabric is treated as an equivalent continuum model composed of thin shell elements. The fabric is made of simple weave Kevlar49, whose behavior is characterized using uniaxial quasi-static and high-strain rate tension tests, picture frame tests, and friction tests. The methodology is general enough to be used with any dry fabric. Test results are used to determine the parameters used in the constitutive and FE models. The constitutive model is programmed as a user-defined material model. Details and results from FE simulations of a suite of 26 high-velocity ballistic test results are presented. Results show that the developed model provides very reasonable predictive capabilities.

In this paper, the effect of temperature and nanoclay reinforcement percentage on nanoclay-reinforced polymeric nanocomposites is studied. First, polypropylene (PP 3371) resin reinforced with various nanoclay percentages is tested at room, elevated, and low temperatures. The tests are conducted on ASTM Type I specimens instrumented with strain gauges in a material testing system machine equipped with an environmental chamber. Next, to ascertain the effect of various PP resins, nanoclay-reinforced Borealis and total petrochemical (TP 3868) tensile specimens are tested from low to elevated temperatures. In addition, nanoclay-reinforced epoxy specimens are tested at room temperature. The test results are plotted as stress-strain curves and the mechanical properties of the nanocomposites including the Young’s modulus, Poisson’s ratio, ultimate stress, and failure strain are determined. The tensile test results indicate that the Young’s modulus of the nanocomposite increases with increasing nanoclay reinforcement percentage. The temperature has even a more significant effect. It was observed that as the temperature decreases the material becomes brittle, has higher stiffness, and fails at lower strains. High temperatures have the opposite effect. As the temperature increases the material loses stiffness and becomes more ductile. Temperature and nanoclay reinforcement also affect the Poisson’s ratio but this effect is less significant. In general, as the temperature increases the Poisson’s ratio also increases. However, an increase in nanoclay reinforcement generally reduces the Poisson’s ratio. It is also noted that the type of resin used may have a significant effect on the mechanical properties of the nanocomposite.

In this paper, the modeling and theoretical prediction for the results given in the companion paper for nanoclay-reinforced polymers subjected to mechanical and thermal loads are presented. In the companion paper, the mechanical properties for three grades of polypropylene and epoxy reinforced with nanoclay were experimentally determined at various temperatures. In this study, the Mori-Tanaka formulations (for oriented particles, two-dimensional randomly distributed particles and three-dimensional randomly distributed particles) are used and the FEM, Young’s modulus, and Poisson’s ratio are calculated and then compared with the experimental results. The Mori-Tanaka formulation is modified to take into account nanoclay particles of varying dimensions and also the effect of voids. In addition, at high temperatures the formulation is further modified to include the effect of temperature in the calculation of the Young’s modulus. It is found that the results obtained from the modified Mori-Tanaka calculations compare well with the experimental results. The finite-element calculations also provide a reasonable estimate for the Young’s modulus; however, the results are less predictive than the Mori-Tanaka results.

Numerical simulation is often used in the design of aircraft structural components subjected to bird-strike loads. To verify the numerical method, a test of bird striking was performed on honeycomb-based sandwich panels made from 3003 aluminum honeycomb and 2024-T3 aluminum skin. The dynamic responses of displacement and strain on the rear skin of the sandwich panel during bird striking were measured using strain gauges and a laser sensor, respectively. A numerical model of bird striking was developed with a nonlinear, finite-element program. A coupling smooth particle hydrodynamics–finite-element analysis algorithm was used to simulate the interaction of the impacting contact between the bird and the target. The numerical simulation results were compared with the tests, and they are in good agreement, which validated the reasonableness of the numerical model established in the present paper. This shows the potential of using the FEM in simulating the dynamic response of aircraft structural components in bird-strike events.

This paper investigates the stochastic nonlinear flexural response of laminated composite plates resting on a two-parameter Pasternak elastic foundation with Winkler cubic nonlinearity and random system properties subjected to transverse uniform lateral pressure and hygrothermal loading. System properties, such as the lamina material properties, coefficients of thermal expansion, coefficients of hygroscopic expansion, and lateral load, are modeled as basic random variables using a micromechanical model. A higher-order shear deformation theory in the von Kármán sense is used to model the system behavior of the laminated plate. A direct iterative-based $C^0$ nonlinear FEM, in conjunction with the first-order perturbation technique previously developed by the authors, is extended for the hygrothermal problem to obtain the second-order response statistics, i.e., the mean and variance of the nonlinear transverse deflection of the plate. Typical numerical results for the second-order statistics of the nonlinear transverse central deflection of composite plates subjected to uniform temperature and moisture distribution over the plate surface and through the plate thickness are obtained for various combinations of foundation parameters, uniform lateral pressures, staking sequences, volume fraction, aspect ratio, plate thickness ratio, and boundary conditions under environmental conditions. The approach has been compared with those available in the literature and an independent Monte Carlo simulation (MCS).

A theory governing the flexural vibration of a conducting two-layer sandwich homogeneous elastic beam in a transverse magnetic field is presented. The physics driving this problem derives from an energy dissipation mechanism through press-fit joints in structural laminates. Recent advances made in the mechanics of sandwich-layered structures have shown that by simulating an environment of nonuniform interface pressure, structural vibration can be attenuated significantly. Equations of mathematical physics governing the stresses and the structural vibration are derived via a laminated beam theory employing the Newtonian form of Cauchy’s stress equations. By restricting mathematical analysis to the case of cantilever architecture, a closed-form polynomial expression is derived for the system response. In particular, the effects of magnetoelasticity, material conductivity, and interfacial pressure gradient on the response characteristics are demonstrated for design analysis and engineering applications. It is shown via integral transforms that each mode of vibration is governed by a two-dimensional family of natural frequency. For special and limit cases, recent theoretical and experimental results are validated from the theory reported in this paper.

High-precision cable net reflectors are widely required for future communications and observations. Concerning the improvement of their surface accuracy, the active cable structure constructed by incorporating a piezoelectric (PZT) actuator into flexible cables has been proposed to achieve active shape control or active correction of the reflector surface. Two design stages of the cable net reflector with active cables, the form-finding analysis and active shape adjustment, are investigated. The form-finding analysis is derived from nodal force equilibrium equations by the minimum norm method-solving linear equations. An active shape adjustment method with the aid of active cables is then proposed to find the actuation voltages of PZT actuators for the prescribed shape. The adjustment method is divided into three parts: a finite-element model of the active cable net reflector, an active adjustment optimal model, and its solving method using the multidimensional advance-retreat algorithm. Finally, numerical examples of a parabolic cable net reflector are analyzed. Simulation results demonstrate that the proposed methods can warrant cable tension forces, which fall into a specified range, and the deformed cable net reflector surface can match the one designed by PZT actuators.

In this paper, a sliding mode reconfigurable control algorithm is developed to deal with nonlinear aircraft system with partial loss fault or total loss failure of actuators. Sliding mode controllers for redundant actuators are combined with those for regular actuators to reconfigure the control system autonomously with the effectiveness information of the regular actuators. The tolerability of the sliding mode control system for faults is defined as the limit of regular actuators under fault conditions. The stability of the control algorithm is proved with Lyapunov method. The effectiveness of the developed control system has been validated on simulation of the longitudinal control of a nonlinear Boeing $747\text{-}100/200$ airplane model.

In this paper, an adaptive neural network controller is proposed for a nonaffine-nonlinear aircraft cabin pressure system with unknown parameters. A multilayer neural network is used to represent the controller structure. The ultimate boundedness of the closed-loop system is guaranteed through a Lyapunov stability analysis by introducing a suitably driven adaptive rule. The effectiveness of the proposed adaptive controller is illustrated by considering an aircraft cabin pressure system, and the simulation results verify the merits of the proposed controller.

In this paper, a backstepping-based nonlinear guidance controller is developed, designed for three-dimensional trajectory tracking for an unmanned aerial vehicle. The guidance law design assumes first-order dynamics for the speed, course/heading, and flight path angles. The three-dimensional kinematic model is simplified to yield a seemingly decoupled system that separates the course/heading from the flight path angle. A closed loop stability of the guidance laws via Lyapunov analysis is demonstrated, and the efficacy of the controller is illustrated for three representative cases.

Navigation of gun-launched precision munitions using affordable technologies is investigated. Estimation algorithms were developed to blend flight dynamic models with measurements from inertial sensors. The launch and flight characteristics of the unique, gun-launched environment were exploited both in the state estimator and for novel heuristic parameter identification. Using the heuristic parameter, the inertial navigation system was calibrated without a global positioning system (GPS), and its performance was compared with a similar calibration performed with a GPS. The experimental results from guided mortar flights indicate that the algorithm with only inertial sensor measurements yields position errors of less than 40 m over a 30-s flight. Trade studies were conducted in simulation to assess algorithm performance over a wider range of conditions such as variation in the flight dynamic model parameters. These results demonstrate that position errors are less than tens of meters for flight times of interest to munitions. Estimation is tolerant to inertial sensor errors because of the novel manner in which known flight dynamics are used to compensate measurements. Overall, this effort shows that navigation error resulting from a low-throughput algorithm using affordable inertial sensors is sufficient to increase system accuracy for munitions in GPS-denied environments.

In this paper, to reduce the computational load of the federated Kalman filter, an expectation-maximization federated Kalman filtering (EM-FKF) algorithm for integrated navigation systems is proposed. First, the states with poor estimate accuracies are removed from local filters to reduce the computational load. Then the EM algorithm is applied. More precisely, the common states for each local filter are estimated in E-step of EM algorithm by using a linear Kalman filtering algorithm, and its own unique sensor biases are updated in M-step of EM algorithm. The M-step in the local filters and the fusion of common states in the master filter are performed simultaneously, so the computational burden is further reduced for the federated Kalman filter. The proposed algorithm was evaluated with simulated data first, then an experiment was conducted on a real inertial navigation system, global positioning system, and star sensor (INS/GPS/SS) integrated navigation system to verify the proposed algorithm. The results of the simulation and the experiment demonstrated that the proposed algorithm effectively reduced the computational load, compared with the standard federated Kalman filtering algorithm.

To realize reliable and autonomous regolith sampling from asteroids in space, the author has developed a new sampling system that uses electrostatic force. This system uses a combination of electrostatic capture and transport of particles. High voltage is applied between parallel screen electrodes of the sampling device. As a result of the electrostatic force, particles are captured by the screen electrodes. The captured particles are then transported by an electrostatic traveling wave and transferred to a collection capsule. It has been demonstrated that lunar soil simulant, iron particles, and crushed ice can be sampled if the end of the screen electrode is in contact with the regolith for a short period of time (within 1 s). Because gravity is extremely low on small asteroids, the process of sampling particles on asteroids will be easier than that on the Earth.

In the present paper, the unsteady oscillatory stagnation point flow of a Jeffrey fluid is discussed. The oscillatory stagnation point flows have been discussed in a fixed frame of reference and a moving frame of reference. The convergence of the obtained analytical solutions has been discussed by plotting $\hslash \text{-curves}$. Lastly, the physical significance of various parameters is discussed through graphs.

This paper describes the study of two-dimensional oscillatory flow inside a rectangular channel for Jeffrey fluid with small suction. The main objective is to investigate the viscoelastic behavior of non-Newtonian fluids subject to time harmonic oscillations. The fluid flow in the channel is generated by suction at the porous walls while time harmonic pressure waves give rise to oscillations in the velocity field. Analytical solution of the time-independent axial velocity and time-dependent oscillatory axial velocity in the channel is obtained. To understand the characteristics of non-Newtonian fluids, the effects of retardation time, and Deborah number on the oscillatory velocity, its amplitude and penetration depth are analyzed. The methodologies adopted to solve this physically complex problem are perturbation, Wentzel-Kramers-Brillouin, and variation-of-parameter. The paper will help in understanding the characteristics of non-Newtonian fluids that are subject to applied oscillatory harmonic strains.